Vasantha R. W. Amarakoon

Electrical properties of ultrafine-grained yttria-stabilized zirconia ceramics

Nanoparticles of yttria-doped tetragonal zirconia polycrystalline ceramics (Y-TZP) with an average crystallite size of less than 9 nm were prepared by a combustion synthesis process. Dense and fine-grained ({lt}200 nm) Y-TZP ceramics were obtained by fast-firing using temperatures lower than 1400{degree}C and dwell times of less than 2 min. Impedance spectroscopy was employed to measure conductivities of oxygen vacancies in the grain and the grain boundary of the fine-grained Y-TZP. The relationships between the concentration of the oxygen vacancies in the grain boundary and measurable physical parameters were determined semiquantitatively. The oxygen vacancy concentrations and activation energies for the oxygen-ion conduction in the grain and the grain boundary of the fine-grained Y-TZP were found to be independent of the average grain size in the average grain-size range of 90{endash}200 nm. These experimental results suggest that, in order to retain the abnormally high oxygen vacancy concentrations of the Y-TZP nanoparticles and thus enhance the oxygen-ion conductivity, it may be necessary to decrease the average grain size to approximately 10 nm.{copyright} {ital 1997 Materials Research Society.}

Sol-gel coating of YBa2Cu3O7− x with TiO2 for enhanced anisotropic grain growth

This investigation focused on the effects of TiO 2 additions (2.5–5 mol%) incorporated via sol-gel coating of powders on sintering behavior, microstructure development, electrical resistivity, and magnetic susceptibility of YBa 2 Cu 3 O 7−x . Specimens were sintered at ≤ 960 °C for 6 h in all cases. TEM analysis indicated the sol-gel coating to be uniformly distributed around each particle and of thickness ∼ 20 to 40 nm. The addition of TiO 2 was seen to reduce the sintering temperature by up to ∼ 200 °C, indicating the formation of a liquidus phase below that normally reported for YBa 2 Cu 3 O 7-x . Grain growth and grain anisotropy behavior were influenced by TiO 2 addition. A maximum in density, grain size, and anisotropy was achieved with 2.5 mol% addition sintered at 930 °C. TiO 2 addition was shown to result in the formation of greater amounts of secondary phases such as Y 2 BaCuO 2 , and BaCuO 2 , particularly at higher TiO 2 levels and sintering temperatures. The use of TiO 2 additions also altered the magnetic susceptibility with the optimum response occurring for samples with 2.5 mol% TiO 2 addition when sintered at 930 °C.

Homogeneous grain growth and fast-firing of chemically modified nanocrystalline MnZn ferrites

Abstract A chemical method utilizing sol-gel reactions was investigated to uniformly incorporate small amounts of additives of Si and Ca into nanocrystalline Mn 0.6 Zn 0.4 Fe 2 O 4 powders (≈14 nm particle size). Sintering behavior of the chemically modified nanocrystalline MnZn ferrites was studied with regard to the effects of the chemical additives and sintering conditions on densification and grain growth. The nanopowder samples without the additives exhibited abnormal grain growth regardless of heating rate after pressing at a relatively low pressure and sintering at 1200°C, while the chemical addition of 2 wt% SiO 2 and 0.5 wt% CaO was found to improve microstructural characteristics, i.e., homogeneous grain growth and less porosity. Fast-firing did not seem to favorably affect densification of the chemically modified MnZn ferrite nanopowder, but was found to contribute to the suppression of grain growth in the nanocrystalline ferrite compacts.

SYNTHESIS OF YTTRIA-STABILIZED ZIRCONIA NANOPARTICLES BY DECOMPOSITION OF METAL NITRATES COATED ON CARBON POWDER

Weakly agglomerated nanoparticles of yttria-stabilized zirconia (YSZ) were synthesized by a novel process which involved the decomposition of metal nitrates that had been coated on ultrafine carbon black powder, after which the carbon black was gasified. The use of ultrafine, high-surface-area carbon black powder apparently allowed the nanocrystalline oxide particles to form and remain separate from each other, after which the carbon black was gasified at a somewhat higher temperature. As a result, the degree of agglomeration was shown to be relatively low. The average crystallite size and the specific surface area of the as-synthesized YSZ nanoparticles were 5{approximately}6 nm and 130 m{sup 2}/g, respectively, for powder synthesized at 650{degree}C. The as-synthesized YSZ nanoparticles had a light brown color and were translucent, which differs distinctly from conventional YSZ particles which are typically white and opaque. The mechanism of the synthesis process was investigated, and indicated that the gasification temperature had a direct effect on the crystallite size of the as-synthesized YSZ nanoparticles. High-density and ultrafine-grained YSZ ceramic articles were prepared by fast-firing, using a dwell temperature of 1250{degree}C and a dwell time of two minutes or less. {copyright} {ital 1996 Materials Research Society.}

Crystallization and microstructural evolution of cordierite-based thick film dielectrics

Abstract Microstructural evolution of glass–ceramic dielectric thick films based on a nonstoichiometric cordierite, 2.4MgO·2Al 2 O 3 ·5SiO 2 , containing B 2 O 3 , P 2 O 5 , and PbO were investigated in conjunction with nucleation and crystal growth. The cordierite thick films were deposited by screen printing on a 96% alumina substrate, and then fired in the temperature range of 850–950°C in a N 2 atmosphere. Surface microstructure characteristics of the thick films depended on PbO content. Heterogeneous nucleation originated predominantly from the interface between the densified thick film and the alumina substrate. The added PbO segregated to the remaining glass during crystallization, and the relative concentration of Pb in the glass was proven to increase with rising temperature. Most of the remaining glass protruded from the film surface at the final temperature of 950°C. Penetration of the alumina substrate by the remaining glass was also observed. The influences of temperature and PbO on the microstructural evolution including nucleation and growth will be discussed with possible explanations for the observed results.

Aqueous powder coating methods for preparation of grain boundary engineered ceramics

Abstract Coated powders provide a mechanism for uniform incorporation of inorganic dopants into ceramics, resulting in improved control of microstructures, grain boundary phases and properties. Incorporation of inorganic dopants into polymeric additive systems is a novel method of coating powders in aqueous suspension. Controlled adsorption of polymer–cation complexes can be used to coat powder surfaces in a suspension, which can be subsequently spray-dried without dopant segregation. This method streamlines the coating process, making it economical for large scale manufacturing. Two alumina formulations with controlled grain boundary chemistry were used as model systems to develop these methods. Alumina with 2.5% magnesium aluminosilicate glass grain boundary phase was prepared and sintered. The conventional mixed powder method was compared with sol–gel coated powder prepared in an alcohol–water solvent, and powder spray-dried from a suspension containing silicone emulsion with added magnesium stearate. The sol–gel coated powder densified at a temperature 100°C below that required for the conventional mixed oxide powder system. Poor mixedness from the emulsion method hindered microstructure development. High purity alumina with Mg-rich grain boundaries was prepared by coating particles with a Mg–polyacrylic acid chelate adsorbed from aqueous solution. Uniform distribution of magnesium at grain boundaries resulted in microstructure with uniform grain size for MgO additions as low as 100 ppm, while 400 ppm MgO was required to obtain similar microstructure using the mixed powder method.

THE EFFECT OF DIELECTRIC PROPERTIES OF SINTERING ADDITIVES ON MICROWAVE SINTERED SILICON NITRIDE CERAMICS

Silicon nitride requires the use of susceptive additives for microwave liquid phase sintering due to the material’s low dielectric loss. In this article, we report the effect of complex dielectric properties of two compositions of sintering aids on 2.45 GHz microwave sintered Si3N4 with respect to power absorption, temperature distribution and densification behavior. The temperature dependent dielectric properties were measured from 25°C to 1400°C using a conventional cavity perturbation technique. Finite Difference Time Domain (FDTD) electromagnetic simulations coupled with a thermal solver was used to predict the microwave power absorption and the corresponding temperature evolution inside the samples. The additive with higher dielectric loss (4 wt% MgO, 6 wt% Y2O3 and 2.5 wt% ZrO2) produces a greater sintered density than the lower loss additive (4 wt% MgO and 6 wt% Y2O3) or pure Si3N4. Although microwave loss at temperatures below 600°C is insignificant with or without the additives, the loss begins to increase at higher temperatures when the additives are present and has a strong upward trend above 1000°C. Above 1200°C the sample containing ZrO2 exhibited the greatest loss. Numerical simulations at the peak sintering temperature show greater microwave power absorption and higher temperature in the sample with the highest loss additive. The simulation results correlate to the difference in densification behavior observed. The simulation was also useful because the material temperature was not accurately provided by optical pyrometer measurements of the crucible sample holder.

Improved magnetic properties and growth anisotropy of chemically modified Sr ferrites

Magnetic properties and microstructural characteristics of SrO⋅5.9Fe2O3 chemically modified with Si and Ca were investigated by changing experimental parameters such as additive composition, the ratio of Ca/Si, and sintering condition. A novel particulate coating method utilizing sol–gel reactions was used to uniformly incorporate the additives of Si and Ca. This method was very successful in obtaining homogeneous grain growth and fine grains. A sample containing the gel additives of 0.6 wt % SiO2 and 0.7 wt % CaO and sintered at 1200 °C for 4 h was found to significantly suppress abnormal grain growth, resulting in submicron-sized grains and high density. A distinct grain boundary phase containing Si and Ca was observed by increasing the sintering temperature to 1250 °C. The resultant microstructural characteristics favorably affected magnetic properties. For example, the chemically modified sample exhibited a higher coercivity of 3530 Oe compared to a value of 2050 Oe obtained for the sample without the...

Grain boundaries and growth kinetics of polycrystalline ferrimagnetic oxides with chemical additives

Grain boundaries and grain growth kinetics of two ferrimagnetic oxides, Y2Gd1Fe5O12 and Li0.3Zn0.4Fe2.3O4, modified by a sol–gel particulate coating process were investigated on the basis of their microstructural characteristics. Interestingly, the addition of small amounts of MnO2 and SiO2 using the sol–gel coating process led to different results for the two magnetic materials. In the case of Y2Gd1Fe5O12, the additives had a role as a grain growth inhibitor because Si-rich precipitates were segregated along the grain boundary and exerted a drag force against grain boundary movement. On the other hand, the same additives acted as an accelerator for grain growth by forming a glassy phase at the grain boundaries for Li0.3Zn0.4Fe2.3O4. The results were analyzed quantitatively by calculating activation energies for grain growth and grain boundary mobilities. For example, the calculated grain boundary mobilities, i.e., 5.6×10−15 m3/N s at 1400 °C for Y2Gd1Fe5O12 and 22.3×10−15 m3/N s at 1150 °C for Li0.3Zn0.4...

The effects of impurity oxides on YBa2Cu3O7−x microstructure development

Abstract Diffusion-couple experiments in conjunction with scanning electron (SEM) and energy dispersive X-ray analysis were used to determine the effect of various oxides on the microstructural development of YBa 2 Cu 3 O 7− x ceramic superconductors. The interaction of a number of impurity oxides including SiO 2 , Bi 2 O 3 , Pr 6 O 11 , TiO 2 , SnO 2 , In 2 O 3 and Sb 2 O 3 resulted in exaggerated grain growth and domain formation in bulk specimens through a liquid-phase wetting mechanism. Reaction sequencing was determined by thermal analysis (DTA) and real-time dynamic X-ray diffraction. The ability and extent of domain formation for samples coupled with SiO 2 , Bi 2 O 3 , Pr 6 O 11 and TiO 2 was determined to be a function of oxygen partial pressure during sintering as well as of the sintering temperature in the range 940°C–980°C. The use of Bi 2 O 3 coupled to pre-oriented samples (i.e. hot forged) resulted in extensive domain growth at 980°C. The presence of SiO 2 in conjunction with hot forging resulted in a lower magnetization response, due to the presence of larger 211 particles and or extensive cracking during sintering.