Fundamental Study and Equipment for Sintering and Testing of Cermet Bodies: VII, Fabrication, Testing, and Properties of 34 Al2O3-66 Cr-Mo Cermets

The properties of a cermet solid body containing 34% Al₂O₃ and 66% 80Cr–20Mo alloy (50Al₂O₃-50 alloy by volume) are given as firing shrinkage, density, modulus of rupture, tensile strength, stress-rupture life, modulus of elasticity, oxidation resistance, thermal-shock resistance, and thermal expansion. It is shown that substitutions of the 80Cr-20Mo alloy for Cr in a similar cermet developed earlier substantially improved thermal-shock resistance.     

Cermets: I, Fundamental Concepts Related to Micro-structure and Physical Properties of Cermet Systems

The microstructure of tungsten carbide and titanium carbide base cermets has been evaluated on the basis of wettability of the carbide phase. Incomplete wetting was found to result in a coalescence of the carbide phase leading to an apparent increase in the carbide particle size. Improvement in the wettability of the nickel-titanium carbide systems by the addition of molybdenum was found to promote the dispersion of the carbide phase and to lead to improved hardness and impact resistance.     

Processing of ZrC–Mo Cermets for High Temperature Applications, Part II: Pressureless Sintering and Mechanical Properties

Pressureless sintering of ZrC–Mo cermets was investigated in a He/H₂ atmosphere and under vacuum. A large density increase was observed for specimens with >20 vol% Mo after heating at 2150°C for 60 min in a He/H₂ atmosphere. The increase in density was attributed to the formation of Mo₂C during heating and its subsequent eutectic reaction with Mo, which produced rounded ZrC grains in a Mo–Mo₂C matrix. Sintering in vacuum did not produce the same increase in density, due to the lack of Mo₂C formation and subsequent lack of liquid formation, which resulted in a microstructure with irregular ZrC grains with isolated areas of Mo. Mechanical properties testing showed a decrease in Young's modulus with increasing Mo content that was consistent with the models presented. Flexure strength of ZrC–Mo cermets sintered in He/H₂ atmosphere materials increased with increasing Mo content from 320 MPa at 20 vol% Mo to 410 MPa at 40 vol% Mo. Strength was predicted by adapting theories developed previously for WC–Co cermets.     

CaTiO3添加剂对氧化铝陶瓷烧结、显微结构及微波介电性能的影响

CaTiO3添加剂通过湿法球磨与Al2O3粉料混合,并通过无压烧结制备了氧化铝陶瓷,研究了CaTiO3添加剂对氧化铝陶瓷烧结性能、相组成、显微结构和微波介电性能的影响.CaTiO3可以使Al2O3的烧结温度降低至1450℃,但在该温度下烧结的样品由于CaTiO3的加入会产生CaAl12O19第二相.样品中存在大、小两种晶粒,根据EDS能谱分析,大晶粒主要是CaAl12O19,而小晶粒为Al2O3和CaAl12O19的混合相.添加CaTiO3有利于Al2O3陶瓷介电常数的提高,1450℃下掺杂2.5wt％ CaTiO3的氧化铝陶瓷具有较好的烧结性能和微波介电性能,相对密度可达到97.74％,εr～10.86,Q×f～ 8061 GHz.     

Dissolution of Alumina, Sintering, and Crystallization in Glass Ceramic Composites for LTCC

Sintering and microstructure evolution of alkali-free calcium–alumo–borosilicate glass/α-Al₂O₃ composites (mean particle size ca. 2 μm) for low-temperature cofired ceramics were studied during heating at 5 K/min by heating microscopy, thermal analysis (DTA), X-ray diffraction (XRD), and electron microscopy (SEM). Composites fully densify at ≈830°C, not essentially influenced by the dissolution of alumina and glass crystallization. Thus wollastonite, as first crystalline phase, was detectable at 840°C. Above 900°C, a pronounced crystallization of anorthite is evident, reaching 60 wt% at 1050°C. Rietveld analyses of XRD data revealed that anorthite precipitates at the expenses of alumina, which declines from ≈33 to <10 wt%, and wollastonite, which fully declines from its maximum of ≈19 wt%. Based on XRD, we discuss the evolution of crystal mass fractions, the residual glass composition, the glass viscosity, and the effective shear viscosity of the composites under study during heating.     

Fundamental Study of Clay: XIII, Drying Behavior and Plastic Properties

The effects of nonionic, anionic, and cationic surface-active agents on yield point, plasticity, drying and firing shrinkage, dry and fired density, and rate of drying have been experimentally determined. These and other factors have been correlated on the basis of a comprehensive theory of the plastic properties and drying behavior of clay-water masses.     

Estimate of the Activation Energies for Boundary Diffusion from Rate-Controlled Sintering of Pure Alumina, and Alumina Doped with Zirconia or Titania

Sintering experiments at constant heating rates were employed to estimate the activation energy for sintering in alumina and in alumina containing 5 vol% zirconia or 5 vol% titania. Grain growth, which can complicate the analysis of sintering kinetics data, was suppressed by using uniformly and densely packed grain compacts prepared by colloidal processing. Grain-boundary diffusion is believed to have been the dominant sintering mechanism. The activation energies were 440 ± 40 kJ/mol for pure alumina, 585 ± 40 kJ/mol for alumina (titania), and 730 ± 60 kJ/mol for alumina (zirconia). The alumina and alumina (titania) results are in agreement with the values reported in the literature. The possibility that the higher activation energies for doped alumina reflect a stronger bonding at alumina interfaces in the presence of zirconium and titanium is discussed.     

Infiltration of Porous Alumina Bodies with Solution Precursors: Strengthening via Compositional Grading, Grain Size Control, and Transformation Toughening

Alumina powder compacts, partially densified with a lowtemperature heat treatment and then cut into bars, were infiltrated with liquid precursors that decomposed to either mullite (3Al₂O₃·2SiO₂), fully stabilized zirconia (cubic Zr(8Y)O₂), or partially stabilized zirconia (tetragonal Zr(4Y)O₂). The specimens were repeatedly infiltrated and pyrolyzed to achieve a higher concentration of the precursor near the surface. The infiltrated bodies were then densified at 1500°C/2 h. Residual stresses developed during cooling from the densification temperature because of the higher concentration of the second phase near the surface and their differential thermal expansion relative to the matrix material. At least 10 bars of each two-phase material were fractured in four-point bending to determine the effect of the second phase on strength. The alumina bars without a second phase had a larger grain size (∼7 μm) and a mean strength of 253 MPa. The intruded phases significantly reduced the Al₂O₃ grain size to ∼1 1μm. Despite their higher concentration near the surface and apparent surface tensile stress, both of the Zr(Y)O₂ phases increased the mean strength to 413 MPa ( c -Zr(8Y)O₂) and 582 MPa ( t -Zr(4Y)O₂, an apparent toughening agent). The mullite second phase produced a high mean strength of 588 MPa, apparently due to its concentration gradient creating a compressive surface stress.     

Synthesis of Nanoparticles in High Temperature Ceramic Microreactors: Design, Fabrication and Testing

Microreactors as a novel concept in chemical technology enable the introduction of new reaction procedures in chemistry, pharmaceutical industry, and molecular biology. These miniaturized reaction systems offer many exceptional technical advantages for a large number of applications. One major application is in the bulk synthesis of nanoparticles. Despite the availability of a plethora of nanoparticle synthesis processes, there exist many difficulties in controlling the shape, size, and purity of nanoparticles in large quantities in a safe and cost-effective manner. These difficulties have been the principal factors adversely limiting the applications of ceramic nanoparticles. Recent experiments have shown that to study the process of growth and formation of nanoparticles, a reactor having much smaller dimensions, namely a microreactor is more appropriate. These studies have also shown that a microchannel reactor provides control over the mean residence time and hence over the nanoparticle size and shape. This paper deals with the design, fabrication, and testing issues related to a high temperature, ceramic microreactor by investigating the use of reactive gas streams in arrays of microchannel reactors. These innovations offer the potential to overcome the barriers associated with synthesis of ceramic nanoparticles in large quantities.     

Preparation of High-Silica Glasses from Colloidal Gels: I, Preparation for Sintering and Properties of Sintered Glasses

A new method for preparing fused silica and high-silica glasses through the sougel process is described. Ordinarily, fused silica is formed by fusion of silica at 1900° to 2000°C. The method described consists of preparation of a porous body by mixing colloidal silica with water, drying, redispersion, molding by casting, followed by gelation, drying, and sintering at 1450° to 1500°C. The sintering is facilitated by addition of 3 to 5 wt% B₂O₃. The process of dispersion-drying-redispersion (so-called "double process") results in formation of a two-mode pore structure which permits drying of cast tubes and rods without breakage. The preparation of the porous bodies, study of pore structure, and properties of resulting glasses are described in Part I, the sintering is considered in Part II, and the mechanism of gel formation is interpreted from infrared spectroscopy studies in Part III.     

用于电子电气产品中无机颜料的六价铬测试方法研究

采用分光光度法对电子电器产品中无机颜料的Cr(VI)进行测试,着重研究无机颜料的Cr(VI)萃取过程中各种因素的影响,通过优化萃取时间、萃取温度、定容pH值、显色时间等参数,确定了无机颜料中Cr(VI)的最佳测试条件为:最佳萃取时间为120 min,萃取温度为90~95℃,定容液最佳pH值为2±0.5,最佳显色时间为5~40 min。该方法在低浓度范围(0~500μg/L)呈良好线性关系,可以满足对用于电子电气产品中无机颜料中痕量Cr(VI)的定量分析。     

体育器材用CNT/2024Al复合材料的制备及性能研究

随着体育用材新材料等领域的飞速发展,对材料的性能提出更严格的要求,传统的金属材料已不能满足需要,CNT/Al基复合材料由于其优秀的力学和物理性能以及良好的可加工性成为上述领域关键的新型材料。但制备CNT/Al基复合材料的主要困难是将CNT的分散和CNT的损伤同时控制在理想的范围内,从而更好地发挥CNT高强、高韧的优势。实验制备CNT/2024Al复合材料,并对球磨后粉末的形貌变化进行研究,同时分析了CNT/2024Al复合材料的微观组织。     

Fibrous Monolithic Ceramics: IV, Mechanical Properties and Oxidation Behavior of the Alumina/Nickel Systein

Fibrous monolithic ceramics were fabricated in the alumina/nickel system. The microstructure consists of high-aspect-ratio polycrystalline cells of alumina separated by thin cell boundaries of nickel. The nickel content in the material is 3 to 8 vol%. The fibrous monolith with uniaxially aligned cells fails noncatastrophically in flexure. Bridging ligaments of nickel, crack deflection along cell boundaries, and crack branching in the axial direction are observed in flexure bars and notched beams. Strength values range from 246 to 375 MPa. Indentations cause controlled damage on the surface but do not introduce strength-degrading flaws. The alumina/nickel fibrous monoliths also show potential for use at high temperatures in oxidizing environments. Noncatastrophic fracture behavior is observed at room temperature after 10 h at 1200°C in air. The Ni cell boundary network is oxidized to a depth of 50 to 100 μm by this heat treatment. The NiO oxidation product in the cell boundaries reacts partly with alumina from the cells to form NiAI₂O₄, which would provide better protection.     

Fabrication and Optical Properties of High-Performance Polycrystalline Nd:YAG Ceramics for Solid-State Lasers

Transparent polycrystalline YAG with nearly the same optical characteristics as those of a single crystal were fabricated by a solid-state reaction method using high-purity powders (>99.99 wt% purity). The average grain size and relative density of the 1.1 at.% Nd:YAG ceramics obtained were about 50 μm and 99.98%, respectively. An oscillation experiment was performed on a cw laser by the diode laser excitation system using the fabricated ceramics. The experimental results indicated an oscillation threshold and a slope efficiency of 309 mW and 28%, respectively. These values were equivalent or superior to those of the 0.9 at.% Nd:YAG single crystal fabricated by the Czochralski method.     

Normal Sintering of (K,Na)NbO3-Based Ceramics: Influence of Sintering Temperature on Densification, Microstructure, and Electrical Properties

Normal sintering of Li-doped and Li/Ta-codoped potassium sodium niobate (KNbO₃–NaNbO₃, KNN)-based ceramics was investigated to clarify the optimal sintering condition for densification, microstructure, and electrical properties. It was found that density increased greatly within a narrow temperature range but tended to decrease when the sintering temperature slightly exceeded the optimal one, accompanied by the appearance of abnormal grain growth, which was considered to be due to the intensified volatilization of alkali metal oxides. Piezoelectric and dielectric properties also showed a similar relationship between the density and sintering temperature, but the highest piezoelectric strain coefficients were obtained at the temperatures lower than that for the highest density, because both densification and composition affect the electrical properties. The highest d ₃₃ value of 206 pC/N was obtained for the Li- and Ta-codoped KNN ceramics prepared at 1090°C.     

Hydrothermal Synthesis of Nanometer-Sized Barium Titanate Powders: Control of Barium/Titanium Ratio, Sintering, and Dielectric Properties

Nanometer-sized BaTiO₃ powders have been synthesized hydrothermally from Ba(OH)₂ and titanium alkoxide at 150°C for 2 h, and the Ba/Ti ratio has been measured with an accuracy of ±0.003. Stoichiometric powders can be obtained by adjusting the Ba/Ti ratio of the reactants to a value of 1.018. At a lower Ba/Ti ratio, the solubility of Ba(OH)₂ prevents full incorporation of barium, and barium-deficient powders result. A higher Ba/Ti ratio leads to the incorporation of excess barium in the powder. K _s(BaTiO₃,-25°C) = 7 × 10^-8 has been calculated for the equilibrium reaction. From this result, two reproducible processes for the synthesis of stoichiometric BaTiO₃ are proposed. The processes rely only on very accurate control of the chemical composition (Ba/Ti ratio) of the precursor suspension. The sintering behavior of powders having Ba/Ti ratio values between 0.965 and 1.011 is described from results of dilatometric measurements and isothermal sintering. Room-temperature dielectric constants as high as 5600 and losses as low as 0.009 have been obtained for a stoichiometry slightly less than 1.000. It is expected that optimum sintering behavior and electrical properties are obtained in the stoichiometry range 0.995-1.000.     

Fabrication,Microstructure, and Luminescent Properties of Ce3+ ‐Doped Lu3Al5O12 (Ce:LuAG) Transparent Ceramics by Low‐Temperature Vacuum Sintering

The fabrication of 0.5 mol% Ce:LuAG transparent ceramics starting from synthetic nanosized Ce:LuAG powders was investigated by low temperature vacuum sintering. It was found that high quality optical Ce:LuAG ceramics could be densified successfully by vacuum sintering (<10^–3 pa) at 1750°C for 10 h. The in‐line optical transmittance of as‐sintered Ce:LuAG ceramics with thickness of 0.7 mm could reach 73.48% at the wavelength of 550 nm. The microstructure observations revealed that transparent Ce:LuAG ceramics were composed of uniform LuAG grains with average size of 9 μm and HRTEM morphology indicated that no impurity segregation existed at grain boundaries or within Ce:LuAG grains. It was also demonstrated that the annealing treatment (at 1450°C for 20 h in air) could greatly enhance the luminescent intensity of as‐sintered Ce:LuAG ceramics under excitation of X‐ray radiation (75 kV, 25 mA), which makes it a potential candidate to be applied in radiation detector.     

Fluoropolymer Fibres: Physicochemical Nature and Structural Dependence of their Unique Properties, Fabrication, and Use. A Review

Fluorine-containing fibre-forming polymers, fibres, and fibre materials have unique properties due to the presence of fluorine atoms. Totally fluorinated polymer materials are distinguished by the highest resistance to all kinds of chemical reagents and physically active media, low wettability by polar liquids, and other specific functional properties. In examining the physicochemical nature of the unique properties of fluoropolymer fibres and fibre materials with respect to the theory of chemical structure and D. I. Mendeleev's Periodic Table of the Elements, we found that they are due to the features of the fluorine atoms, which have the highest electronegativity of all elements, high ionization energy, high energies of bonds with carbon atoms, and simultaneously shield the molecular chain from external effects. Fibre-forming fluoropolymers, fibres, and fibre materials made from them have high resistance to external effects, biostability and bioinertness, and highly effective protective and other functional characteristics. This relates to the totally fluorinated materials PTFE, HFP, and CP-TFE-HFP to the greatest degree. Fibres and fibre materials based on totally fluorinated polymers (PTFE, HFP, CP-TFE-HFP) are placed in a special group for use in articles utilized in extreme conditions in exposure to especially aggressive media and other external active effects where the use of other kinds of fibres and fibre materials is difficult or impossible because of their low stability and rapid destruction.     

Fabrication of Ceramic Hip Implant Composites: Influence of Silicon Nitride on Physical,Mechanical and Wear Properties

This study examined the effects of silicon nitride reinforcement on physical, mechanical and wear properties of different ceramic (zirconium oxide, magnesium oxide, chromium oxide and aluminum oxide) containing hip implant composites. The hip implant composites were produced using conventional mixing and spark plasma sintering methods by substituting aluminum oxide (68, 70.5, 73 and 75.5 wt.%) with silicon nitride (0, 2.5, 5 and 7.5 wt.%). Experimental results showed that silicon nitride content had significant effect on the evaluated physical, mechanical and wear properties. The density of the composites found to decrease whereas void content, Young’s modulus, hardness, wear resistance and fracture toughness first decreased (for 2.5 wt.%) and then increased with the increasing amount of silicon nitride content. The maximum hardness, Young’s modulus, wear resistance and fracture toughness values of 28.64 GPa, 280.18 GPa, 0.0076 mm³/million cycles and 11.84 MPa.m^1/2, respectively were registered for 2.5 wt.% silicon nitride additions that also had the lowest void content (0.38%).

     

Sintering Behavior of Gehlenite. Part I: Self-Forming, Macro-/Mesoporous Gehlenite—Pore-Forming Mechanism, Microstructure, Mechanical, and Physical Properties

A novel kind of pore self-forming macro-/mesoporous gehlenite (2CaO·Al₂O₃·SiO₂) ceramic (abbreviated C₂AS) having a highest porosity of 80% corresponding to a volume expansion of 134% during sintering has been developed. The pore self-forming ability, microstructure, mechanical, and thermal physical properties of the porous ceramic are related to the sintering temperature. The gehlenite ceramic shows a very good pore self-forming ability over a very wide range of temperature from 900° to 1450°C. No vesicant is required and no hydrothermal treatment is needed, as is generally the case for other kinds of porous ceramics or glasses. The pore self-forming ability of the C₂AS porous ceramic can be attributed to the escape of the adsorbed water vapor during the sintering process, due to automatic hydration of the fine, amorphous, flakey-shaped starting C₂AS powder particles synthesized by the organic steric entrapment (PVA) method, as well as to their fine, porous microstructure. The pores of the ceramics can be either open or closed, and the average pore size ranges from 0.6 to 1.1 μm, corresponding to a porosity of 75%–80%, respectively. The porous ceramic can preserve nanometer-sized (26–50 nm) crystallites up to 1000°C. Sintered or thermally treated under different conditions, the porous ceramics exhibit relatively high flexural strengths ranging from 9.1 to 15.4 MPa, with a standard deviation of 0.3 and 4.2 MPa, respectively. Thermal properties of the porous ceramic up to 1000°C, including thermal expansion coefficient, thermal diffusivity, specific heat, and thermal conductivity, were investigated, and the stability of the porous ceramic in boiling water was also studied.