CONDUCTIVE DRIlNG CHARACTERISTICS OF GELATINIZED RICE STARCH

ABSTRACT

The drying mechanism of shaped ceramics is reviewed and some methods for eliminating defects produced by drying are discussed in this report. The types of defects depend upon the shaping method, shape, properties of the raw materials, drying process and other items. Most defects of dried ceramics are produced during the initial or middle drying stage when large shrinkage of the ceramic body occurs. These defects may be successfully eliminated by heating the body from the inside. Self-deformation caused by weight is also a serious defect. The near net shaping of fine ceramics to reduce machining requires uniform body shrinkage and hence, highly controlled drying. Examples of electric current drying, dielectric drying, and dewaxing are also shown in this report.     

Wet‐Route Synthesis and Characterization of Yb:CaF2 Optical Ceramics

This paper reports a new strainless fabrication method for ytterbium‐doped CaF₂ laser ceramics involving no drying step before green body casting. The nanoparticles were kept in aqueous solution until green body shaping. Centrifugation was used to obtain correct compactness of the green body before sintering. Characterizations were conducted at different steps of the fabrication process. No grain boundaries oxidation was observed in the sintered ceramics although the nanoparticles were permanently maintained in water until they were sintered. Finally, these ceramics are more homogeneous and have less light scattering defects (no porosity), and present improved optical properties when compared to ceramics obtained from dried nanopowders.     

复杂形状陶瓷件的凝胶注模成型

为降低成本，简化工艺，对Al2O3复杂形状陶瓷件采用凝胶注模工艺成型。本文着重就有机单体含量与配比对固化胶体性状的影响、料浆加入引发剂后的凝胶固化行为、低粘度高固相含量浆料的调制、坯体复杂外形与内腔的形成以及坯体在干燥脱脂过程中的收缩变形等进行了研究。     

对W.B.B.商品用球粘土注浆性能的研究

本采用XRD,DTA－TG,沉降法颗粒分析,粘土－水系统流动性,触变性和Zeta电位测定,试样干燥强度,干燥收缩和烧成收缩测定等手段对WBB清远华建陶瓷厂矿物有限公司生产的卫生瓷专用球粘土的化学组成,矿物组成,颗粒组成及工艺性能进行了系统的研究,结果表明该球粘土性能优良,完全适用于卫生瓷注浆成型工艺,有利于提高成型性能和生坯强度,有利于提高生产效率和产品质量。     

Preparation of porous SiC-Al2O3 ceramics via gelcasting utilising a shrinkable pore-forming agent and oxidised coarse-grained SiC

By utilising soaked millet as a shrinkable pore-forming agent, porous silicon carbide-alumina (SiC-Al₂O₃) ceramics were prepared via gelcasting. The fabrication of SiC-Al₂O₃ ceramics based on oxidised and unoxidised coarse-grained SiC was also studied. The water swelling, drying shrinkage, and low-temperature carbonisation of the millet were investigated. We found that the shrinkage of the soaked millet was greater than that of gel body during drying, which left large gaps that prevented shrinkage stresses from destroying the gel body. Low-temperature carbonisation of the millet should be performed slowly at 220–240?°C because its expansion rate increases to 45% at 250?°C, resulting in the cracking of samples. At a constant sintering temperature, the flexural strength of the SiC-Al₂O₃ ceramics prepared with SiC powders oxidised at 1000?°C was the highest, indicating that oxidised powders can successfully decrease the required sintering temperature and improve the flexural strength of composite ceramics. Based on our optimised process, porous SiC-Al₂O₃ ceramics were sintered at 1500?°C for 2?h. When their skeletons were fully developed, their pore sizes were in the range of 1.5–2?mm. Their porosity and flexural strength were 60.2–65.1% and 8.3–10.5?MPa, respectively.     

Preparation of porous Y2SiO5 ceramics with high porosity and extremely low thermal conductivity for radome applications

It is well known that aqueous gel-casting is challenging to prepare high-porosity ceramics due to the considerable drying shrinkage, cracking, and deformation of green bodies during drying caused by the high surface tension of water. Porous Y₂SiO₅ ceramics with high porosity were prepared by introducing carbon fibers as a support material in the drying process of aqueous gel-casting to reduce shrinkage during drying. Burning out the carbon fibers after drying does not negatively affect the properties of the porous ceramic. As prepared green bodies by aqueous gel-casting have low shrinkages of 8.69%–6.81% during drying processes and high compressive strength of 13.73 ± 1.55–10.66～0.49 MPa. The higher compressive strength of the green body has a positive significance for processing porous ceramics into special-shaped structures. As prepared porous Y₂SiO₅ ceramics have high porosity of 73.94%–87.71%, lightweights of 1.16–0.55 g?cm³, extremely low thermal conductivities of 0.134 ± 0.006 to 0.051 ± 0.001 W?m^?1?k^?1, relatively low dielectric constants of 2.34–1.58, and tan δ are lower than 1.25 × 10^?3. Porous Y₂SiO₅ ceramics with excellent dielectric properties and thermal insulation properties meet the requirements of thermal insulation and wave transmission integration of radome materials. Aqueous gel-casting also enriches the preparation methods of high-porosity Y₂SiO₅ ceramics.     

Some technological laws and properties for highly porous ceramic concretes

Conclusions Using ceramic concrete made from zirconia as an example we formulated conditions and established the main technological laws for obtaining highly porous ceramic concretes.We analyzed the composition and microstructure of the ceramic concrete from the point of view of the existence in the system of noncompensated shrinkage. It is shown that for ceramic concrete with a high bond shrinkage during drying and heat treatment, with the purpose of reducing the shrinkage stresses in the system, it is necessary to use multifraction (coarse, medium, and fine) fillers.We show the effectiveness of using, as one of the filler components, removable poreforming additive, based on foamed polystyrol.The optimum region of the grain-size distribution of the components was established; this comes within the true specific volumes in the structure of the shaped ceramic concrete and amounts to 20–30% bond, 20–25% fine (0.1–0.4 mm) and 25–40% coarse (5–10 mm) filler made from waste zirconia foamed ceramics, and 20–25% foamed polystrol (0.63–1.6 mm).We studied certain properties of the resulting materials with a porosity of 60–75% and a compressive strength of 5–20 MPa.Compared with other highly porous materials, for example, foamed ceramics, the highly porous ceramic concretes have technological advantages (much lower water capacity of the shaping system, reduced shrinkage in drying and firing) and also improved operating characteristics, for instance, thermal conductivity, thermal-shock resistance and volume constancy.Translated from Ogneupory, No. 2, pp. 20–25, February, 1984.     

酯化淀粉原位凝固成型Al2O3陶瓷的研究

研究了以淀粉作凝胶剂原位凝固成型Al2O3陶瓷的新方法.当淀粉大分子在水溶液中加热到一定温度时,淀粉颗粒吸水溶胀,陶瓷浆料脱水并固化成坚硬的坯体;而且溶胀后的淀粉可作为粘结剂,有助于坯体固化并增加其强度.详细探讨了酯化淀粉添加量不同时,对陶瓷浆料流变特性的影响,以及对成型出素坯的线收缩、相对密度、干坯强度以及显微结构的影响.     

DETERMINING THE SOURCE OF CHANGING SHRINKAGE RATES DURING KILN DRYING

The conclusions drawn by researchers on the source of changing shrinkage rates are discussed. The objective of this study was to determine the source of changing shrinkage rates early in drying and whether shrinkage would make a useful parameter for an automated kiln control system. In this study, four loads of red oak and one of maple were kiln dried using established schedules Shrinkage of the lumber was continuously recorded, and moisture content and released elastic strain were periodically recorded. The moisture content and strain data showed that the changing shrinkage rates early in drying boards were caused by reduced internal stress levels, not the occurrence of fiber saturation point or temperature as reported by some. Results of this study indicate it may be possible to develop an automated kiln system using shrinkage as the controlling parameter. This would reduce the drying time while avoiding additional drying defects.     

DETERMINING THE SOURCE OF CHANGING SHRINKAGE RATES DURING KILN DRYING∗

ABSTRACT

The conclusions drawn by researchers on the source of changing shrinkage rates are discussed. The objective of this study was to determine the source of changing shrinkage rates early in drying and whether shrinkage would make a useful parameter for an automated kiln control system. In this study, four loads of red oak and one of maple were kiln dried using established schedules Shrinkage of the lumber was continuously recorded, and moisture content and released elastic strain were periodically recorded. The moisture content and strain data showed that the changing shrinkage rates early in drying boards were caused by reduced internal stress levels, not the occurrence of fiber saturation point or temperature as reported by some. Results of this study indicate it may be possible to develop an automated kiln system using shrinkage as the controlling parameter. This would reduce the drying time while avoiding additional drying defects.     

Hydrophobic coagulation of alumina slurries

This study reports a new wet forming method by hydrophobic modification of short-chain anionic dispersants. The influence of the content and type of hydrophobic chains on the rheology of alumina slurries, the drying shrinkage rates of wet bodies, the density and strength of the green bodies, and the density and microstructure of the resultant ceramics were intensively investigated. It is revealed that hydrophobic modification can endow short-chain dispersants with coagulation ability, when 0.3 wt% short-chain PIBM (a copolymer of isobutylene and maleic anhydride) was combined with 0.1 wt% TMAC (tetramethylammonium chloride), solid loading of the slurry was increased to 58 vol% and the slurry can coagulate to a wet green body in a short time. The resultant green body showed a uniform microstructure and was successfully sintered into translucent ceramics.     

CONTRACTION CHARAClFRlSTlCS OF MOLDED CERAMICS DURING DRYING

The characteristics of drvine and contraction (shrinkage) of molded ceramics were expenmentally studied to clarify the effects of drying conditions on contraction behaviors or deformation during drying processes. Mikawa-Shanshu clay molded into spheres or slabs was used as a sample which underwent convective drying by hot air.

Three periods of preheating, constanl rate and falling rate existed, and the contraction was observed mainly in the preheating and constanl rate periods for all samples. It is also observed that the behavior of ihe contraction of molded clay is inflienced significantly by drying conditions. The higher drying rate makes the contraction of clay more remarkable in the preheating and constant rate periods. This behavior could not be explained from the difference of the moisture content distribution among runs. However, it is suggested that the inside parts of molded ceramics may be subjected to the strong compressional stresses and the strain-stress in the sample would Influence the contraction behavior when he gradient of moisture content distribution becomes steeper with increase in the drying rate.     

CONTRACTION CHARAClFRlSTlCS OF MOLDED CERAMICS DURING DRYING

ABSTRACT

The characteristics of drvine and contraction (shrinkage) of molded ceramics were expenmentally studied to clarify the effects of drying conditions on contraction behaviors or deformation during drying processes. Mikawa-Shanshu clay molded into spheres or slabs was used as a sample which underwent convective drying by hot air.

Three periods of preheating, constanl rate and falling rate existed, and the contraction was observed mainly in the preheating and constanl rate periods for all samples. It is also observed that the behavior of ihe contraction of molded clay is inflienced significantly by drying conditions. The higher drying rate makes the contraction of clay more remarkable in the preheating and constant rate periods. This behavior could not be explained from the difference of the moisture content distribution among runs. However, it is suggested that the inside parts of molded ceramics may be subjected to the strong compressional stresses and the strain-stress in the sample would Influence the contraction behavior when he gradient of moisture content distribution becomes steeper with increase in the drying rate.     

The influence of shaping and firing technology on ceramic properties of calcareous and non-calcareous illitic–chloritic clays

Two very different illitic–chloritic clays (calcareous and non-calcareous), both currently used in the Italian brickmaking industry, were used. Technological testing consisted in a simulation of the industrial processing performed at a laboratory scale. The pieces were obtained by three shaping techniques, pressing, extrusion and moulding, and fired at 1000 and 1100 °C, in fast and slow heating cycles. In each case their technological properties were studied. The microstructure and technological parameters of the ceramic pieces vary greatly, depending on the clay composition and the processing techniques. Pressing always produces pieces made from non-calcareous clay with the lowest drying and firing shrinkage, open porosity and water absorption. As it regards the calcareous clay, the lowest values of drying and firing shrinkage and water absorption are obtained by extrusion and moulding, respectively. The mechanical resistance, pore size distribution and critical pore diameter do not reflect clearly the influence of shaping techniques. Independently from the shaping technique adopted, all non-calcareous bodies show higher Maage's indexes than the calcareous ones. In any case, the Maage's durability factor is higher in extruded samples.     

陶瓷部件挤出成型工艺

本文以蜂窝陶瓷生产工艺为例，讨论了陶瓷部件挤出成型过程中，塑性泥料的制备、成型、干燥和烧成等工艺技术和装备问题。     

Aqueous Gelcasting of Fused Silica Using Colloidal Silica Binder

An aqueous‐based gelcasting of fused silica ceramics by using colloidal silica binder was developed. Fused silica slurries having different volume percentage of solid loading from 63 to 74 vol% in colloidal silica were made and the rheological properties were evaluated. It was found that the slurry with 73 vol% of solid loading with viscosity 0.70 Pa.s is suitable for this gelcasting system. The influence of solid loading on physical and mechanical properties of gelcast green and sintered bodies has been studied. The fabricated green body by using colloidal silica binder exhibited a flexural strength of 9 MPa and 88% of theoretical density with 2.2% of drying shrinkage while the sintered sample exhibited flexural strength of 60 MPa and 95% of theoretical density with 4.3% of sintering shrinkage. It was observed that, the nano silica particles from the colloidal silica binder is filling the interstitial positions in the consolidated fused silica green body and enhancing physical and mechanical properties.     

Fabrication of high-density NiFe2O4 ceramics by slip casting and pressureless sintering

High-density NiFe₂O₄ ceramics with homogeneous microstructure were produced by slip casting and pressureless sintering. The slurry stability, sintering behavior, and microstructure of NiFe₂O₄ ceramics were investigated. A stable slurry can be obtained by adding 12.5 wt% NiFe₂O₄ nanoparticle and 5 wt% nano-binder at a slurry pH around 11.0. The linear shrinkage and linear shrinkage rate for both NiFe₂O₄ ceramic green bodies shaped by cold press molding and slip casting showed nearly the same trends. The temperature associated with the maximum linear shrinkage rate of slip casted green body was 1263.5°C, which was lower than that of cold press molded sample (1272.0°C). The sintering activation energy of slip casted green body was also lower than that of cold press molded sample (279.18 vs 288.47 kJ mol⁻¹), owing to high density and homogeneity of slip-casted green compact. A high-density NiFe₂O₄ ceramics with uniform grain size distribution can be produced by slip casting and pressureless sintering at 1350°C for 6 hours, attributed to the ability of slip casting to minimize agglomerates and micropores. It demonstrated that slip casting was more suitable to prepare high-density NiFe₂O₄ ceramics with good homogeneity.     

Starch consolidation of alumina: Fabrication and mechanical properties

A consolidation technique based on gelling property of starch was used to prepare alumina ceramics. Slurry containing alumina powder, dispersant and small amount of starch (2–3.5 wt.% of powder weight) was cast into a nonporous mould and heated to gelation temperature to produce a rigid green body. A defect free green body was obtained and the total linear shrinkage during drying was 2–3% and the green density observed was 64% of theoretical value. After complete drying, ceramic compacts were sintered without debinding operation. Sintered density of 99.4% was achieved after sintering at 1600 °C for 2 h. Flexural strength values of dried and sintered alumina were ～10 and 247 MPa, respectively. The sintered ceramics showed an extremely dense microstructure.     

Development of ethylene glycol-based gelcasting for the preparation of highly porous SiC ceramics

Aqueous gelcasting is inappropriate for the preparation of highly porous ceramics, due to the large drying shrinkage of green bodies caused by the high surface tension of water. To solve this problem, non-aqueous gelcasting using organic solvents with much lower surface tension was developed. However, for most organic solvents, the precipitation polymerization of gels led to the low strength of green bodies, which was inconvenient for the fabrication of large size workpieces. In this work, a novel ethylene glycol-based gelcasting was developed to prepare highly porous SiC ceramics. Ethylene glycol induced the solution polymerization of gels and increased the strength of green bodies effectively. In addition, the high flexibility of the ethylene glycol-based gels could release the inner stress in the drying process. Highly porous SiC ceramics with large size were successfully prepared by the optimized gelcasting method.     

Mo-(Fe-B)-Fe混合粉末流延成型薄层坯体的干燥机理分析

流延成型Mo-(Fe-B)-Fe混合粉末流延成型薄层坯体的干燥过程非常重要，本文根据现有理论对流延坯体干燥过程的各个阶段，特别是坯体孔隙中充满液相的恒速干燥阶段（CRP）进行了分析。在恒速干燥阶段，产生了坯体的最大收缩和最大干燥应力，对引起干燥收缩的力和液相传输机制进行了分析。通过分析液相流动和固体网络之间的相互作用。可以计算孔隙液相中的压力分布。在靠近坯体干燥表面处，液相压力具有最大值，导致坯体在该处承受较大的压应力，并由此产生了不同大小的坯体收缩，从而引起干燥过程中的开裂现象，坯体开裂的可能性主要与坯体厚度和干燥速率有关，提出了避免干燥开裂的几个技术措施。