DRYING OF FINE CERAMICS

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.     

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.     

Modern high-density oxide ceramics with controlled microstructure. Part II. Substantiation of the choice of modifying additives that affect the degree of sintering of oxide ceramics

It is established that additives that form solid solutions provide substantial densification of oxide ceramics due to defects of the crystal lattice. The use of combined additives that form solid solutions of isovalent and nonisovalent substitution makes it possible to create oxide ceramics with an exceptionally high density, which may result in the formation of optical transparency.Translated from Ogneupory, No. 4, pp. 2 – 13, April, 1996.The first two articles of the series have been published in Nos. 1 and 2, 1996.     

Effect of the loading rate of the strength of GB-7 ceramics

The effect of the loading rate on the strength of an alumina ceramics of grade GB-7 in air is studied. It is established that the strength of the ceramics decreases substantially with increase in the loading rate due to the interaction between the calcium borosilicate vitreous phase and steam, which causes substantial growth of the microstructural defects existing in the ceramics.     

Effect of CIP on the strength of ceramics produced from ZrO2 + 3 mol.% Y2O3 powder

It is shown that the strength of ceramics from zirconia partially stabilized with yttria (PSZ) is determined substantially by the nature of the surface or subsurface defects. The kind of defects is connected with the fullness of formation of grain boundaries. It is shown that growth of the CIP pressure causes a change in the nature of the defect responsible for fracture, which in turn causes substantial hardening of the ceramics. When PSZ powder is pressed at 0.4 GPa the resulting ceramics has an ultimate bending strength of 1700 MPa. As a rule ceramics with such a strength are obtained only with the use of hot isostatic pressure.     

Anomalous Defects and Dynamic Failure of Armor Ceramics

The ballistic performance of state-of-the-art silicon carbide armor material can exhibit a fairly wide variability in certain test configurations, which, it is proposed, may be due to the presence of large (>0.1 mm), rare defects, termed, herein, "anomalous" defects. SiC rubble resulting from ballistic tests was examined, as were quasi-static test samples. Ballistic fragment fracture surfaces revealed large carbonaceous defects that seemed to affect fracture path and mode. Low-strength biaxial flexure samples demonstrated similar defects (>0.1 mm) as failure origins. Carbonaceous defects similar in appearance but smaller in size were also found at the fracture origins of SiC bend bars. Frequently, alumina inclusions were found within the carbonaceous discontinuities. These alumina inclusions may cause the graphitic regions to form during sintering. The random distribution of such large, rare carbonaceous discontinuities from sample-to-sample, as well as batch-to-batch variability, may explain high ballistic variability for SiC armor ceramics.     

多元稳定剂复合掺杂对增韧ZrO_2陶瓷性能的影响

Y_2O_3、CeO_2,MgO分别单独作为稳定剂制备增韧ZrO_2陶瓷材料,各有优点和不足。为了发挥不同稳定剂的优点而抑制其不足,本文研究了多元稳定剂复合添加对增韧ZrO_2陶瓷性能的影响。试验发现Y、Ce、Mg三种稳定剂复合掺杂的(Y、Ce、Mg)—PTZ陶瓷保持了Y稳定剂使材料强度、硬度较高的优点,发挥了Ce稳定剂使材料高韧性的优点,而Mg的加入实现了液相促进下的低温烧结,克服了Ce稳定剂使材料晶粒粗大,穿晶断裂等缺点,可获得高韧性、高强度ZrO_2陶瓷材料。     

Strength-Processing Defects Relationship Based on Micrographic Analysis and Fracture Mechanics in Alumina Ceramics

Characterization of processing defects in structural ceramics is very important for the investigation of mechanical property, because processing defects are origin of fracture and govern the strength of products. The relationship between defects and strength is represented by simple equation which contains factor of defect size. In this study, size of defects in alumina ceramics was measured by optical microscopy with sintered body thinned to about 150 μm. The size distribution was used for strength calculation based on linear fracture mechanics. Average and Weibull modulus of calculated strength were very agreed with experimentally measured them. The result shows that defects in sintered body govern the strength and are quantitatively related to strength by fracture mechanics.     

Application of Thermoluminescence and Reflectance Methods to Study of Lattice Defects in Alumina Ceramics

Alumina ceramics irradiated with gamma rays emit light of varying intensity as they are warmed. The resulting glow curve may be resolved into discrete peaks, each of which corresponds to a separate electron trapping level. The energy depth of each trap may be estimated from the glow-peak temperature and the temperature at which the light intensity reaches half its maximum value. Resolved glow curves are presented for a series of alumina ceramics including specimens having less than 100 p.p.m. of impurities as well as others contaminated with low concentrations of Si, Ti, Fe, Mg, and Ca oxides. Curves are also given for high-purity aluminas which were heated in oxygen and in hydrogen after firing. Where possible, glow peaks are identified with the causative impurity ion or annealing treatment, and estimated trap depths are tabulated. Because of its inverse relation to optical absorption, reflectance is also sensitive to changes in the number and type of lattice defects. Annealing in oxygen and introduction of Ca²⁺ ions produce a significant lowering of the reflectance of irradiated alumina ceramics, whereas exposure to hydrogen has the opposite effect.     

Ultrafast synthesis and densification of ZrO2 doped KNN ceramics by reactive flash sintering

Fabrication of dense KNN-based lead-free piezoelectric ceramics at low temperatures in short time through a cost-effective way remains a challenge. Herein, this challenge could be addressed by using reactive flash sintering. It is demonstrated that the phase transformation of KNbO₃-NaNbO₃ into (K,Na)NbO₃ and densification occur simultaneously during the flash event. Most importantly, ZrO₂ doping can greatly decrease the onset flash temperature, which is ascribed to the increased conductivity of sample. In addition, the current limit has a significant effect on the phase transformation and densification. The flash-sintered KNN ceramics exhibit the good ferroelectric and piezoelectric properties. Furthermore, the ZrO₂ doped and undoped KNN ceramics show a comparable coercive field E_c, which may be related to the residual point defects after the flash. Besides the Joule heating, the avalanche generation of point defects is suggested to be responsible for the ultrafast solid-state reaction and densification rates.     

Flaw Size Distribution in High-Quality Alumina

A difference in fracture strength was noted among commercial alumina ceramics. The origin of this difference was examined using optical microscopy in the transmission mode. Large defects measuring several tens of micrometers or more were clearly observed, and a direct correlation was found between the size of the large defects and the strength of the ceramics. The results indicated that a low concentration of large defects controlled the strength distribution of the ceramics studied.     

The influences of stoichiometry on the sintering behavior,optical and scintillation properties of Pr:LuAG ceramics

Sintering aids may enter the host lattice, create defects, and seriously deteriorate the scintillation properties of ceramic scintillators. In this study, the 0.3at%Pr:LuAG ceramics with different excess of Lu were fabricated by the solid-state reactive sintering without aids. The influences of stoichiometry on the sintering behaviors, optical and scintillation properties of the ceramics are systematically studied. The results show that the stoichiometric ceramics experience an abnormal grain growth during sintering. The Lu excess can restrain the abnormal grain growth of the ceramics because of the impurity drag effect. The excess Al₂O₃ as optical scattering centers, can also pin in the grain boundary to limit the fast migration of the grain boundaries. The light yield value of the as-sintered ceramics decreases with the increase of Lu content. After air-annealing, the ceramics with appropriate excess of Lu can achieve better optical quality and higher light yield compared to the stoichiometric Pr:LuAG ceramics.     

Ultrastrong tough zirconia ceramics by defects-engineering

We report the preparation of a category of ultrastrong tough zirconia ceramics by engineering defects using an oscillatory pressure during pressure assisted sintering. The introduced oscillatory pressure enhances the dynamic grain rearrangement, plastic deformation, mass transportation, and pore removal, leading to the formation of pore-free ceramics characterized by the rich coherent grain boundaries among individual mesocrystalline grains with intragranular quasi-interfaces. As a proof of concept, the pressure required by oscillatory pressure sintering (OPS) for preparing fully dense 3Y-TZP ceramics is significantly reduced, which is just ∼1/5 of that required by hot isostatic pressing. The OPS-prepared 3Y-TZP ceramics reached a record-breaking high bending strength and fracture toughness, being up to 1.8 GPa and 16 MPa·m^1/2, respectively. This success illustrates a universal principle of engineering defects for making breakthrough in exploring other ultrastrong tough ceramics.     

Effect of oxidation on flexural strength and thermal properties of AlN ceramics with residual stress and impedance spectroscopy analysis of defects and impurities

The effect of oxidation in air on the phase composition, microstructure, flexural strength and thermal property of AlN ceramics was investigated. Oxidation was found to produce a continuous oxide layer on the surface of AlN samples. The flexural strength and thermal conductivity of AlN ceramics significantly improved after oxidation at 1000 °C and 1100 °C. Residual stress in the AlN ceramic matrix was enhanced by oxidation. The enhanced residual compressive stresses inhibited crack propagation and reduced interfacial thermal resistance, thereby improving the flexural strength and thermal conductivity of AlN ceramics. Electrochemical impedance spectroscopy was further used to analyze the defects in AlN ceramics. The increase in fitting grain resistance revealed a decrease in aluminum vacancy concentration in oxidized AlN sample, which resulted in high thermal conductivity. Therefore, oxidation at a certain temperature is pretty effective to obtain excellent performance for AlN ceramics.     

Solubility of Hydrogen Defects in Doped and Undoped BaTiO3

Thermal desorption studies and electrical conductivity measurements have been used to investigate the solubility of hydrogen defects in barium titanate ceramics in oxidizing atmospheres. Hydrogen is found to dissolve in the perovskite lattice of undoped and acceptor-doped BaTiO₃ by the formation of hydroxide ions on regular oxygen ion sites. The hydrogen defects act as mobile donors and may substitute the oxygen vacancies in compensating fixed acceptors. From the dependence of the solubility on the water vapor pressure and the dopant concentration, the incorporation/desorption equilibrium reaction is deduced. A defect model is derived which consistently extends previously proposed models. The influence of Ba₂TiO₄ and Ba₆Ti₁₇O₄₀ second phases on the solubility of hydrogen defects is discussed.     

Characterization of Fine-Grained Bismuth Vanadate Ceramics Obtained Using Nanosized Powders

Nanocrystalline bismuth vanadate (n-BiV) powders with a crystallite size of <50 nm have been prepared, at room temperature, by subjecting a stoichiometric mixture of bismuth oxide and vanadium pentoxide to mechanical activation. The n-BiV powders show enhanced sinterability, in comparison to the conventionally prepared micrometer-sized bismuth vanadate (m-BiV) powders and yield ceramics with a uniform microstructure. High-density (∼98% of the theoretical value), fine-grained (average grain size of ∼2 μm) ceramics, obtained using n-BiV have a high dielectric constant and a high pyroelectric coefficient and are associated with low dielectric loss, both at room temperature and at the transition temperature. These fine-grained ceramics show diffused phase transition and relaxor behavior, which are attributed to the irregular distribution of defects and/or compositional inhomogeneities in these ceramics. The fine-grained ceramics exhibit ferroelectric hysteresis loops with higher remanent polarization and lower coercive field values than the coarse-grained ceramics.     

反应烧结氮化硅陶瓷模具材料的研究

用反应烧结方法制备氮化硅陶瓷材料模具,然后将其安装在ＹＡ－１００型四柱液压拉深机上进行拉深不锈钢实验。结果证明：与水基润滑剂配合,反应烧结氮化硅陶瓷材料模具拉深ＳＵＳ３０４不锈钢时,拉深件不产生划痕、划伤等表面缺陷,拉深件废品低于０．１％模具表面也不产生拉深棱缺陷,是可以替代金属模具的新一代模具材料,开发前景广阔。     

Correlation between crystal grain sizes of transparent ceramics and scintillation light yields

Light yields of Y₂O₃ ceramics are different from specimen to specimen. Nature of this phenomenon is not clear yet. Furthermore, origin of emission peaks for Y₂O₃ is not well understood. The results reported suggest that the emission derived from trapped excitons originates from the defects accumulated in the grain boundaries of the ceramics. In this paper, average grain size is introduced in order to evaluate defects inside the ceramics. Relationship between average crystal grain size and scintillation light yield is demonstrated.     

先进陶瓷胶态成型新工艺的研究进展

总结了先进陶瓷胶态成型新工艺在清华大学的研究进展.简要介绍了几种高固相含量的浓悬浮体和高质量坯体的制备新技术,提出胶态成型新工艺固化机制的新见解.采用新工艺去除坯体中缺陷,实现高可靠性、复杂形状高性能陶瓷部件低成本的自动化、批量化制备,为先进陶瓷产业化铺平了道路.     

Antisite defect in nonstoichiometric yttrium aluminum garnet: Experimental and first-principles calculation

Antisite defects in nonstoichiometric yttrium aluminum garnet (YAG) were investigated systematically with experiments and first-principles calculation based on density functional theory. Transparent YAG ceramics with different deviations from stoichiometry have been prepared and the lattice constants increase with the increase of deviation. Calculations show that Y_Al,16a is the most preferred antisite defect and the concentration of defects increases as the sintering temperature increased. High sintering temperature results in high concentration of Y_Al defects, however Y_Al defects may avoid secondary phase in Y₂O₃-rich YAG. It is found that formation energy of Al_Y antisite defect is very high, therefore the concentration of Al_Y antisite defects is very low even at high temperature. For Al₂O₃-rich YAG, it is impossible to avoid the formation of secondary phase. The deviation from stoichiometry has great influence on the transmittance and optical quality of transparent ceramics.