Evidence for a non-thermal microwave effect in the sintering of partially stabilized zirconia

A hybrid furnace, which allows the simultaneous application of microwave and radiant energy, has been used to investigate the sintering of partially stabilized zirconia (doped with 3 mol % yttria). Microwave-enhanced sintering is clearly demonstrated with densification occurring at a lower temperature when a high-frequency electric field is applied. By considering the variation of electric field strength with furnace temperature, this enhancement is shown to be non-thermal in nature, being dependent on the electric field strength and not the power density (heating) of the microwaves. This dependence on electric field is consistent with an additional driving force term in the equation which describes the diffusion of vacancies through the material during sintering.     

Sintering and grain growth of 3 mol% yttria zirconia in a microwave field

A comparative study of the sintering and grain growth of 3 mol% yttria zirconia using conventional and microwave heating was performed. Extensive measurements of grain size were performed at various stages of densification, and following isothermal ageing at 1500 °C for 1, 5, 10 and 15 h. Microwave heating was found to enhance densification processes during constant rate heating. The grain size/density relationship for the microwave-sintered samples was shifted in the direction of increased density for density values less than 96% of the theoretical value when compared to conventionally heated samples. This suggests that there may be a difference in the predominant diffusion mechanisms operating during the initial and intermediate stages of sintering. Results of the ageing experiments showed that once densification was near completion, grain growth was accelerated in the microwave field, and exaggerated grain growth occurred.     

Electrical Properties and Grain Growth Kinetics of PZN-based Ceramics Using Microwave Sintering

Effectiveness of microwave sintering process through investigation of microstructural characteristics and electricalrproperties of x（0.94PbZn1/3Nb2/3O3 ＋ 0.06BaTiO3 ） ＋ （1 - x）PbZryTi1-xO3 （PBZNZT） ceramics with x = 0.6 and y = 0.52 was evaluated. The relative density of 95% was achieved with sintering at 800℃for 2 h. The small grain growth exponents indicate how easy the grain growth in these materials sintered using microwave radiation. Grain growth rate increases abruptly and is higher than that of conventional sintering at a temperature higher than 1050℃. This is attributed to the lower activation energy and higher grain boundary mobility. The activation energy required for the grain growth is found to be 132kJ/mol. Higher remanent polarization （Pr = 50. ltLC/cm2） and increase in remanent polarization with sintering temperature are observed in microwave sintering process when compared to that of conventional sintering process, due to fast increase in grain growth rate and homogeneity in the specimen. The results indicate lower sintering energy and reduction of PbO pollution in the working environment by microwave sintering process.     

Fabrication of hydrothermal ageing resistant of 3 mol % yttria-stabilised tetragonal zirconia polycrystalline via microwave sintering

The influence of microwave sintering on the densification, mechanical performances, microstructure evolution and hydrothermal ageing behaviour of pure 3 mol % yttria-stabilised tetragonal zirconia polycrystalline (3Y-TZP) ceramics was compared with conventional sintered samples. Green bodies were sintered via conventional pressure-less and microwave sintering method between 1200 °C to 1400 °C with dwelling time and firing rate at 120 min, 10 °C/min and 1 min, 20 °C/min. Result showed that reduced processing temperature and holding time is possible with microwave sintering technique for fabricating good resistant zirconia sample with bulk density, Young's modulus, and Vicker's hardness that are comparable to samples sintered with conventional method. However, the microwave sintered samples suffered from hydrothermal ageing where their average grain size is above critical size. The enhancement of hydrothermal ageing resistance of the sintered samples is associated with the decreasing grain size of the sintered samples instead of sintering method.     

Phase evolution and sintering kinetics of hydroxyapatite synthesized by solution combustion technique

Solution combustion technique has been used to prepare hydroxypatite (HAp) powder from calcium nitrate, di-ammonium hydrogen phosphate and citric acid precursors. Phase evolution has been studied as a function of calcination temperatures. The crystal structure, phase purity and stiochiometry of phase have been studied by Rietveld analysis of the calcined powder. It was observed that the prepared Hap powder was phase pure and stoichiometric. The sintering behaviour and sintering kinetics of the HAp compact has been studied by dilatometer. Activation energy for sintering has been calculated from the dilatometer results. Grain boundary diffusion was found to be the dominant densification mechanism during the initial stage of sintering. The activation energy for sintering (438 kJ/mol) was found to be in excellent agreement with reported value.     

Effect of composition on the enhanced microwave sintering of alumina-based ceramic composites

A series of comparative experiments were performed in which a number of alumina-zirconia compositions were sintered in both microwave and conventional furnaces, using identical heating profiles. Measurement of sample end-point densities showed an enhancement of the sintering process associated with the use of microwave heating for all compositions studied. The associated microstructures examined using scanning electron microscopy showed slightly larger grain sizes for the microwave-sintered compacts, as would be expected from their higher densities. The design of a high-temperature sintering dilatometer has allowed continuousin situ monitoring of the densification process in both the microwave and conventional environment. Data obtained in this way have shown that there is an effect of composition on microwave densification. This appears to be related to the increased lossiness of the composite, (increased zirconia content), rather than the effect of zirconia as a sintering aid. In addition the dilatometer results suggest that the microwave enhancement of the sintering process may be due to a reduction in the activation energy for grain-boundary diffusion.Electricity Association Technology Ltd. Registered Office: 30 Millbank, London SW1P 4RD.     

微波烧结氧化锌压敏电阻的致密化和晶粒生长

研究了微波烧结的ZnO压敏电阻的致密化和生长动力学, 微波烧结温度从900～1200℃, 保温时间从20min～2h. 研究表明, 微波烧结ZnO压敏电阻的物相组成和传统烧结的样品没有区别; 微波烧结有助于样品的致密化, 并降低致密化温度. 随着烧结温度的升高, 致密化和反致密化作用共同影响样品的密度, 其中Bi的挥发是主要影响因素. 微波烧结ZnO压敏电阻的晶粒生长动力学指数为2.9～3.4, 生长激活能为225kJ/mol, 传统烧结的ZnO压敏电阻的晶粒生长动力学指数为3.6～4.2, 生长激活能为363kJ/mol. 液相Bi₂O₃、尖晶石相和微波的“非热效应”是影响微波烧结ZnO压敏电阻陶瓷晶粒生长的主要因素.     

Pressureless sintering curve and sintering activation energy of Fe–Co–Cu pre-alloyed powders

The kinetic characteristics of Fe–Co–Cu pre-alloyed powders in the pressureless sintering process have been investigated. The expansion ratio, linear shrinkage, densification rate and effect of heating rate on the sintering have been analyzed. Based on the classical Arrhenius curve, the sintering activation energy has been calculated. Results show that the samples have a smaller expansion ratio before contracting when the Fe content is higher, and the final linear shrinkage ratio is larger too. The sintering carries out more efficiently and the final linear shrinkage ratio is larger when the samples at a lower heating rate. In the initial and final stage of sintering, the Arrhenius curve is suitable for the Fe–Co–Cu pre-alloyed powders and diffusion is the main transport mechanism. At the initial stage of sintering the sintering activation energy of Fe25%–Co15%–Cu60% powder is 453.11 kJ/mol, Fe45%–Co15%–Cu40% powder is 638.28 kJ/mol and Fe65%–Co15%–Cu20% powder is 504.6 kJ/mol, respectively. At the final stage of sintering the sintering activation energy of Fe25%–Co15%–Cu60% powder is 31.17 kJ/mol, Fe45%–Co15%–Cu40% powder is 20.09 kJ/mol and Fe65%–Co15%–Cu20% powder is 35.13 kJ/mol, respectively. The sintering activation energy in the middle stage is dominated by not only one diffusion mechanism so it is not suitable for the Arrhenius curve.     

利用控制烧结曲线及其拓展预测ZrO2陶瓷烧结过程

将钇稳定氧化锆(3Y-PSZ)冷等静压素坯在原位测量仪中进行恒速无压烧结, 升温速率分别为2、5、8℃/min, 通过原位测量仪保存图像, 并用软件Image-Pro Plus 6.0对图像进行处理, 得到收缩数据, 建立氧化锆的控制烧结曲线(MSC), 成功计算出其烧结活化能Q为685.7 kJ/mol。并对控制烧结曲线进行扩展(EMSCE), 模拟出恒定加热速率下整个烧结过程中温度与相对密度的关系, 而不仅仅是预测最终密度。该研究提供了一种预测材料烧结制度的可能性, 确保了所选烧结时间和烧结温度的高精确度和可重复性。     

Microwave induced plasma processing of nuclear waste calcines

Microwave induced plasma processing was used to sinter synthetic Idaho Chemical Processing Plant (ICPP) alumina and zirconia based high level nuclear waste calcines in a nitrogen atmosphere. The microwave densification behaviour of these nuclear waste calcines was observed parallel with identification of the phases formed after sintering. Sintered densities of > 3.20 g cm^–3 were obtained within 10 min of microwave sintering of pure calcines. Glass frit containing calcines showed lower sintering densities (< 2.0 g cm^–3) due to reactions between the frit and volatile substances in both zirconia based and alumina based calcines; prior removal of frit volatiles increased the sintered density. Phases formed in the microwave sintered calcines were identified by X-ray diffraction.     

Effect of sintering temperature and holding time on the properties of 3Y-ZrO2 microfiltration membranes

Sintering behavior of supported and unsupported microfiltration membranes prepared from 3 mol% yttria doped zirconia powder was investigated as a function of temperature and holding time in non-isothermal and isothermal densification. Shrinkage that started at 1000°C showed the highest rate between 1200°C and 1300°C although the rate decreased above 1300°C. The activation energy of sintering was calculated at 735 kJ/mol, assuming the grain boundary diffusion mechanism for mass transport. Mean pore size decreased in unsupported membranes and increased in supported ones as the sintering temperature increased up to 1200°C. Dimensional shrinkage of unsupported membrane slabs showed an increase in shrinkage first in the lateral dimension and then in the thickness as the sintering temperature increased. Pore growth and lower hardness in supported membranes, can be explained due to the restricted lateral shrinkage in the supported membranes. Removal of porosity was pronounced above 1100°C and the density increased linearly as a function of holding time. Microhardness of membranes sintered above 1100°C increased as a function of sintering temperature and was higher in unsupported membranes. Samples sintered above 975°C had a100% tetragonal phase structure. Permeability of supported membranes increased as a function of sintering temperature due to pore growth despite a decrease in porosity.     

Enhancing densification of zirconia-containing ceramic-matrix composites by microwave processing

Various ceramic-matrix composites containing zirconia were sintered using a 2.45 GHz microwave field. The effects of the addition of zirconia and the processing parameters on the sintering and microstructure development were investigated. The results showed that microwave processing enhanced the densification of these composites considerably. The enhancement in sintered density was up to 46% over conventional sintering, depending on the systems. This revised version was published online in November 2006 with corrections to the Cover Date.     

Mechanically alloyed Ni/8YSZ powder mixtures: preparation, powder characterization and sintering behavior

A state of the art anode for the solid oxide fuel cell (SOFC)consists of a mixture of 8 mol % Y2O3-stabilized zirconia (8YSZ) andnickel particles, which form an interconnected porous structure after sintering. Coarsening of the Ni particles under SOFC workingconditions has to be avoided, hence it leads to a deterioration of the anode's performance. In the present work the aim was to improve thestability of the Ni particles by a reduction of the sinteringactivity of nickel. For this purpose between 10 and 50 % by volume of nano-sized zirconia particleshave been dispersed in the nickel matrix by dry ball milling in a planetary mill.Forpressed samples made of mechanically alloyed Ni with 10 vol % of 8YSZ,a homogeneous dispersion of 8YSZ particles in the Ni matrix wasconfirmed by transmission electron microscopy. It was confirmed bymercury porosity penetration and optical microscopy that thisdispersoid structure leads to a retardation of recrystallization.Also, an essentially lowered densification during sintering was found, compared to samples made of the pure Ni powder. Samples made of mechanically alloyed Ni with a higher zirconia amountshowed a higher densification during sintering and annealing thansamples containing 10 vol % 8YSZ. It is assumed that this results frominsufficient dispersion in these systems. The results show that mechanically alloyed nickel,with a homogeneous dispersion of 8YSZ crystallites, is a promising candidate for high temperature catalysts.     

Sintering mechanism of fine zirconia powders with alumina added by powder mixing and chemical processes

The present study examined the sintering behavior of fine zirconia powders (containing 2.9 mol% Y₂O₃) with and without small amounts of Al₂O₃ added by different ways: powder mixing (PM), homogeneous precipitation (HP), and hydrolysis of alkoxide (HA). The initial sintering behavior was examined by both measurement methods of a constant rate of heating and isothermal shrinkage. In the PM process, Al₂O₃ particles pinned the shrinkage of zirconia powder compact at the initial stage and diffuse toward zirconia surface to enhance the sintering. This initial sintering mechanism was explained by the grain-boundary diffusion for the Al₂O₃-free powder and the volume diffusion for Al₂O₃-added powder. When Al₂O₃ was added to zirconia powder by HP and HA processes, the densification rate was more stimulated compared to the PM process. The initial sintering mechanism did not change by the way for Al₂O₃ addition. The Al₂O₃ addition by chemical processes of HP and HA tended to enhance the grain growth of zirconia, while the uniform microstructure was achieved because of homogeneous addition of Al₂O₃ by those processes.     

Reaction sintering: correlation between densification and reaction

By studying simultaneous densification kinetics and reaction kinetics during the sintering and hot pressing of alumina-zircon mixtures, it has been possible to distinguish between the particle rearrangement and particle reshaping (diffusive) stages of the densification process. Particle rearrangement is found to be more significant during hot pressing (20 MN m^–2) and is the dominant mechanism at relative densities up to 72%. In pressureless sintering, diffusive processes become dominant above relative densities of about 62%.     

Densification and α-β transformation mechanisms during hot pressing of Si3N4 — Y2O3 — SiO2 composition

Densification of Si₃N₄-Y₂O₃ — SiO₂ mixtures under hot pressing occurs by a complex multicomponent mechanism including as main material transport processes particle rearrangement by rheologic flow and solution/diffusion/reprecipitation. Grain boundary sliding, Ostwald ripening and dislocation flow are also suggested to take place, especially in the final stage of densification. Stress and time analyses of densification and shrinkage rate according to the known models for liquid phase sintering and stationary creep have been used to identify the possible rate-governing mechanisms at the various sintering stages. The apparent activation energy for densification was found to depend on the actual density of the material being hot pressed. The apparent activation energy for the α-β transformation (E_a = ?544 KJmol^?1 is similar to the apparent activation energy for densification in the intermediate stage where the solution/precipitation process is most active. Overall, the sintering behaviour is largely dominated by the flow properties of the liquid viscous grain boundary phase.     

超细/纳米粉末改进Ti(C,N)基金属陶瓷性能研究进展

综述了近几年超细或纳米粉末改进Ti(C,N)基金属陶瓷性能的方法,简要分析了含超细或纳米粉末Ti(C,N)基金属陶瓷的致密化问题.总结了真空烧结 热等静压处理和放电等离子烧结的特点,并分析了微波烧结和等离子活化烧结制备Ti(C,N)基金属陶瓷的可能性.     

Processing of alumina-based composites via conventional sintering and their characterization

The present work relates to the processing of dense alumina-based composites, their microstructural characterization and study of mechanical properties. Alumina ceramic material and alumina-based composites with m-Zirconia and Ceria addition are sintered at 1600°C, 1650°C and 1700°C temperatures via conventional sintering. Solid-state diffusion during sintering led to volume diffusion in alumina, and volume and grain boundary diffusion in alumina composite. In the present sintering conditions alumina is found to be the least dense as improper solid-state diffusion resulted in porosity, whereas alumina–zirconia composite achieved the highest density of 97%. Scanning electron microscope (SEM) micrograph shows homogeneous distribution of fine zirconia particles inside the alumina matrix, filling the voids of the alumina skeletal structure. Zirconia connects to alumina particles, restricting its abnormal grain growth. It results in strong bonding and grain refinement. Alumina–zirconia composite exhibits the highest hardness and fracture toughness of 14.37?GPa and 4.6?MPa?·?m^1/2 at 1700°C. Alumina suppresses the transformation of m-t zirconia, resulting in high toughness of alumina composites. Alumina–zirconia–ceria composite revealed the presence of porosity, which led to less densification and low mechanical properties.     

Experimental and Numerical Studies of Densification and Grain Growth of 8YSZ during Flash Sintering

As a promising sintering technique, flash sintering utilizes high electric fields to achieve rapid densification at low furnace temperatures. Various factors can influence the densification rate during flash sintering, such as ultrahigh heating rates, extra-high sample temperatures, and electric field. However, the determining factor of the densification rate and the key mechanism during densification are still under debate. Herein, the densification and grain growth kinetic during flash sintering of 8 mol% Y₂O₃-stabilized ZrO₂ (8YSZ) is studied experimentally and numerically using finite element method (FEM). The roles of Joule heating and heating rate on the densification are investigated by comparing flash sintering with conventional sintering. An apparently smaller activation energy for the material transport resulting in densification is obtained by flash sintering ($Q_{\mathrm{d}}$ =424 kJ mol⁻¹) compared to the conventional sintering ($Q_{\mathrm{d}}$ = 691 kJ mol⁻¹). In addition, a constitutive model is implemented to study both the densification and the grain growth during flash and conventional sintering. Furthermore, the effect of electrical polarity on the density and the grain size evolution during flash sintering of 8YSZ is also investigated. The simulation results of average density and grain size inhomogeneity agree well with the experimental data.     

Effect of starting powders on the sintering of nanostructured ZrO2 ceramics by colloidal processing

The effect of starting powders on the sintering of nanostructured tetragonal zirconia was evaluated. Suspensions were prepared with a concentration of 10 vol.% by mixing a bicomponent mixture of commercial powders (97 mol.% monoclinic zirconia with 3 mol.% yttria) and by dispersing commercially available tetragonal zirconia (3YTZ, Tosoh). The preparation of the slurry by bead-milling was optimized. Colloidal processing using 50 μm zirconia beads at 4000 rpm generated a fully deagglomerated suspension leading to the formation of high-density consolidated compacts (62% of the theoretical density (TD) for the bicomponent suspension). Optimum colloidal processing of the bicomponent suspension followed by the sintering of yttria and zirconia allowed us to obtain nanostructured tetragonal zirconia. Three different sintering techniques were investigated: normal sintering, two-step sintering and spark plasma sintering. The inhibition of grain growth in the bicomponent mixed powders in comparison with 3YTZ was demonstrated. The inhibition of the grain growth may have been caused by inter-diffusion of cations during the sintering.