Microwave sintered Si3N4-MgO system that contains 2, 4 and 10 wt% of ZrO2 as secondary particulates were investigated with respect to phase transformation and microstructure development. The experimental results of microwave sintered samples were compared with conventional methods. Complete α to β phase transformation was observed in the case of microwave sintered samples due to the volumetric nature of microwave heating. High temperature X-ray diffraction (HTXRD) analysis was performed to study in-situ the oxidation behavior of Si3N4 specimens. Si3N4 specimens with 10 wt % ZrO2 were exposed to air at temperature between 25°C and 900°C for up to 24 hours. Microwave sintered sample were structurally stable in air 25°C and 900°C for up to 24 hours of testing.

Tin oxide powders with and without Sb as a dopant were prepared by coprecipitation from aqueous chloride solutions, A new technique called sol-gel coating was utilized to adjust the surface chemistry of Sb-doped SnO2 powders. Powders coated with a TiO2-SiO2-Na2O sol-gel composition sintered to almost theoretical density and very low resistivity when fired at 1350°Cfor 2 h. Thick films processed using coated Sb-doped SnO2 powders were very sensitive to ethyl alcohol, decreasing the resistance at room temperature by 900 ohms.

Barium hexaferrite is a well-known ceramic permanent magnet and due to its high coercivity, remanence, and large uniaxial magnetic anisotropy, finds applications that compete with metallic magnets. Even though a number of the high-temperature properties of barium hexaferrite have been studied extensively, its anisotropic thermal expansion has not been reported so far. Dynamic high-temperature x-ray diffraction (HT-XRD) is one powerful method to obtain thermal expansion data for anisotropic polycrystalline materials in a very short period of time. In this paper the anisotropic nature of the thermal expansion coefficient of the barium hexaferrite phase is reported with the use of a dynamic HT-XRD setup. The thermal expansion coefficient (linear fit) was determined to be 8.36 × 10^−6 K^−1 along the a – b plane to 1.4 × 10^−5 K^−1 along the c axis between the temperature range of 293 to 1343 K.

Dense, nanocrystalline ceramic articles of doped ZrO{sub 2} (for use in solid electrolytes, oxygen sensors, electrode materials, thermal barrier coatings, etc.), BaTiO{sub 3} (for capacitor applications), and YBa{sub 2}Cu{sub 3}O{sub 7-x} (a high-temperature superconductor with uses, e.g., in magnetic flux trapping and high-speed capacitor applications) were prepared by the new nanofabrication process that has been developed in this research program. The process consists of two steps: synthesis of ceramic nanoparticles, and fabrication of dense ceramic articles that possess nanocrystalline features. The synthesis step is capable of producing 10-nanometer-diameter crystallites of doped ZrO{sub 2}, and of being scaled up to kilogram/hour production rates. The fabrication step produced dense, ultrafine-grained articles at significantly reduced sintering temperatures and times--representing a factor of 10-100 reduction in process energy requirements. The process has thus been shown to be technically feasible, while a preliminary engineering cost analysis of a pilot plant-scale version of the process indicates that it is both a cost- and an energy-efficient method of producing nanoparticles and nanocrystalline ceramics from those nanoparticles. One U.S. patent for this process has been allowed, and an additional five (continuation-in-part) applications have been filed. Technology transfer efforts have begun, through ongoing discussions with representatives from three manufacturing concerns.

Nanocrystalline copper ferrite (Cu0.5Fe2.5O4) was synthesized using a forward strike gelation method with polyacrylic acid (PAA) as a gelating agent. The dried gel was calcined at a low temperature of 400 °C to get the final powder. The effect of pH and the ratio of the cation to the carboxylic group in the initial gel were studied with respect to both the phases and the crystallite size of the final powders synthesized. Phase and crystallite size analysis were done using x-ray diffraction and TEM. Saturation magnetization results were obtained using a SQUID magnetometer. The reactions occurring in the nano-size copper ferrite, in air as a function of temperature, were tracked using adynamic high temperature x-ray diffraction (HTXRD) system.

Results are presented concerning the formation kinetics of Y/sub 2/BaCuO/sub 5/ (211) and YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (123) phases during melt processing of melt quench (MQ) precursors and phase pure 123. The influence of Pt and 211 additions on microstructural development within YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// system are examined quantitatively via XRD analysis and DTA. Real time dynamic XRD analysis showed the phase reaction sequences for the MQ and phase pure 123 (SSC) precursor systems, and revealed the activation energy (E/sub a/) for 211 formation to be lower for MQ powders upon peritectic decomposition. The SSC precursors developed greater amounts of 211 at 1100/spl deg/C when compared to MQ precursors, (/spl sim/28 wgt% vs. 10 wgt% respectively). The addition of 0.5 m/o 211 additions in SSC resulted in lower 123 temperatures and E/sub a/ than that without 211 addition. Pt additions did alter the residual 211 content upon texturing. The presence of Pt with excess 211 additions was seen to inhibit 211 formation upon texturing.<>

Grain growth and texturing of YBa2Cu3O7−δ is greatly influenced by the presence of liquid phase additives during sintering. Oxides such as TiO2, SiO2, Bi2O3, and Pr6O11 were incorporated into the liquid phase during the sintering of YBa2Cu3O7−δ (123) by use of grain boundary diffusion couples and the microstructure was analyzed using scanning electron microscopy/electron dispersive spectroscopy. Exaggerated grain growth and domain formation was observed in bulk specimens. Differential thermal analysis and real time dynamic x-ray diffraction were used to determine reaction sequencing. The ability and extent of domain formation was determined for 123 samples coupled with impurity oxides to be a function of sintering temperature (940–980°) and oxygen partial pressure. Enhanced texturing was observed at low PO2 atmospheres. The addition of Bi2O3 and TiO2 was shown to degrade dc magnetic susceptibility of 123 whereas SiO2 and Pr6O11 enhanced it. The domain formation and texturing takes place in the bulk for 123 at temperatures of 980°C or below (i.e. well below the peritectic decomposition temperature) by the interaction of an impurity doped liquid phase followed by a precipitation and exaggerated grain growth.

Fully dense, nanocrystalline ceramic articles were prepared by the new nanofabrication process developed in this research program. The process consists of two steps: synthesis of ceramic nanoparticles and fabrication of dense, nanocrystalline ceramic parts. The synthesis step produced 10-nanometer-diameter crystallites and is capable of being scaled up to kilogam/hour production rates. The fabrication step produced dense articles at significantly reduced sintering temperatures and times-representing a factor of 10-100 reduction in process energy requirements. The process was demonstrated by producing ultrafine-gained yttria-doped ZrO{sub 2}, an important material with a variety of energy-related applications (e.g., solid electrolytes, oxygen sensors, electrode materials, thermal barrier coatings, etc.); BaTiO{sub 3} (for capacitor applications); and YBa{sub 2}Cu{sub 3}O{sub 7-x}(a high-temperature superconductor with uses, e.g., in magnetic flux trapping and high-speed capacitor applications). Results from this reporting period (September 1993--May 1994) clearly illustrate the capabilities of this energy-efficient and directly commercializable process for producing dense, nanocrystalline, multicomponent oxide ceramics.

The high temperature (1100°C) coarsening of Y2BaCuO5 (211) and subsequent YBa2Cu3O7−δ (123) growth kinetics using melt quenched and 123 precursor powders were examined via quenching. Fine scale excess yttrium addition by sol gel coating was employed up to 20 mol percent. X-ray diffraction identified 211 fraction between 123 (∼30 wt.%) and melt quenched (∼10 wt.%) in the precursor powders. The addition of yttrium was seen to shift the 211 weight fraction to higher levels for the MQ powders. Refinement of the 211 particle size was seen in the presence of Pt but not with yttrium addition. The coarsening behavior of 211 in either powder did not appear to significantly change with excess yttrium addition at 1100°C. Differential thermal analysis showed that the 123 phase solidification temperature increased in the presence of Pt and reduced with yttrium addition up to 10 mol %. Dilatometric measurements showed the influence of yttrium addition on the densification behavior of 123 due to the presence of low temperature liquid phase formation. Directly inserting samples at soak teperatures were seen to significantly alter the 211 weight fraction for 123 powder in contrast to slower thermal heating to temperature. However, this effect was not seen in the case of the melt quenched precursor powder.

Fully dense, nanocrystalline ceramic articles were prepared by the new nanofabrication process. The process consists of two steps: synthesis of ceramic nanoparticles and fabrication of dense, nanocrystalline ceramic parts. The synthesis step produced 10-nanometer-diameter crystallites, and is capable of being scaled up to kilogram/hour production rates. The fabrication step produced dense parts at significantly reduced sintering temperatures and times -- representing a factor of 10--100 reduction in process energy requirements. The process was demonstrated by producing ultrafine-grained yttria-stabilized ZrO{sub 2}, an important material with a variety of energy-related applications (solid electrolytes, oxygen sensors, electrode materials, thermal barrier coatings, etc.). Results from this period clearly illustrate the capabilities of this energy-efficient and directly commercializable process for producing dense, nanocrystalline, multicomponent oxide ceramics.

This investigation focused on the effects of TiO2 additions (2.5–5 mol%) incorporated via sol-gel coating of powders on sintering behavior, microstructure development, electrical resistivity, and magnetic susceptibility of YBa2Cu3O7−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 TiO2 was seen to reduce the sintering temperature by up to ∼ 200 °C, indicating the formation of a liquidus phase below that normally reported for YBa2Cu3O7-x. Grain growth and grain anisotropy behavior were influenced by TiO2 addition. A maximum in density, grain size, and anisotropy was achieved with 2.5 mol% addition sintered at 930 °C. TiO2 addition was shown to result in the formation of greater amounts of secondary phases such as Y2BaCuO2, and BaCuO2, particularly at higher TiO2 levels and sintering temperatures. The use of TiO2 additions also altered the magnetic susceptibility with the optimum response occurring for samples with 2.5 mol% TiO2 addition when sintered at 930 °C.

AbstractA new technique, sol‐gel coating of powders, is used to control microstructure and properties of grain‐boundary‐controlled electronic ceramics such as Sb‐doped SnO2 sensor material and positive temp. coefficient of resistance (PTCR) BaTiO3.