多元纳米复合氧化锌压敏陶瓷的晶粒生长

研究了多元纳米复合ZnO电压敏粉体在高温下的烧结行为.应用晶粒生长的动力学方程Gn-G0=K0texp(-Q/RT),确定了晶粒生长的动力学指数n和激活能Q.实验结果表明,随着烧结温度的提高和保温时间的延长,ZnO压敏陶瓷的晶粒不断长大,其动力学指数n=3.2,激活能Q=(185±28)kJ/mol.     

Dielectric relaxation in Ba(Ti0.8Zr0.2)O3 ceramics prepared from sol-gel and solid state reaction powders

In order to correlate dielectric relaxation properties with chemical homogeneity and microstructure in ferroelectric ceramics, powders are prepared both by sol-gel route and solid-state reaction. This comparative study concerns Ba(Ti_0.8Zr_0.2)O₃ ceramics, with maximum permittivity close to room temperature. Dielectric relaxation is reported in the frequency and temperature ranges 10⁶ to 10⁹ Hz and 250–350 K, respectively. A minimum of the relaxation frequency occurs close to the phase transition temperature. The variations off _r are correlated with chemical homogeneity and microstructure.     

Pechini法制备铌酸锂陶瓷的结晶性能研究

以Li2CO3、Nb2O5、K2CO3为初始原料,采用Pechini法制备了铌酸锂(LiNbO3)陶瓷。用XRD、SEM等分析了LiNbO3陶瓷的结晶特性。分析结果表明Li2O对于铌酸锂陶瓷的烧结及致密度有着重要的影响,经980℃烧结得到的LiNbO3陶瓷晶粒发育良好,当烧结温度>1100℃时,Li2O有较大的损失,所得到的LiNbO3陶瓷组分与化学计量比有一定偏差。因此应采用多种技术方法来减少Li2O的损失以得到致密、结晶性良好的铌酸锂陶瓷。     

Preparation and properties of ceramics from magnesium spinel by sol-gel technology

A dense electroinsulating ceramic was prepared at lower temperature (1400–1450 °C) compared to the solid phase synthesis. The synthesis of the spinel was carried out at 1000 °C reaching over 97% spinel by applying the sol-gel method and the inclusion of one of the components as a soluble salt. Combinations of additives like B₂O₃ and TiO₂ were included as alcoholates during the process of hydrolysis. These additives caused an intensification in the course of the process of spinel-formation.     

Characterization and adsorption properties of powders prepared from cactus pulp

Two cactaceous powders, labelled CACMM1 and CACMM2, are shown to contain calcium oxalate and to consist of globular or cubosome particles. The samples were characterized by X-ray diffractometry, spectroscopic, thermic, nitrogen and water adsorption methods and compared with other organic compounds. The pore systems have been analysed from adsorption isotherms, t-plots and pore size distribution curves. The bioadsorbents CACMM1 and CACMM2 exhibited low specific surface area values and presented pores in the mesopore range. Composite adsorption isotherms for the binary mixtures benzene-hexane and benzene-cyclohexane on cactaceous powders have been established and revealed preferential adsorption of hexane and cyclohexane in the respective system. Also the adsorption behavior of dyes from aqueous solution was studied to complete the diversity of adsorbate molecules. Linear adsorption isotherms were obtained up to 6 mM dye equilibrium concentration.     

Microstructure and properties of alumina-SiC nanocomposites prepared from ultrafine powders

Alumina-Silicon Carbide nanocomposites were produced and studied under different aspects: characteristics of the starting materials, processing, microstructure and mechanical properties. The raw materials were two kinds of fine SiC powders (30 and 45 nm) and two Al₂O₃ powders (60 and 140 nm). Different compositions (amounts of SiC in the range 0.5–5 vol%) were performed and the characteristics of the resulting materials compared. The oxygen enrichment in SiC nanopowder due to specific powder treatments was controlled, in order to optimize powder processing routes. Densification tests of Al₂O₃-SiC powder mixtures were performed both by pressureless sintering and hot pressing route. The addition of SiC reduced the densification rate and favoured a refinement of the matrix. Improvement of mechanical properties over monolithic alumina was obtained in composites with the 45 nm SiC. The study pointed out that the critical factor for the success of these materials is the choice of the raw SiC powders in terms of grain size and state of agglomeration. The addition of this ultrafine SiC strongly affected the microstructural evolution, even at low volumetric fractions. The results do not substantiate any remarkable effect by dispersoids in the tested nanosize range.     

Effects of sintering conditions on the microstructure and mechanical properties of SiC prepared using powders recovered from kerf loss sludge

The effects of sintering conditions on the microstructure and mechanical properties of the sintered SiC prepared using the SiC powder recovered from the kerf loss sludge were investigated. The recovered SiC powders were consolidated by spark plasma sintering (SPS) and conventional sintering methods. The effects of sintering temperature, time and methods (SPS and conventional sintering) on the phase, grain size and density of SiC were systematically studied. The Vickers hardness of spark plasma-sintered (SPSed) samples was higher than that of conventional sintered samples due to small grain size. When holding time was increased from 10 to 30 min, the grain size and relative density of SPSed samples were also increased, which lead to the almost constant Vickers hardness by competing effects of grain size and relative density. When holding time was over 30 min, no appreciable change of the relative density and grain size were observed, which can lead to similar values of Vickers hardness. SPS process can be used to make SiC with high density and hardness at relatively low temperature compared with the conventional sintering process.     

Effect of sintering parameters on the microstructure and properties of strontium modified PZT ceramics prepared using spray-dried powders

PZT powders of the composition Pb_0.94Sr_0.06 (Zr_0.53Ti_0.47)O₃, prepared by spray drying and calcining techniques, were processed to sintered ceramics by conventional cold pressing and sintering at various temperatures and periods between 1000 to 1250°C for 0.5 to 12h. Sintered ceramics were evaluated for their microstructure and electromechanical properties. Highly dense ceramics having bulk density of the order of 97% of the theoretical value could be obtained after sintering at a considerably lower temperature of 1000°C in comparison to the 1300°C generally required for powders prepared by conventional ceramic processing. However, the increase in sintering temperature of reactive spray-dried powders causes the entrapment of closed pores as a result of exaggerated grain growth and subsequent pore coarsening thereby leading to a decrease in the bulk density of the ceramics. It has been observed that minor variations in the sintering parameters influence the porosity, grain size and electromechanical properties. Values of the dielectric constant, piezoelectric strain coefficient and electromechanic coupling factor increase with the increase in grain size and decrease with the increase in porosity of the sintered ceramic whereas the dielectric dissipation factor decreases with the increase in sintering temperature.     

The mechanical properties of ceramics from bauxite waste

This paper summarizes the development of engineering ceramics made from bauxite waste (red mud) produced in the alumina industry in Jamaica. Test specimens are fabricated from powders by sintering. For a particle size distribution of less than 75m in the starting powders and firing temperatures in the range of 1000 to 1100° C various mechanical properties are measured. These include fracture toughness (K _Ic), modulus of rupture (MOR), compression strength ( _c) and Brinell hardness. While apparent porosity varies between 40 and 48%, K _Ic is found to vary between 0.39 and 0.68 MN m^–3/2. The values are compared with those measured for commercial ceramic tiles and also with ceramics of magnesia and calcium zirconate. Within the fabrication temperature range studied the MOR ranges between 17.23 and 27.09 MN m^–2, compressive strength between 42.0 and 83.9 MN m^–2 and Brinell hardness between 26.2 and 59.9 kg mm^–2. With the aid of scanning electron microscopy and a basic knowledge of the physicochemical properties of the mud an attempt is made to explain the high strength and toughness achieved. The ready availability of raw material and the relatively low firing temperatures required for suitable engineering products should keep the production costs low for red mud ceramics.     

Physicochemical properties of tungsten carbide powders prepared from ionic melts

Fine-particle tungsten carbide powders were prepared by high-temperature electrochemical synthesis, and their physicochemical properties were studied. The ceramics fabricated from these powders showed superior mechanical properties. The powders were found to exhibit high catalytic activity for hydrogen electrogeneration and electrodissolution.     

Role of sintering time,crystalline phases and symmetry in the piezoelectric properties of lead-free KNN-modified ceramics

Lead-free KNN-modified piezoceramics of the system (Li,Na,K)(Nb,Ta,Sb)O₃ were prepared by conventional solid-state sintering. The X-ray diffraction patterns revealed a perovskite phase, together with some minor secondary phase, which was assigned to K₃LiNb₆O₁₇, tetragonal tungsten–bronze (TTB). A structural evolution toward a pure tetragonal structure with the increasing sintering time was observed, associated with the decrease of TTB phase. A correlation between higher tetragonality and higher piezoelectric response was clearly evidenced. Contrary to the case of the LiTaO₃ modified KNN, very large abnormal grains with TTB structure were not detected. As a consequence, the simultaneous modification by tantalum and antimony seems to induce during sintering a different behaviour from the one of LiTaO₃ modified KNN.     

Synthesis and sintering properties of cerium oxide powders prepared from oxalate precursors

Fine cerium oxide powders obtained by low-temperature decomposition from oxalate and hydrazinate oxalate are compared. Studies of thermal behaviour and disagglomerating capability are given. The preparation of oxide powders by chemical and physical methods is able to give a dispersed suspension, and leads with appropriate recovery to increased green density and final density after sintering.     

Sol-gel制备PLCT薄膜的结晶特性与电畴结构研究

运用sol-gel技术制备了(Pb,La,Ca)TiO3(简写为PLCT)铁电薄膜;利用XRD、SEM、AFM和EDAX分析了PLCT薄膜的结构、表面形貌和组分。XRD衍射结果表明,PLCT薄膜呈钙钛矿结构。随着退火时间的增加,PLCT薄膜的XRD衍射峰的强度也随之增加。SEM、AFM分析表明,PLCT铁电薄膜表面平整、致密、无裂缝。EDAX分析表明,PLCT薄膜的实际组分十分接近设计组分。利用PFM分析了PLCT薄膜的电畴结构,发现随着退火时间的增加,PLCT薄膜的电畴由细小圆点状逐渐增大并形成片状电畴。     

柠檬酸溶胶-凝胶法合成ZnGa2O4:Mn2+纳米粉末的结构和发光性能研究

采用柠檬酸溶胶-凝胶法合成了ZnGa2O4∶Mn2 粉末.利用XRD、SEM、IR、EPR和PL光谱手段,表征了样品的结构、形貌和发光性能.结果表明,柠檬酸溶胶-凝胶法制备的ZnGa2O4∶Mn2 颗粒均匀,在650℃热处理7h即可得到结晶性好、粒径约为50nm的粉末;掺杂的锰是 2价的,不仅存在于四面体间隙中,也存在于八面体间隙中;当Mn2 的掺杂浓度为0.3%(原子分数),退火温度为800℃时,样品的绿色发光强度最大.     

PZT纳米晶的改进型溶胶-凝胶法制备及其生长过程研究

以乙二醇为溶剂,硝酸锆为锆源,采用改进的溶胶-凝胶法,通过优化工艺参数制备了纳米Pb(Zr0.52Ti0.48)O3(PZT).通过对工艺条件研究, 揭示了体系的凝胶化可通过升高温度和增加pH值来实现.通过对溶胶的红外光谱分析给出了溶胶、凝胶形成的机理.通过TG-DTA、XRD、SEM、TEM等手段对从凝胶到PZT纳米晶的生长过程及性能进行了表征.实验证明,改进后工艺简单易行,溶胶稳定,干凝胶在650℃热处理2h,获得较完整的钙钛矿型PZT,粒径约为50～100nm.     

Mechanical properties of zirconia-alumina composite ceramics prepared from Zr-Al metallo-organic compounds

The mixture of a Zr-Al metallo-organic compound and Al₂O₃ powder yields dense ZrO₂-Al₂O₃ composite ceramics. The fraction of the tetragonal ZrO₂ phase in as-sintered ZrO₂-Al₂O₃ ceramics is almost 100% and the ZrO₂ grains at about 500 nm in diameter are dispersed in the matrix. The ceramics have high fracture toughness and bending strength.     

Optical properties and characterization of zinc nitride nanoneedles prepared from ball-milled Zn powders

Zinc nitride nanoneedles (ZNNs) with diameters at stem and tip parts as 200-300 nm and 30-70 nm respectively have been prepared by the nitridation of ball-milled zinc powders at 600 °C for 120 min under NH₃ gas environment. The structural, compositional and morphological characterizations of the product were conducted by X-ray diffraction, energy dispersive X-ray spectroscopy, scanning electron microscopy and transmission electron microscopy. From transmission spectrum data, an indirect band gap of 2.72 eV has been calculated for ZNNs whereas photoluminescence studies exhibited a strong UV excitonic mission band at 395 nm as well as two weak defect related blue emissions at 453 and 465 nm. A vapor-solid (VS) process based growth mechanism for the formation of ZNNs has also been discussed briefly.     

Size effect on piezoelectric properties of barium stannate titanate ceramics prepared from nanoparticles

Piezoelectric properties of Ba(Ti_1?x Sn _x )O₃ ceramics with x = 0.025, 0.045 and 0.065, prepared from 16 nm powders, were compared with those of the corresponding ceramics obtained from 86 nm powders to see the effect of tin content and particle size of the starting powders. Ba(Ti_1?x Sn _x )O₃ powders were synthesized by solid state reaction of BaCO₃, TiO₂ and SnO₂ at 1,050 °C. The powders were high energy ball milled to produce nanocrystalline powders having average particle size of 16 nm. The milled powders were sintered at 1,350 °C for 4 h to yield ceramics. For these ceramics, increasing Sn content from x = 0.025–0.065 produces a decrease in (1) unipolar strain level s from 0.084 to 0.027 %, and (2) electromechanical coupling factor k _p from 33.6 to 19.3 %. However, the bulk density, room temperature dielectric constant and piezoelectric charge constant d ₃₃ exhibit an increase from 5.03–5.84 g/cm³, 1,342–2,156 and 7–110 pC/N, respectively, with increasing Sn content. The increasing trend of density and d ₃₃ presently observed is in sharp contrast to the result of corresponding ceramics prepared from 86 nm nanopowders. The present study reveals a cooperative mechanism involving both the nanoscale size of the starting particles and optimum tin content which results in the enhancement of d ₃₃ with tin content.     

Effect of cerium addition on phase transformation and microstructure of cordierite ceramics prepared by sol-gel method

Low-temperature sintering of cordierite ceramic depends on the phase transformation into cordierite and the properties depend on its microstructure. In the present work, the effect of cerium on the phase transformation and microstructure of cordierite ceramics prepared by sol-gel method is studied by X-ray diffraction (XRD), differential thermal analysis (DTA) and scanning electron microscopy (SEM) in order to lower the sintering temperature and improve the properties of cordierite ceramic with the addition of cerium. It is observed that the cerium addition obviously lowers the crystallization temperature of -cordierite while slightly raises that of -cordierite. The lowest temperature for cordierite transformation, which approaches the crystallization temperature of -cordierite, is achieved in the sample containing 4 wt% of cerium, implying a possibility to lower the sintering temperature of cordierite ceramics. The Ce-contained ceramics show a biphasic microstructure that is dependent on sintering temperature. Sintered below 1300°C, a cordierite-CeO₂ microstructure is present; while sintered at the temperature above 1300°C, appears a cordierite-glass microstructure, of which the amount of glass phase is limited to a small extent. Since the addition of 4 wt% cerium to this MgO-Al₂O₃-SiO₂ system substantially enhances the densification of cordierite ceramics and lowers the sintering temperature to the level of around 1000°C, it makes the ceramics suitable for such applications, where the low-temperature sintering is required, as the substrates for electronic circuit and the catalytic supports (with oxygen storage capacity) for cleaning of automotive exhaust emissions.