The tensile creep behavior of two ceramic composite systems exhibiting duplex microstructures was studied relative to their single-phase constituents in the temperature and stress ranges of 1,100--1,350 C and 35--75 MPa. The equi-volumetric compositions in the Al[sub 2]O[sub 3]:c-ZrO[sub 2] (8 mol% Y[sub 2]O[sub 3]) and Al[sub 2]O[sub 3]:Y[sub 3]Al[sub 5]O[sub 12] systems both exhibit lower creep rates than either of their single-phase constituents. This effect is attributed to Y[sup 3+] (and possibly Zr[sup 4+]) present in the Al[sub 2]O[sub 3] as a segregant which lowers the creep rate by [approximately]2 orders of magnitude. It is believed that the segregation of Y[sup 3+] to the Al[sub 2]O[sub 3] grain boundaries hinders the interface reaction believed to control the creep. If one of the single-phase constituents is taken to be the Y[sup 3+]-doped Al[sub 2]O[sub 3], the creep of the duplex microstructures can be modeled using standard composite theory applied to flow.

Creep of duplex microstructures

Transmission electron microscopy (at 100 and 1000 kV potential) and analytical scanning transmission electron microscopy were used to study α-Al203 second-phase particles and their interactions with grain boundaries in two high-conductivity Y203/Yb203 stabilized zirconia ceramics containing deliberate additions of the alumina as a sintering aid. Most of the Al203 particles were intragranular and microanalysis showed that they contained inclusions rich in Zr or Si plus Zr. Al2O3 particles at grain boundaries were frequently associated with amorphous cusp areas rich in Si and Al. The results suggest that the Al203 acts as a scavenger for SiO2, removing it from grain-boundary localities. A model is proposed whereby this process occurs as the boundaries meet the second-phase particles, assisted by rapid grain-boundary diffusion. Such an ZrO2-Al2O3-SiO2 interaction and partitioning is predicted thermodynamically and offers a possible explanation for the improvements in ionic conductivity brought about by Al2O3 additions, as reported in the literature.

Microstructural Analysis of Sintered High-Conductivity Zirconia with Al203 Additions

Role of segregating dopants on the improved creep resistance of aluminum oxide

Experimental evidence determined by scanning transmission electron microscopy, scanning Auger electron spectroscopy, and lattice parameter measurements is reported for interfacial segregation in pure and doped perovskite materials, e.g., BaTiO3 and SrTiO3. The segregation behavior for isovalent dopants (e.g., Sr2+, Ca2+, or Pb2+ at a Ba2+ site) is mainly associated with a strain energy contribution from ionic misfit. However, both strain energy and space charge contributions are important in determining the segregation behavior of aliovalent ions (e.g., La3+ or Nd3+ at a Ba2+ site and Fe3+ or Ni3+ at a Ti4+ site). Segregation depths for aliovlent dopants are typically 15 to 20 nm thick. Besides dopants, constituent alkaline-earth ions also segregate to interfaces. Their segregation behavior is promoted by high temperatures, low partial pressures of oxygen, and the presence of acceptors. The results are explained in terms of a proposed theory of segregation presented in part I of the present series.

Interfacial Segregation in Perovskites: II, Experimental Evidence

The electrical conductivity of several sintered yttria ceramics with different levels of lower valent cation impurities was studied as a function of the partial pressure of water vapor (3 to 1600 Pa) at 500° to 1300°C in oxygen or air. At the higher temperatures yttria is a P-type conductor, but at the lowest temperatures and highest water vapor pressures, an ionic contribution becomes significant. The p-type conductivity decreases with increasing water vapor pressure. This is interpreted in terms of a model involving dissolution of hydrogen as interstitial protons and which counterbalance the effective charges of the lower valent impurities. At the higher water vapor pressures and lower impurity levels excess protons are probably compensated by interstitial hydroxide or oxygen ions. Indications of grain boundary segregation of impurities are reported.

Electrical Conductivity and Defect Structure of Y2O3 as a Function of Water Vapor Pressure

Analytical scanning transmission electron microscopy (AEM) was used to study grain-boundary solute segregation in the systems MgO-NiO, A12O3-Y, O3, NiO-Cr2O3, and NiO-Al2O3. Electron beam spreading within the specimen was incorporated into a model to quantitatively measure solute segregation. Grain-boundary segregation occurred in A12O3-Y2O3 and NiO-Cr2O3 but was not detected in MgO-NiO and NiO-Al2O3 specimens. These results and the quantitative measurements agree with equilibrium solute segregation theories. Microhard-ness measurements indicate no difference in hardness between the grain boundary and the matrix for Cr-doped NiO, a system in which segregation was detected.

Investigation of Grain‐Boundary Segregation in Ceramic Oxides by Analytical Scanning Transmission Electron Microscopy

A scanning transmission electron microscope (STEM) equipped with an X-ray energy dispersive spectrometer (EDS) was used for quantitative X-ray tnicroanalysis of an MgO-10 mol% NiO ceramic. Using the Cliff-Lorimer standardless ratio technique for quantitative X-ray microanalysis, absorption of the Mg characteristic X rays, particularly by oxygen in the specimen, was found to be a critical problem and corrections were required. This problem is characteristic of low atomic number oxide ceramics. However, preferential absorption of characteristic X rays by carbon contamination buildup during X-ray analysis was negligible. A grain-boundary composition profile, corrected for absorption, showed no segregation of the Ni to the grain boundary, in agreement with current segregation theory.

Absorption Effects in STEM Microanalysis of Ceramic Oxides

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Investigation of Grain‐Boundary Segregation in Ceramic Oxides by Analytical Scanning Transmission Electron Microscopy

Absorption Effects in STEM Microanalysis of Ceramic Oxides