Rapid and Safe Drying of Large-Sized Alumina Ceramic Gel Bodies

Rapid and Safe Drying of Large-Sized Alumina Ceramic Gel Bodies | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Rapid and Safe Drying of Large-Sized Alumina Ceramic Gel Bodies Mingyue Wang, Lixin Wang, Zhiyi Wang This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6587948/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Large-sized or complex-shaped precision ceramic components are crucial in high-end industries such as aerospace, deep-sea exploration, and medical technology. However, traditional ceramic forming techniques encounter several challenges in manufacturing these components. The gel casting technique presents new opportunities to overcome these difficulties. However, it demands strict drying conditions, leading to lengthy drying cycles and low yield rates, which significantly limit its industrial application. This study aims to investigate a safe and efficient drying system for large alumina ceramic gel bodies, focusing specifically on the drying safety of water-based gel systems. Experimental results indicate that when the monomer addition amounts to 6 wt% and the ratio of monomer to crosslinking agent is 75:1, employing a two-stage drying method leads to optimal outcomes. The initial drying stage occurs at a temperature of 50°C with a humidity level between 80% and 72% RH. In the second stage, air drying is conducted directly once the moisture content drops below 17%. This process successfully produces a ceramic green body with a uniform microstructure, fully compliant with machining requirements. Consequently, rapid and safe drying of large-sized alumina ceramic gel bodies is achieved. Physical sciences/Engineering Physical sciences/Materials science Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction Alumina ceramics possess exceptional properties such as high strength, high hardness, resistance to high temperatures, wear resistance, and corrosion resistance to acids and bases. These qualities have led to their widespread applications in industries like aerospace, petrochemicals, national defense, and marine development. As technology advances, the demand for advanced ceramics is increasingly shifting towards large-sized or complex-shaped precision components. However, the production of these components presents significant challenges, as it requires high precision along with excellent wear resistance, strength, and hardness. Traditional ceramic forming techniques often fall short in meeting these stringent requirements. Fortunately, the introduction of gelcasting technology of ceramics has successfully addressed these challenges 1 – 8 , paving the way for the efficient production of large-sized precision ceramic components. Gel casting technology combines organic chemistry with traditional ceramic forming processes, allowing for the near-net-shape production of ceramic green bodies that exhibit high strength, low organic content, easy debonding, and some plasticity 9 – 16 . This technology meets the demands of precision mechanical processing effectively. Gel casting offers several significant advantages 17 – 20 : a) Wide Adaptability: It does not have specific requirements for ceramic powders and can quickly form large, complex-shaped components with varying wall thicknesses. b) Diverse Mold Material Options: Various materials can be used for molds, and the forming cycles are relatively short. c) Easy Precision Machining: The ceramic green bodies possess high strength and some plasticity, making them suitable for precision mechanical processing. d) Excellent Environmental Performance: The organic content in the green bodies is low, especially in water-based gel systems. e) Simple Equipment Structure: The equipment used in this process is straightforward, contributing to lower manufacturing costs. Gel casting technology has a significant drawback: it necessitates extremely strict drying conditions. The ceramic gel bodies produced through this method contain a considerable amount of moisture. During the drying phase, the rapid evaporation of water leads to a higher shrinkage rate, making these bodies susceptible to cracking and collapse. Moreover, the drying process is complex and slow, particularly for larger bodies, where uneven shrinkage can create concentrated structural stress and residual stress, resulting in deformation or warping. In summary, the drying process for ceramic gel bodies is lengthy and vulnerable to issues like deformation, cracking, and collapse, leading to a low yield rate. This problem is especially pronounced for larger, thick-walled bodies, significantly limiting the large-scale use of gel casting in the ceramics industry. In recent years, researchers have conducted extensive studies on various drying methods for ceramic gel bodies 21 – 25 . These methods include hot air drying, high-humidity drying, room-temperature drying, electric heating drying, radiation drying, liquid drying, and freeze drying, each yielding specific results 26 – 28 . However, most of these studies have primarily focused on small-sized products (thickness ≤ 30 mm) and those with regular shapes. There have been no publicly reported methods for the safe and rapid drying of large-sized ceramic gel bodies (thickness ≥ 50 mm) or those with complex shapes, which presents a significant challenge. The objective of this study is to achieve safe and efficient drying of large-sized alumina ceramic gel bodies. To accomplish this, the low-toxicity N-hydroxymethylacrylamide was used as the monomer and N, N-methylene bisacrylamide as the crosslinking agent. This research thoroughly examined the drying safety issues associated with the water-based gel system. Based on this, the study investigated how various factors—such as the ratio of monomer to crosslinking agent, the amount of monomer used, and the different drying methods—affect the drying safety, strength of the green body, bulk density, and water absorption rate of the large-sized alumina ceramic gel bodies. Ultimately, qualified large-sized ceramic green bodies were successfully prepared. Additionally, complex structures of alumina ceramic threads and bolts (diameter×height = 80×145 mm) with precise fitting were produced through machining and an appropriate firing schedule. Methods The 95Al 2 O 3 ceramic powder used in the experiment consisted of 95% Al 2 O 3 and 5% flux (mass ratio: talc: alumina: quartz = 63:17:20). After thorough mixing, the materials were finely ground to control the particle size distribution with D50 = 1 µm. The preparation process of alumina ceramic gel bodies was as follows: a) Preparation of Premixed Solution: The monomer N-hydroxymethylacrylamide (NMAM), the crosslinking agent N,N-methylene bisacrylamide (MBA), the dispersant ammonium polyacrylate, and the pH adjuster ammonia were dissolved in deionized water to create a premixed solution. b) Preparation of Slurry: The 95Al 2 O 3 ceramic powder was mixed with the premixed solution and ball-milled for 120 minutes to prepare the ceramic slurry. c) Gel Casting: The ball-milled slurry was subjected to vacuum degassing, and a 10 wt% ammonium persulfate solution was added as the initiator, along with tetramethylethylenediamine (TMEDA) as the catalyst. After mixing thoroughly, the slurry was injected into the silicone mold with dimensions of 50×50×50 mm (shown in Fig. 1 A) to obtain the gel body. d) Drying of Gel Body: The gel body was dried under a specified drying regime to obtain the alumina ceramic green body. The monomer content was maintained at 6% wt (based on Al 2 O 3 ceramic powder). The study examined the effects of various mass ratios of the monomer to the crosslinking agent (10:1, 25:1, 50:1, 75:1, and 100:1) on the drying process of alumina gel bodies to identify the optimal monomer-to-crosslinking agent ratio. Additionally, the research investigated how different monomer amounts (2%, 6%, 8%, 10%, and 12%) influence the drying process of the alumina gel bodies. The experiment also investigated the effects of two different drying methods on the drying process of alumina gel bodies: room temperature air drying and high-humidity constant temperature drying. a) Room Temperature Air Drying: This method was conducted at a temperature of 21°C, with humidity (definition marked in Supplementary Information) maintained at 62–65% RH. b) High-Humidity Constant Temperature Drying: In this method, temperatures were set at 25°C, 50°C, and 80°C, with humidity gradually decreasing from 82–85% RH. Furthermore, large ceramic green bodies were created through gel casting by carefully choosing the right amounts of monomers and the appropriate ratios of monomers to crosslinking agents, as well as selecting suitable drying methods for the gel bodies. Once formed, these bodies underwent CNC machining and were fired according to a specific firing schedule. This process resulted in large-sized ceramic bolts and nuts with complex structure and tight fit. Results and Discussion Drying Process and Macroscopic Morphology of NMAM System Hydrogel in Air The preparation method of the hydrogel of NMAM system is the same as that of the alumina ceramic gel body, except that no alumina ceramic powder is added. Figure 1 B illustrates the morphology and size of the hydrogel after demolding. Figure 1 C depicts the high-temperature drying process of the hydrogel in air and its macroscopic morphology. Samples (a), (b), (c), (d), and (e) each contain a monomer content of 6 wt%, with varying monomer to crosslinker ratios of 10:1, 25:1, 50:1, 75:1, and 100:1, respectively. (X 1 ) represents the demolded hydrogel, (X 2 ) indicates the hydrogel that cracked during drying, and (X 3 ) corresponds to the dried gel. From Fig. 1 C, it is observed that at a constant air temperature of 50°C, the hydrogels (a 1 ), (b 1 ), (c 1 ), and (d 1 ) cracked after one hour, two hours, 20 hours, and 24 hours of drying, respectively. Their morphologies are shown in photos (a 2 ), (b 2 ), (c 2 ), and (d 2 ). As the ratio of monomer to crosslinker increases, the degree of cracking in the hydrogel gradually decreases, transitioning from complete fragmentation in (a 3 ), (b 3 ), and (c 3 ) to an overall cracked state in (d 3 ). Notably, the hydrogel (e 1 ) did not crack during the drying process, and the dried gel remained completely crack-free. As the number of crosslinkers decreases, the degree of crosslinking in the hydrogel also diminishes. This results in a loose three-dimensional network structure that allows moisture to escape more easily. Additionally, the moisture gradient between the surface and the interior of the hydrogel decreases, which reduces the likelihood of cracking during the drying process. Experimental results indicate that when the monomer-to-crosslinker ratio is 100:1, the hydrogel can be safely dried in air at 50°C. Figure 1 D illustrates the room-temperature drying process of the hydrogel in air after demolding, along with its macroscopic morphology. The samples labeled (f), (g), (h), (i), and (j) all contain a monomer content of 6 wt%, with monomer-to-crosslinker ratios of 10:1, 25:1, 50:1, 75:1, and 100:1, respectively. Here, (X 1 ) refers to the de-molded hydrogel, (X 2 ) indicates the hydrogel that cracked during drying, and (X 3 ) represents the dried gel. From Fig. 1 D, it is evident that the drying and cracking rates of the hydrogel at room temperature are significantly lower than those at a constant temperature of 50°C. Furthermore, the extent of drying cracks is notably less severe. The hydrogels (f 1 ), (g 1 ), and (h 1 ) experienced cracking after 61 hours, 72 hours, and 120 hours of drying, respectively. The other samples displayed some cracking, except for (f 3 ), which was fragmented. Notably, the hydrogels (i 1 ) and (j 1 ) remained intact and crack-free throughout the drying process. This demonstrates that when the monomer-to-crosslinker ratios are 75:1 and 100:1, the hydrogel can be safely dried at room temperature without cracking. The ratio of monomers to crosslinkers should not be too low. If the ratio is low, the resulting gel primarily consists of linear polymer chains formed by monomer polymerization, with only a few branched chains created by crosslinkers. Although cracking does not occur during the drying process, the strength of the wet gel remains low, making it difficult to shape. The weight of the gel can lead to deformation and stress concentration, which may ultimately cause cracking during drying. For instance, when the monomer-to-crosslinker ratio is set at 100:1, the strength of the resulting gel is insufficient, making it unsuitable for the preparation of large ceramic bodies. Therefore, subsequent experiments opted for a monomer-to-crosslinker ratio of 75:1. Effects of Different Drying Methods on the Drying Process and Safety of Alumina Ceramic Gel Bodies Alumina ceramic gel bodies were prepared using silicone molds with dimensions of 50×50×50 mm, featuring a controlled monomer content of 6 wt% and a monomer-to-crosslinker ratio of 75:1. The gel bodies underwent drying using various methods: air drying at room temperature, high-humidity drying at 25℃, high-humidity drying at 50℃, and high-humidity drying at 80℃. Figure 4 (a)(b) illustrates the water loss rates (definition marked in Supplementary Information) and moisture content curves for the alumina ceramic gel bodies subjected to different drying methods. The data indicates that during the early drying stage (up to 78 hours), the water loss rates of the gel bodies were ranked as follows: room temperature drying > high-humidity drying at 80℃ > high-humidity drying at 25℃ > high-humidity drying at 50℃. In the later drying stage (after 78 hours), the ranking changed to: high-humidity drying at 80℃ > room temperature drying > high-humidity drying at 50℃ > high-humidity drying at 25℃. Figure 2 illustrates the appearance of alumina ceramic green bodies after drying using various methods. Under room temperature drying for 24 hours, the alumina ceramic body (a) exhibited significant cracks and central collapse, with a moisture content of 17.4%. With high humidity drying at 25°C, the alumina ceramic body (b) showed noticeable cracks and central collapse after 138 hours, having a moisture content of 13.1% and a humidity of 75% relative humidity (RH). After 72 hours of high moisture drying at 50°C, the body (c) showed no defects. When the sample was placed directly in a hot air dryer at 50°C, the water loss rate curve quickly aligned with and then surpassed the curve for high humidity drying at 25°C after 72 hours, resulting in no defects by the end of the drying process. Under high moisture drying at 80°C, the alumina ceramic green body (d) developed numerous small cracks after 72 hours, but there was no structural collapse, with a moisture content of 11.4% and humidity ranging between 80% and 82% RH. The early stage of the drying process (before 78 hours) is critical from both a drying and safety perspective. According to Fig. 4 (a) , the sample experienced the fastest water loss rate under room temperature drying conditions; however, cracks and collapses were observed after 24 hours. The next fastest drying method is high humidity drying at 80°C, where cracks appeared after 72 hours. In comparison, high humidity drying at 25°C showed cracks and collapses after 138 hours. Conversely, high humidity drying at 50°C had the slowest water loss rate. After 72 hours, when the high humidity protection was removed and the sample was directly subjected to 50°C hot air drying, no defects occur, resulting in safe drying. Therefore, for thick-walled ceramic gel bodies, it is essential to strictly control the moisture removal rate in the early drying stage to ensure safe drying. This study investigated the optimal timing for transitioning from high-humidity thermal drying to air thermal drying, considering safety at a high-humidity level of 50°C. To evaluate this transition, the moisture content of the gel body was used as a key indicator. Gel bodies with moisture contents of 20%, 19%, 18%, 17%, and 15% were placed directly in 50°C air drying conditions. The results indicated that gel bodies with moisture contents of 17% and 15% showed no defects. Through repeated experimental validation, it was determined that a moisture content of 17% is the threshold for concluding the high-humidity thermal drying process. Furthermore, when comparing various drying methods under initial temperatures of 50°C and humidity levels between 80 − 72% RH, direct air drying proved to be the safest and fastest method for large-sized ceramic gel bodies when the later moisture content falls below 17%. Effect of Monomer to Crosslinker Ratio on the Drying Rate of Alumina Ceramic Gel Bodies Under the condition of maintaining a monomer content of 6 wt%, the effect of the monomer to crosslinker ratio on the drying rate of gel bodies was investigated. The experiments were conducted using a safe drying mode, with the water loss rate serving as the characterization index, as illustrated in Fig. 4 (c) . The figure demonstrates that during the early drying stage (within the first 72 hours in a high-humidity environment), an increase in the ratio of monomers to crosslinkers leaded to a gradual rise in the water loss rate. However, in the later drying stage (after 72 hours, with air drying at 50°C), the drying rate exhibited an opposite trend. In the high-humidity environment of the early drying stage, the drying rate was limited by the internal diffusion rate of moisture migrating from the interior to the surface. As the number of crosslinkers increased, the three-dimensional network structure within the gel body became denser, resulting in a slower internal diffusion rate of moisture. This ultimately reduced the overall water loss rate of the body. When transitioning to the later drying stage with 50°C air drying, the drying rate was constrained by the external diffusion rate, which pertained to the surface moisture evaporating and diffusing into the surrounding atmosphere. With an increased number of crosslinkers, the hydrophilic groups on the linear polymer of the monomer became largely crosslinked in three dimensions, significantly diminishing their ability to absorb or retain moisture. Consequently, this enhanced the efficiency of surface moisture evaporation and diffusion into the surrounding medium, thereby increasing the overall water loss rate of the gel body. Effect of Monomer to Crosslinker Ratio on the Strength of Ceramic Green Bodies With the monomer content maintained at 6 wt%, the effect of varying the monomer-to-crosslinker ratio (mass fractions of 10, 25, 50, 75, and 100) on the bending strength of ceramic green bodies was investigated, as illustrated in Fig. 4 (d) . The results indicate that as the ratio of monomer to crosslinker increases, the strength of the green body gradually improves. The maximum strength of 36.6 MPa was achieved at a ratio of 75:1. However, beyond this point, an increase in the ratio leaded to a decline in strength. The bending strength of the body is influenced by the density of the three-dimensional network structure within the gel, which is determined by the degree of crosslinking or gel strength, as well as the integrity of this network structure. A higher density contributes to greater strength, but it also relies on the completeness of the three-dimensional network. If this network structure experiences significant breakage during the drying process, the strength will be compromised. At a monomer-to-crosslinker ratio of 75:1, a balance was established between the density and integrity of the three-dimensional network structure. When the ratio was below 75:1, the gel became more susceptible to cracking during drying, which reduced its integrity and, consequently, its strength. In contrast, when the ratio exceeded 75:1, although the gel could dry safely and maintained the integrity of its three-dimensional network structure, the lower density resulted in a decrease in strength. Effect of Monomer Amount on the Volume Density and Water Absorption Rate of Ceramic Green Bodies With a monomer-to-crosslinker ratio set at 75:1, the effect of varying monomer amounts (mass fractions of 3%, 5%, 8%, 10%, and 12%) on the volume density and water absorption rate (definition marked in Supplementary Information) of alumina ceramic green bodies were investigated, as illustrated in Fig. 4 (e) . The results indicate that as the monomer content increases, the volume density of the green body gradually rises, while the water absorption rate declines. By maintaining a constant monomer-to-crosslinker ratio, which ensures a consistent degree of crosslinking, an increase in the monomer amount significantly enhances the density of the three-dimensional network within the body. This enhancement leads to greater compactness and volume density, resulting in a corresponding decrease in the water absorption rate. Additionally, the increase in compactness and volume density contributes positively to the bending strength of the green body, demonstrating a consistent trend. However, it’s important to note that the number of monomers should not be increased indefinitely; it should be limited to the minimum required for gel casting. This approach facilitates the removal of excess material during the ceramic firing process and helps prevent significant organic volatilization, which could harm the environment. As shown in Fig. 4 (e) , the volume density curve began to plateau after the monomer amount reached 6%. Therefore, this experiment identified 6 wt% as the optimal monomer concentration, achieving a volume density of 1.72 g/cm³. Preparation of Large-Sized Alumina Ceramic Components Alumina ceramic gel bodies were prepared by controlling the monomer content to 6% and maintaining a monomer-to-crosslinker ratio of 75:1. A two-stage drying process was implemented: the initial stage involved high-humidity drying at a temperature of 50°C, with humidity levels ranging from 80–72% relative humidity. Once the moisture content of the body dropped below 17%, the process transitioned to direct air drying, resulting in alumina ceramic green bodies, as shown in Fig. 2 (e) . The ceramic green bodies were then precisely machined into shape using CNC machinery, ensuring accurate dimensions. After following the established firing regimen, the final products (bolts and nuts) were obtained. The fired bolts and nuts displayed stable and consistent shrinkage, allowing for a perfect fit, as illustrated in Fig. 3 . Microstructural Analysis of Alumina Ceramic Green Bodies Figures 5 (a), (b), (c) , and (d) display microstructural images of alumina ceramic green bodies at magnifications of 1000x, 2000x, 5000x, and 10000x, respectively. In these images, the smaller particles are α-alumina, while the larger particles represent flux materials. The particle distribution within the microstructure of the green body is relatively uniform, and the pore distribution is also consistent. Most of these pores result from moisture being expelled from the body, while some are remnants of air that were not removed during the molding process. Conclusion This study conducted a thorough investigation into the drying process of hydrogels from the NMAM system. When the crosslinker content is low (such as ratios of 75:1 and 100:1), it is possible to dry larger hydrogels in the air without needing a high-humidity environment. This approach significantly enhances drying efficiency, enabling rapid and safe drying. However, a lower crosslinker content may decrease the strength of the hydrogel, which can affect its moldability. Additionally, the study examined the drying process of alumina ceramic green bodies under various drying conditions, including room temperature air drying and drying in high-humidity environments at 25°C, 50°C, and 80°C. A two-stage drying method was utilized, beginning with a high-humidity drying phase (drying temperature at 50°C, with humidity ranging from 80–72% relative humidity), followed by direct air drying (ensuring that the body moisture content does not exceed 17%). This drying method is regarded as the most suitable and safe approach for the rapid drying of large-sized ceramic green bodies. The ratio of monomers to crosslinker has a significant impact on the bending strength of alumina ceramic green bodies. At a ratio of 75:1, the green body attained its highest strength of 36.6 MPa. When the monomer-to-crosslinker ratio was kept constant, increasing the monomer content gradually enhanced the volume density of the green body while concurrently reducing the water absorption rate. With a monomer addition of 6 wt% and a monomer-to-crosslinker ratio of 75:1, the green body exhibited a strength of 36.6 MPa and a volume density of 1.72 g/cm³. A two-stage safe drying method was employed to prepare a ceramic green body with a uniform microstructure that fully meets machining requirements. Large threaded products were machined with controlled dimensional accuracy, and high-temperature sintering produced the final ceramic products. The fired bolts and nuts displayed stable and consistent shrinkage, ensuring a perfect fit. Declarations Competing interests The authors declare no competing financial interests. Data and materials availability All data is available in the main text and the Supplementary Information. Author Contribution Z. W. proposed the research direction and supervised the project. M. W. designed and performed the experiments. L.W. carried out the SEM characterization and data analysis, discussed all experimental results and drafted the manuscript. All authors checked the manuscript and agreed with its content. Acknowledgement Z. W. thanks the support from the Guangdong Provincial Department of Science and Technology (Yang Fan's Project No. 2015YT02C089) and Shandong Provincial Committee for Natural Science Research Fund (Project No. ZR2020MB113). References Omatete, O. O., Janney, M. A., Nunn, S. D. & Gelcasting From laboratory development toward industrial production. J. Eur. Ceram. Soc. 17 , 407–413 (1997). Liu, X. et al. 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Porous alumina ceramics obtained by particles self-assembly combing freeze drying method. Materials 12 , 897 (2019). Ha, J. S. Effect of atmosphere type on gelcasting behavior of Al2O3 and evaluation of green strength. Ceram. Int. 26 , 251–254 (2000). Miao, K., Lu, Z., Cao, J., Zhang, H. & Li, D. Effect of polydimethylsiloxane on the mid-temperature strength of gelcast Al2O3 ceramic parts. Mater. Des. 89 , 810–814 (2016). Xie, R., Zhang, D., Zhang, X., Zhou, K. & Button, T. W. Gelcasting of alumina ceramics with improved green strength. Ceram. Int. 38 , 6923–6926 (2012). Additional Declarations No competing interests reported. Supplementary Files Supplementaryinformation.docx Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6587948","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":451633001,"identity":"2c4c479c-3ee3-4c9e-83a5-3b912b6492b4","order_by":0,"name":"Mingyue Wang","email":"","orcid":"","institution":"Qingdao University of Science and Technology","correspondingAuthor":false,"prefix":"","firstName":"Mingyue","middleName":"","lastName":"Wang","suffix":""},{"id":451633002,"identity":"ec9c9751-4e35-442e-b1c7-3ce6e1212317","order_by":1,"name":"Lixin Wang","email":"","orcid":"","institution":"City University of Hong Kong","correspondingAuthor":false,"prefix":"","firstName":"Lixin","middleName":"","lastName":"Wang","suffix":""},{"id":451633003,"identity":"0ac2d52f-184e-4f2c-9053-c4110da90e08","order_by":2,"name":"Zhiyi Wang","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA70lEQVRIiWNgGAWjYLACxgYJBjb2BijvANFaeEBKE4jXAiQkEojUYnD87OGXP3dYJPZJPt4m+fMHgxzfjQTGzwX4tJzJS7OQPCOR2CadVibNk8BgLHkjgVl6Bh4tZgdyzAwM20BacsykgQ5L3HAjgY2ZB5+W82/MDBJBWiTPmEn+SGCoJ6zlRo7xg4MgLRI8ZhJAhyUYENJif+ONGWNjm4RxG09asTVPmoThzDMPm6XxaZHszzH++LOtTnZ+++GNN3/Y2MjzHU8++BmfFiBgkwASjg3A0APSIDY4mvAC5g8gBzJAtIyCUTAKRsEowAQAkfJLFWFvdwQAAAAASUVORK5CYII=","orcid":"","institution":"Qingdao University of Science and Technology","correspondingAuthor":true,"prefix":"","firstName":"Zhiyi","middleName":"","lastName":"Wang","suffix":""}],"badges":[],"createdAt":"2025-05-04 10:38:14","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6587948/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6587948/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":82258130,"identity":"3b93a717-d4d3-4b57-8911-4f1085601c58","added_by":"auto","created_at":"2025-05-08 11:33:31","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":1152664,"visible":true,"origin":"","legend":"\u003cp\u003eMicroscopic image of the hydrogel samples. (A) 50 mm × 50 mm × 50 mm silicone mold for gel casting process; (B) The size of the hydrogel after demolding; (C) Macroscopic morphological changes of hydrogels with different ratios of monomer to crosslinking agents during a constant-temperature drying process at 50℃ in the air; (D) Macroscopic morphological changes of hydrogels with different ratios of monomer to crosslinking agents during a room temperature drying process in the air.\u003c/p\u003e","description":"","filename":"image1.png","url":"https://assets-eu.researchsquare.com/files/rs-6587948/v1/a334b357e30ce61b475e29a0.png"},{"id":82258129,"identity":"1644fe63-078c-4c67-8aef-fb555413e7b9","added_by":"auto","created_at":"2025-05-08 11:33:31","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":768295,"visible":true,"origin":"","legend":"\u003cp\u003eThe green bodies of alumina ceramic. (a) The macroscopic morphology of the alumina ceramic gel body after drying at room temperature; (b) The macroscopic morphology of the alumina ceramic gel body after drying in a high-humidity environment at 25℃; (c) The macroscopic morphology of the alumina ceramic gel body after drying in a high-humidity environment at 50℃; (d) The macroscopic morphology of the alumina ceramic gel body after drying in a high-humidity environment at 80℃; (e) The green bodies of the alumina ceramic prepared.\u003c/p\u003e","description":"","filename":"image2.png","url":"https://assets-eu.researchsquare.com/files/rs-6587948/v1/0e5c8381024f742511af99ba.png"},{"id":82259664,"identity":"e86aeb24-af30-4e66-9c7d-52e7fb63796c","added_by":"auto","created_at":"2025-05-08 11:49:31","extension":"jpeg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":191992,"visible":true,"origin":"","legend":"\u003cp\u003eThe ceramic bolt and nut products prepared.\u003c/p\u003e","description":"","filename":"image3.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6587948/v1/c9fa9a0637f42428fa1685f2.jpeg"},{"id":82259211,"identity":"0a6d0fe5-126c-4a75-b691-fd90dfd28629","added_by":"auto","created_at":"2025-05-08 11:41:31","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":297169,"visible":true,"origin":"","legend":"\u003cp\u003eThe performance variation trend of the alumina ceramic gel body during the drying process. (a) Changes of water loss rate of alumina ceramic gel bodies under different drying methods; (b) Changes of water content of alumina ceramic gel bodies under different drying methods; (c) The influence of the different ratios of monomer to crosslinking agent on the drying rate of gel bodies; (d) The effect of monomer to crosslinking agent ratio on the bending strength of green bodies; (e) The effect of monomer addition on the volume density and water absorption of green bodies.\u003c/p\u003e","description":"","filename":"image4.png","url":"https://assets-eu.researchsquare.com/files/rs-6587948/v1/22ed0910359f32e59e7f13b7.png"},{"id":82258136,"identity":"1a2bf79a-2c81-4760-8633-0cb5cc3f1e6c","added_by":"auto","created_at":"2025-05-08 11:33:31","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":7891358,"visible":true,"origin":"","legend":"\u003cp\u003eSEM images of alumina ceramic green bodies at different magnifications\u003c/p\u003e\n\u003cp\u003e(a) ×1000; (b) ×2000; (c) ×5000; (d) ×10000.\u003c/p\u003e","description":"","filename":"image5.png","url":"https://assets-eu.researchsquare.com/files/rs-6587948/v1/4b1f12ae0b5946a8ccc087ec.png"},{"id":83493540,"identity":"3b1eadb1-69fb-463f-aae0-82f68b83e371","added_by":"auto","created_at":"2025-05-27 11:02:14","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":13021729,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6587948/v1/677cadee-3005-43b0-89ab-57dc9123b85b.pdf"},{"id":82258128,"identity":"3bc96f7e-6e4c-4d31-8fca-74a90fb22fd9","added_by":"auto","created_at":"2025-05-08 11:33:31","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":27937,"visible":true,"origin":"","legend":"","description":"","filename":"Supplementaryinformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-6587948/v1/67c2cf51cf3b6535e7354307.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Rapid and Safe Drying of Large-Sized Alumina Ceramic Gel Bodies","fulltext":[{"header":"Introduction","content":"\u003cp\u003eAlumina ceramics possess exceptional properties such as high strength, high hardness, resistance to high temperatures, wear resistance, and corrosion resistance to acids and bases. These qualities have led to their widespread applications in industries like aerospace, petrochemicals, national defense, and marine development. As technology advances, the demand for advanced ceramics is increasingly shifting towards large-sized or complex-shaped precision components. However, the production of these components presents significant challenges, as it requires high precision along with excellent wear resistance, strength, and hardness. Traditional ceramic forming techniques often fall short in meeting these stringent requirements. Fortunately, the introduction of gelcasting technology of ceramics has successfully addressed these challenges\u003csup\u003e\u003cspan additionalcitationids=\"CR2 CR3 CR4 CR5 CR6 CR7\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e–\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e, paving the way for the efficient production of large-sized precision ceramic components.\u003c/p\u003e \u003cp\u003eGel casting technology combines organic chemistry with traditional ceramic forming processes, allowing for the near-net-shape production of ceramic green bodies that exhibit high strength, low organic content, easy debonding, and some plasticity\u003csup\u003e\u003cspan additionalcitationids=\"CR10 CR11 CR12 CR13 CR14 CR15\" citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e–\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e. This technology meets the demands of precision mechanical processing effectively. Gel casting offers several significant advantages\u003csup\u003e\u003cspan additionalcitationids=\"CR18 CR19\" citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e–\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e: a) Wide Adaptability: It does not have specific requirements for ceramic powders and can quickly form large, complex-shaped components with varying wall thicknesses. b) Diverse Mold Material Options: Various materials can be used for molds, and the forming cycles are relatively short. c) Easy Precision Machining: The ceramic green bodies possess high strength and some plasticity, making them suitable for precision mechanical processing. d) Excellent Environmental Performance: The organic content in the green bodies is low, especially in water-based gel systems. e) Simple Equipment Structure: The equipment used in this process is straightforward, contributing to lower manufacturing costs.\u003c/p\u003e \u003cp\u003eGel casting technology has a significant drawback: it necessitates extremely strict drying conditions. The ceramic gel bodies produced through this method contain a considerable amount of moisture. During the drying phase, the rapid evaporation of water leads to a higher shrinkage rate, making these bodies susceptible to cracking and collapse. Moreover, the drying process is complex and slow, particularly for larger bodies, where uneven shrinkage can create concentrated structural stress and residual stress, resulting in deformation or warping. In summary, the drying process for ceramic gel bodies is lengthy and vulnerable to issues like deformation, cracking, and collapse, leading to a low yield rate. This problem is especially pronounced for larger, thick-walled bodies, significantly limiting the large-scale use of gel casting in the ceramics industry.\u003c/p\u003e \u003cp\u003eIn recent years, researchers have conducted extensive studies on various drying methods for ceramic gel bodies\u003csup\u003e\u003cspan additionalcitationids=\"CR22 CR23 CR24\" citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e–\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e. These methods include hot air drying, high-humidity drying, room-temperature drying, electric heating drying, radiation drying, liquid drying, and freeze drying, each yielding specific results\u003csup\u003e\u003cspan additionalcitationids=\"CR27\" citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e–\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e. However, most of these studies have primarily focused on small-sized products (thickness ≤ 30 mm) and those with regular shapes. There have been no publicly reported methods for the safe and rapid drying of large-sized ceramic gel bodies (thickness ≥ 50 mm) or those with complex shapes, which presents a significant challenge.\u003c/p\u003e \u003cp\u003eThe objective of this study is to achieve safe and efficient drying of large-sized alumina ceramic gel bodies. To accomplish this, the low-toxicity N-hydroxymethylacrylamide was used as the monomer and N, N-methylene bisacrylamide as the crosslinking agent. This research thoroughly examined the drying safety issues associated with the water-based gel system. Based on this, the study investigated how various factors—such as the ratio of monomer to crosslinking agent, the amount of monomer used, and the different drying methods—affect the drying safety, strength of the green body, bulk density, and water absorption rate of the large-sized alumina ceramic gel bodies. Ultimately, qualified large-sized ceramic green bodies were successfully prepared. Additionally, complex structures of alumina ceramic threads and bolts (diameter×height = 80×145 mm) with precise fitting were produced through machining and an appropriate firing schedule.\u003c/p\u003e "},{"header":"Methods","content":"\u003cp\u003eThe 95Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e ceramic powder used in the experiment consisted of 95% Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and 5% flux (mass ratio: talc: alumina: quartz = 63:17:20). After thorough mixing, the materials were finely ground to control the particle size distribution with D50 = 1 µm. The preparation process of alumina ceramic gel bodies was as follows: a) Preparation of Premixed Solution: The monomer N-hydroxymethylacrylamide (NMAM), the crosslinking agent N,N-methylene bisacrylamide (MBA), the dispersant ammonium polyacrylate, and the pH adjuster ammonia were dissolved in deionized water to create a premixed solution. b) Preparation of Slurry: The 95Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e ceramic powder was mixed with the premixed solution and ball-milled for 120 minutes to prepare the ceramic slurry. c) Gel Casting: The ball-milled slurry was subjected to vacuum degassing, and a 10 wt% ammonium persulfate solution was added as the initiator, along with tetramethylethylenediamine (TMEDA) as the catalyst. After mixing thoroughly, the slurry was injected into the silicone mold with dimensions of 50×50×50 mm (shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eA) to obtain the gel body. d) Drying of Gel Body: The gel body was dried under a specified drying regime to obtain the alumina ceramic green body. The monomer content was maintained at 6% wt (based on Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e ceramic powder). The study examined the effects of various mass ratios of the monomer to the crosslinking agent (10:1, 25:1, 50:1, 75:1, and 100:1) on the drying process of alumina gel bodies to identify the optimal monomer-to-crosslinking agent ratio. Additionally, the research investigated how different monomer amounts (2%, 6%, 8%, 10%, and 12%) influence the drying process of the alumina gel bodies.\u003c/p\u003e\u003cp\u003eThe experiment also investigated the effects of two different drying methods on the drying process of alumina gel bodies: room temperature air drying and high-humidity constant temperature drying. a) Room Temperature Air Drying: This method was conducted at a temperature of 21°C, with humidity (definition marked in Supplementary Information) maintained at 62–65% RH. b) High-Humidity Constant Temperature Drying: In this method, temperatures were set at 25°C, 50°C, and 80°C, with humidity gradually decreasing from 82–85% RH. Furthermore, large ceramic green bodies were created through gel casting by carefully choosing the right amounts of monomers and the appropriate ratios of monomers to crosslinking agents, as well as selecting suitable drying methods for the gel bodies. Once formed, these bodies underwent CNC machining and were fired according to a specific firing schedule. This process resulted in large-sized ceramic bolts and nuts with complex structure and tight fit.\u003c/p\u003e"},{"header":"Results and Discussion","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eDrying Process and Macroscopic Morphology of NMAM System Hydrogel in Air\u003c/h2\u003e \u003cp\u003eThe preparation method of the hydrogel of NMAM system is the same as that of the alumina ceramic gel body, except that no alumina ceramic powder is added. Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eB illustrates the morphology and size of the hydrogel after demolding. Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eC depicts the high-temperature drying process of the hydrogel in air and its macroscopic morphology. Samples (a), (b), (c), (d), and (e) each contain a monomer content of 6 wt%, with varying monomer to crosslinker ratios of 10:1, 25:1, 50:1, 75:1, and 100:1, respectively. (X\u003csub\u003e1\u003c/sub\u003e) represents the demolded hydrogel, (X\u003csub\u003e2\u003c/sub\u003e) indicates the hydrogel that cracked during drying, and (X\u003csub\u003e3\u003c/sub\u003e) corresponds to the dried gel. From Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eC, it is observed that at a constant air temperature of 50\u0026deg;C, the hydrogels (a\u003csub\u003e1\u003c/sub\u003e), (b\u003csub\u003e1\u003c/sub\u003e), (c\u003csub\u003e1\u003c/sub\u003e), and (d\u003csub\u003e1\u003c/sub\u003e) cracked after one hour, two hours, 20 hours, and 24 hours of drying, respectively. Their morphologies are shown in photos (a\u003csub\u003e2\u003c/sub\u003e), (b\u003csub\u003e2\u003c/sub\u003e), (c\u003csub\u003e2\u003c/sub\u003e), and (d\u003csub\u003e2\u003c/sub\u003e). As the ratio of monomer to crosslinker increases, the degree of cracking in the hydrogel gradually decreases, transitioning from complete fragmentation in (a\u003csub\u003e3\u003c/sub\u003e), (b\u003csub\u003e3\u003c/sub\u003e), and (c\u003csub\u003e3\u003c/sub\u003e) to an overall cracked state in (d\u003csub\u003e3\u003c/sub\u003e). Notably, the hydrogel (e\u003csub\u003e1\u003c/sub\u003e) did not crack during the drying process, and the dried gel remained completely crack-free. As the number of crosslinkers decreases, the degree of crosslinking in the hydrogel also diminishes. This results in a loose three-dimensional network structure that allows moisture to escape more easily. Additionally, the moisture gradient between the surface and the interior of the hydrogel decreases, which reduces the likelihood of cracking during the drying process. Experimental results indicate that when the monomer-to-crosslinker ratio is 100:1, the hydrogel can be safely dried in air at 50\u0026deg;C.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eD illustrates the room-temperature drying process of the hydrogel in air after demolding, along with its macroscopic morphology. The samples labeled (f), (g), (h), (i), and (j) all contain a monomer content of 6 wt%, with monomer-to-crosslinker ratios of 10:1, 25:1, 50:1, 75:1, and 100:1, respectively. Here, (X\u003csub\u003e1\u003c/sub\u003e) refers to the de-molded hydrogel, (X\u003csub\u003e2\u003c/sub\u003e) indicates the hydrogel that cracked during drying, and (X\u003csub\u003e3\u003c/sub\u003e) represents the dried gel. From Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eD, it is evident that the drying and cracking rates of the hydrogel at room temperature are significantly lower than those at a constant temperature of 50\u0026deg;C. Furthermore, the extent of drying cracks is notably less severe. The hydrogels (f\u003csub\u003e1\u003c/sub\u003e), (g\u003csub\u003e1\u003c/sub\u003e), and (h\u003csub\u003e1\u003c/sub\u003e) experienced cracking after 61 hours, 72 hours, and 120 hours of drying, respectively. The other samples displayed some cracking, except for (f\u003csub\u003e3\u003c/sub\u003e), which was fragmented. Notably, the hydrogels (i\u003csub\u003e1\u003c/sub\u003e) and (j\u003csub\u003e1\u003c/sub\u003e) remained intact and crack-free throughout the drying process. This demonstrates that when the monomer-to-crosslinker ratios are 75:1 and 100:1, the hydrogel can be safely dried at room temperature without cracking.\u003c/p\u003e \u003cp\u003eThe ratio of monomers to crosslinkers should not be too low. If the ratio is low, the resulting gel primarily consists of linear polymer chains formed by monomer polymerization, with only a few branched chains created by crosslinkers. Although cracking does not occur during the drying process, the strength of the wet gel remains low, making it difficult to shape. The weight of the gel can lead to deformation and stress concentration, which may ultimately cause cracking during drying. For instance, when the monomer-to-crosslinker ratio is set at 100:1, the strength of the resulting gel is insufficient, making it unsuitable for the preparation of large ceramic bodies. Therefore, subsequent experiments opted for a monomer-to-crosslinker ratio of 75:1.\u003c/p\u003e \u003cp\u003e \u003cb\u003eEffects of Different Drying Methods on the Drying Process and Safety of Alumina Ceramic Gel Bodies\u003c/b\u003e \u003c/p\u003e \u003cp\u003eAlumina ceramic gel bodies were prepared using silicone molds with dimensions of 50\u0026times;50\u0026times;50 mm, featuring a controlled monomer content of 6 wt% and a monomer-to-crosslinker ratio of 75:1. The gel bodies underwent drying using various methods: air drying at room temperature, high-humidity drying at 25℃, high-humidity drying at 50℃, and high-humidity drying at 80℃. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e\u003cb\u003e(a)(b)\u003c/b\u003e illustrates the water loss rates (definition marked in Supplementary Information) and moisture content curves for the alumina ceramic gel bodies subjected to different drying methods. The data indicates that during the early drying stage (up to 78 hours), the water loss rates of the gel bodies were ranked as follows: room temperature drying\u0026thinsp;\u0026gt;\u0026thinsp;high-humidity drying at 80℃ \u0026gt; high-humidity drying at 25℃ \u0026gt; high-humidity drying at 50℃. In the later drying stage (after 78 hours), the ranking changed to: high-humidity drying at 80℃ \u0026gt; room temperature drying\u0026thinsp;\u0026gt;\u0026thinsp;high-humidity drying at 50℃ \u0026gt; high-humidity drying at 25℃.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e illustrates the appearance of alumina ceramic green bodies after drying using various methods. Under room temperature drying for 24 hours, the alumina ceramic body (a) exhibited significant cracks and central collapse, with a moisture content of 17.4%. With high humidity drying at 25\u0026deg;C, the alumina ceramic body (b) showed noticeable cracks and central collapse after 138 hours, having a moisture content of 13.1% and a humidity of 75% relative humidity (RH). After 72 hours of high moisture drying at 50\u0026deg;C, the body (c) showed no defects. When the sample was placed directly in a hot air dryer at 50\u0026deg;C, the water loss rate curve quickly aligned with and then surpassed the curve for high humidity drying at 25\u0026deg;C after 72 hours, resulting in no defects by the end of the drying process. Under high moisture drying at 80\u0026deg;C, the alumina ceramic green body (d) developed numerous small cracks after 72 hours, but there was no structural collapse, with a moisture content of 11.4% and humidity ranging between 80% and 82% RH.\u003c/p\u003e \u003cp\u003eThe early stage of the drying process (before 78 hours) is critical from both a drying and safety perspective. According to Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e\u003cb\u003e(a)\u003c/b\u003e, the sample experienced the fastest water loss rate under room temperature drying conditions; however, cracks and collapses were observed after 24 hours. The next fastest drying method is high humidity drying at 80\u0026deg;C, where cracks appeared after 72 hours. In comparison, high humidity drying at 25\u0026deg;C showed cracks and collapses after 138 hours. Conversely, high humidity drying at 50\u0026deg;C had the slowest water loss rate. After 72 hours, when the high humidity protection was removed and the sample was directly subjected to 50\u0026deg;C hot air drying, no defects occur, resulting in safe drying. Therefore, for thick-walled ceramic gel bodies, it is essential to strictly control the moisture removal rate in the early drying stage to ensure safe drying.\u003c/p\u003e \u003cp\u003eThis study investigated the optimal timing for transitioning from high-humidity thermal drying to air thermal drying, considering safety at a high-humidity level of 50\u0026deg;C. To evaluate this transition, the moisture content of the gel body was used as a key indicator. Gel bodies with moisture contents of 20%, 19%, 18%, 17%, and 15% were placed directly in 50\u0026deg;C air drying conditions. The results indicated that gel bodies with moisture contents of 17% and 15% showed no defects. Through repeated experimental validation, it was determined that a moisture content of 17% is the threshold for concluding the high-humidity thermal drying process. Furthermore, when comparing various drying methods under initial temperatures of 50\u0026deg;C and humidity levels between 80\u0026thinsp;\u0026minus;\u0026thinsp;72% RH, direct air drying proved to be the safest and fastest method for large-sized ceramic gel bodies when the later moisture content falls below 17%.\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eEffect of Monomer to Crosslinker Ratio on the Drying Rate of Alumina Ceramic Gel Bodies\u003c/h3\u003e\n\u003cp\u003eUnder the condition of maintaining a monomer content of 6 wt%, the effect of the monomer to crosslinker ratio on the drying rate of gel bodies was investigated. The experiments were conducted using a safe drying mode, with the water loss rate serving as the characterization index, as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e\u003cb\u003e(c)\u003c/b\u003e. The figure demonstrates that during the early drying stage (within the first 72 hours in a high-humidity environment), an increase in the ratio of monomers to crosslinkers leaded to a gradual rise in the water loss rate. However, in the later drying stage (after 72 hours, with air drying at 50\u0026deg;C), the drying rate exhibited an opposite trend. In the high-humidity environment of the early drying stage, the drying rate was limited by the internal diffusion rate of moisture migrating from the interior to the surface. As the number of crosslinkers increased, the three-dimensional network structure within the gel body became denser, resulting in a slower internal diffusion rate of moisture. This ultimately reduced the overall water loss rate of the body. When transitioning to the later drying stage with 50\u0026deg;C air drying, the drying rate was constrained by the external diffusion rate, which pertained to the surface moisture evaporating and diffusing into the surrounding atmosphere. With an increased number of crosslinkers, the hydrophilic groups on the linear polymer of the monomer became largely crosslinked in three dimensions, significantly diminishing their ability to absorb or retain moisture. Consequently, this enhanced the efficiency of surface moisture evaporation and diffusion into the surrounding medium, thereby increasing the overall water loss rate of the gel body.\u003c/p\u003e\n\u003ch3\u003eEffect of Monomer to Crosslinker Ratio on the Strength of Ceramic Green Bodies\u003c/h3\u003e\n\u003cp\u003eWith the monomer content maintained at 6 wt%, the effect of varying the monomer-to-crosslinker ratio (mass fractions of 10, 25, 50, 75, and 100) on the bending strength of ceramic green bodies was investigated, as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e\u003cb\u003e(d)\u003c/b\u003e. The results indicate that as the ratio of monomer to crosslinker increases, the strength of the green body gradually improves. The maximum strength of 36.6 MPa was achieved at a ratio of 75:1. However, beyond this point, an increase in the ratio leaded to a decline in strength. The bending strength of the body is influenced by the density of the three-dimensional network structure within the gel, which is determined by the degree of crosslinking or gel strength, as well as the integrity of this network structure. A higher density contributes to greater strength, but it also relies on the completeness of the three-dimensional network. If this network structure experiences significant breakage during the drying process, the strength will be compromised. At a monomer-to-crosslinker ratio of 75:1, a balance was established between the density and integrity of the three-dimensional network structure. When the ratio was below 75:1, the gel became more susceptible to cracking during drying, which reduced its integrity and, consequently, its strength. In contrast, when the ratio exceeded 75:1, although the gel could dry safely and maintained the integrity of its three-dimensional network structure, the lower density resulted in a decrease in strength.\u003c/p\u003e \u003cp\u003e \u003cb\u003eEffect of Monomer Amount on the Volume Density and Water Absorption Rate of Ceramic Green Bodies\u003c/b\u003e \u003c/p\u003e \u003cp\u003eWith a monomer-to-crosslinker ratio set at 75:1, the effect of varying monomer amounts (mass fractions of 3%, 5%, 8%, 10%, and 12%) on the volume density and water absorption rate (definition marked in Supplementary Information) of alumina ceramic green bodies were investigated, as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e\u003cb\u003e(e)\u003c/b\u003e. The results indicate that as the monomer content increases, the volume density of the green body gradually rises, while the water absorption rate declines. By maintaining a constant monomer-to-crosslinker ratio, which ensures a consistent degree of crosslinking, an increase in the monomer amount significantly enhances the density of the three-dimensional network within the body. This enhancement leads to greater compactness and volume density, resulting in a corresponding decrease in the water absorption rate. Additionally, the increase in compactness and volume density contributes positively to the bending strength of the green body, demonstrating a consistent trend. However, it\u0026rsquo;s important to note that the number of monomers should not be increased indefinitely; it should be limited to the minimum required for gel casting. This approach facilitates the removal of excess material during the ceramic firing process and helps prevent significant organic volatilization, which could harm the environment. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e\u003cb\u003e(e)\u003c/b\u003e, the volume density curve began to plateau after the monomer amount reached 6%. Therefore, this experiment identified 6 wt% as the optimal monomer concentration, achieving a volume density of 1.72 g/cm\u0026sup3;.\u003c/p\u003e\n\u003ch3\u003ePreparation of Large-Sized Alumina Ceramic Components\u003c/h3\u003e\n\u003cp\u003eAlumina ceramic gel bodies were prepared by controlling the monomer content to 6% and maintaining a monomer-to-crosslinker ratio of 75:1. A two-stage drying process was implemented: the initial stage involved high-humidity drying at a temperature of 50\u0026deg;C, with humidity levels ranging from 80\u0026ndash;72% relative humidity. Once the moisture content of the body dropped below 17%, the process transitioned to direct air drying, resulting in alumina ceramic green bodies, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e\u003cb\u003e(e)\u003c/b\u003e. The ceramic green bodies were then precisely machined into shape using CNC machinery, ensuring accurate dimensions. After following the established firing regimen, the final products (bolts and nuts) were obtained. The fired bolts and nuts displayed stable and consistent shrinkage, allowing for a perfect fit, as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e.\u003c/p\u003e\n\u003ch3\u003eMicrostructural Analysis of Alumina Ceramic Green Bodies\u003c/h3\u003e\n\u003cp\u003eFigures \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e \u003cb\u003e(a), (b), (c)\u003c/b\u003e, and \u003cb\u003e(d)\u003c/b\u003e display microstructural images of alumina ceramic green bodies at magnifications of 1000x, 2000x, 5000x, and 10000x, respectively. In these images, the smaller particles are α-alumina, while the larger particles represent flux materials. The particle distribution within the microstructure of the green body is relatively uniform, and the pore distribution is also consistent. Most of these pores result from moisture being expelled from the body, while some are remnants of air that were not removed during the molding process.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eThis study conducted a thorough investigation into the drying process of hydrogels from the NMAM system. When the crosslinker content is low (such as ratios of 75:1 and 100:1), it is possible to dry larger hydrogels in the air without needing a high-humidity environment. This approach significantly enhances drying efficiency, enabling rapid and safe drying. However, a lower crosslinker content may decrease the strength of the hydrogel, which can affect its moldability.\u003c/p\u003e \u003cp\u003eAdditionally, the study examined the drying process of alumina ceramic green bodies under various drying conditions, including room temperature air drying and drying in high-humidity environments at 25\u0026deg;C, 50\u0026deg;C, and 80\u0026deg;C. A two-stage drying method was utilized, beginning with a high-humidity drying phase (drying temperature at 50\u0026deg;C, with humidity ranging from 80\u0026ndash;72% relative humidity), followed by direct air drying (ensuring that the body moisture content does not exceed 17%). This drying method is regarded as the most suitable and safe approach for the rapid drying of large-sized ceramic green bodies. The ratio of monomers to crosslinker has a significant impact on the bending strength of alumina ceramic green bodies. At a ratio of 75:1, the green body attained its highest strength of 36.6 MPa. When the monomer-to-crosslinker ratio was kept constant, increasing the monomer content gradually enhanced the volume density of the green body while concurrently reducing the water absorption rate. With a monomer addition of 6 wt% and a monomer-to-crosslinker ratio of 75:1, the green body exhibited a strength of 36.6 MPa and a volume density of 1.72 g/cm\u0026sup3;. A two-stage safe drying method was employed to prepare a ceramic green body with a uniform microstructure that fully meets machining requirements. Large threaded products were machined with controlled dimensional accuracy, and high-temperature sintering produced the final ceramic products. The fired bolts and nuts displayed stable and consistent shrinkage, ensuring a perfect fit.\u003c/p\u003e"},{"header":"Declarations","content":" \u003ch2\u003eCompeting interests\u003c/h2\u003e \u003cp\u003eThe authors declare no competing financial interests.\u003c/p\u003e \u003ch2\u003eData and materials availability\u003c/h2\u003e \u003cp\u003eAll data is available in the main text and the Supplementary Information.\u003c/p\u003e \u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eZ. W. proposed the research direction and supervised the project. M. W. designed and performed the experiments. L.W. carried out the SEM characterization and data analysis, discussed all experimental results and drafted the manuscript. All authors checked the manuscript and agreed with its content.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eZ. W. thanks the support from the Guangdong Provincial Department of Science and Technology (Yang Fan's Project No. 2015YT02C089) and Shandong Provincial Committee for Natural Science Research Fund (Project No. ZR2020MB113).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eOmatete, O. O., Janney, M. A., Nunn, S. D. \u0026amp; Gelcasting From laboratory development toward industrial production. \u003cem\u003eJ. Eur. Ceram. Soc.\u003c/em\u003e \u003cb\u003e17\u003c/b\u003e, 407\u0026ndash;413 (1997).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLiu, X. et al. 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Int.\u003c/em\u003e \u003cb\u003e38\u003c/b\u003e, 6923\u0026ndash;6926 (2012).\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-6587948/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6587948/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eLarge-sized or complex-shaped precision ceramic components are crucial in high-end industries such as aerospace, deep-sea exploration, and medical technology. However, traditional ceramic forming techniques encounter several challenges in manufacturing these components. The gel casting technique presents new opportunities to overcome these difficulties. However, it demands strict drying conditions, leading to lengthy drying cycles and low yield rates, which significantly limit its industrial application. This study aims to investigate a safe and efficient drying system for large alumina ceramic gel bodies, focusing specifically on the drying safety of water-based gel systems. Experimental results indicate that when the monomer addition amounts to 6 wt% and the ratio of monomer to crosslinking agent is 75:1, employing a two-stage drying method leads to optimal outcomes. The initial drying stage occurs at a temperature of 50\u0026deg;C with a humidity level between 80% and 72% RH. In the second stage, air drying is conducted directly once the moisture content drops below 17%. This process successfully produces a ceramic green body with a uniform microstructure, fully compliant with machining requirements. Consequently, rapid and safe drying of large-sized alumina ceramic gel bodies is achieved.\u003c/p\u003e","manuscriptTitle":"Rapid and Safe Drying of Large-Sized Alumina Ceramic Gel Bodies","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-05-08 11:33:27","doi":"10.21203/rs.3.rs-6587948/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"09bb4feb-9047-445f-901b-9d518038516d","owner":[],"postedDate":"May 8th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":48040203,"name":"Physical sciences/Engineering"},{"id":48040204,"name":"Physical sciences/Materials science"}],"tags":[],"updatedAt":"2025-05-27T10:53:51+00:00","versionOfRecord":[],"versionCreatedAt":"2025-05-08 11:33:27","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-6587948","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6587948","identity":"rs-6587948","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}