Influence of Ionic Charge on Diffusion in the Surface Layer of Magnesia

The diffusion of ⁵⁷Co isotope on the MgO (100) surface was investigated by the edge-source method. The surface diffusion parameter, αD_sδ, where α is the segregation factor, D_s the surface diffusion coefficient, and δ the thickness of the high-diffusivity layer, was determined over the temperature region 750° to 1250°C. An Arrhenius plot shows a break at ∼1100°C. Below this temperature ionic or localized transport predominates and above it nonlocalized transport seems to predominate. The divalent Co ion diffuses faster than the trivalent Cr ion in the surface layer. The apparent activation energies for the localized surface diffusion of ⁵⁷Co and ⁵⁹Cr are 59±12 and 110±12 kj/mol, respectively.     

The existence and stability of Ca6Zr19O44 compound in the system ZrO2CaO

The development of ordered phases in the system ZrO₂CaO was studied during prolonged heating of reactive powders derived from gels. The pure phase ф₂ was obtained at 1250°C only with gels containing 24 m/o CaO which indicates that the correct composition of this phase i$\text{s}$ Ca₆Zr₁₉O₄₄. This phase has rhombohedral symmetry, space group R3c, and is analogous to the ф₂ phase in the HfO₂CaO system. The pure phase ф₁ (CaZr₄O₉) was obtained by heating gels containing 20 m/o CaO at 1180°C. This phase often appears as a precursor to the formation of the ф₂ phase. At 1355±15°C the ф₂ phase decomposes to a cubic ZrO₂ solid solution and CaZrO₃. At 1235±15°C the ф₁ phase decomposes to a cubic ZrO₂ solid solution and the phase ф₂. The inability to synthesize a phase analogous to the phase ф (Ca₂Hf₇O₁₆) in the HfO₂CaO system can be explained by the low cation diffusivity in the ZrO₂CaO system below 1150°C.     

Mechanism of Material Removal from Silicon Carbide by Carbon Dioxide Laser Heating

The mechanism of material removal from SiC by CO₂ laser heating was studied using sintered and single-crystal α-SiC. Removal rate and width of the groove showed maxima when plotted as a function of translation speeds. Groove depth decreased as the translation speed of samples increased. Similar results were obtained if argon or air was used as gas assist, which indicated that the material removal mechanism is induced dissociation of SiC. Microstructure of the material deposited in and outside of the groove was studied by SEM. At low scanning speeds, columnar grains 10 to 50 μm long appeared. As the scanning speed increased, columnar grains became smaller and finally only irregular polycrystalline particles were observed. By using Raman spectroscopy, Auger analysis, and X-ray diffraction, phases inside and outside the groove were identified as Si, β-SiC, C, and SiO₂. Columnar grains were identified as β-SiC covered with thin layers of C, Si, and SiO₂. Slow scanning speeds enhanced the growth of β-SiC. At slow scanning speed, free silicon was always found in the grooves of lased single crystals but not in the grooves of lased sintered SiC. It can be concluded that the mechanism of material removal from silicon carbide by CO₂ laser heating is a vaporization process, and material found in the groove and on the surface near the groove is formed by condensation from the vapor.     

Ionic Conductivity of the Fluorite-Type Hafnia–R2O3 Solid Solutions

The ionic conductivity of the hafnia-scandia, hafnia-yttria, and hafnia-rare earth solid solutions with high dopant concentrations of 8, 10, and 14 mol% was measured in air at 600° to 1050°C. Impedance spectroscopy was used to obtain lattice conductivity. A majority of the investigated samples exhibited linear Arrhenius plots of the lattice conductivity as a function of temperature. For all investigated dopant concentrations the ionic conductivity was shown to decrease as the dopant radius increased. The activation enthalpy for conduction was found to increase with dopant ionic radius. The fact that the highest ionic conductivity among 14-mol%-doped systems was obtained with HfO₂─Sc₂O₃ suggested that the radius ratio approach should be used to predict the electrical conductivity behavior of HfO₂─R₂O₃ systems. A qualitative model based on the Kilner's lattice parameter map does not seem to apply to these systems. For the three systems HfO₂─Yb₂O₃, HfO₂─Y₂O₃, and Hf₂O₃─Sm₂O₃ a conductivity maximum was observed near the dopant concentration of 10 mol%. Deep vacancy trapping is responsible for the decrease in the ionic conductivity at high dopant concentrations. Formation of microdomains of an ordered compound cannot explain the obtained results. A comparison between the ionic conductivities of doped HfO₂ and ZrO₂ systems indicated that the ionic conductivities of HfO₂ systems are 1.5 to 2.2 times lower than the ionic conductivities of ZrO₂ systems.     

Neutron diffraction investigation of Zr3Y4O12

Ordering of Zr₃Y₄O₁₂ has been investigated by x-ray and neutron diffraction techniques. Ordering is very sluggish as shown by the presence of both disordered cubic and ordered rhombohedral phases after annealing for three months at 1100°C. Refinement of neutron data for ordered Zr₃Y₄O₁₂ by the Rietveld profile technique confirms the structure reported by Scott.     

Synthesis and sintering of Ca0.5Zr2P3O12 -a low thermal expansion material

The single phase compound Ca_0.5Zr₂P₃O₁₂ (CZP) was prepared by solid state reaction technique. This material shows a negative thermal expansion in the temperature region of 30°–500°C. The effect of MgO and ZnO addition on the sintering behavior and thermal expansion characteristics of Ca_0.5Zr₂P₃O₁₂ was investigated. Mg₃(PO₄)₂ and Zn₃(PO₄)₂ were observed as minor phases responsible for improving the overall thermal expansion of CZP + MgO, ZnO systems. SEM studies and density data are also discussed. Observed sintering kinetics suggest that a liquid phase is promoting the sintering reaction. 98+% of theoretical density and near zero expansion behavior in certain compositions were observed.     

Phase relations and ordering in the system HfO2CaO

The chemical equilibrium for the system HfO₂CaHfO₃ was determined for temperatures above ～1200°C by heating reactive powders for extended periods of time. The eutectoid decomposition of the cubic solid solution was found to occur at 19.0 ± 0.5 m/o CaO and 1415 ± 7°C. The solid solubility of CaO in tetragonal hafnia is very low. The tetragonal solid solution was shown to form from monoclinic hafnia solid solution at a eutectoid temperature of ～1775 ± 50°C. Three ordered compounds exist in this system. The ordered compound CaHf₄O₉(ф₁) was found to be unstable at all temperatures investigated. It does, however, form metastably. The compound Ca₂Hf₇O₁₆(ф) is stable with an upper limit of 1473 ± 5°C. No lower limit of stability for this compound could be determined. The compound Ca₆Hf₁₉O₄₄(ф₂) is stable from ～1350 ± 50°C to its upper limit of 1463 ± 5°C.     

Fluorite related ordered compounds in the ZrO2CaO and ZrO2Y2O3 systems

ZrO₂:CaO and ZrO₂:Y₂O₃ systems have been examined for possible ordered compounds. Fluorite related ordered compounds CaZr₄O₉ and Zr₃Y₄O₁₂ have been successfully prepared and identified. While it is believed that the former is monoclinic with $\text{a}\text{= 17.813 ± 0.005}\text{A}\text{?}$, $\text{b}\text{= 14.612 ± .004}\text{A}\text{?}$, $\text{c}\text{= 12.065 ± .003}\text{A}\text{?}$ and β = 119.5 ± .02° and isostructural with CaHf₄O₉, the latter appears to be isostructural with M₇O₁₂ type of compounds with $\text{a}\text{= 9.723 ± .001}\text{A}\text{?}$ and $\text{c}\text{= 9.090 ± 0.002}\text{A}\text{?}$, which is formed commonly in compounds containing tri and tetravalent cations and is characterized by a chain oxygen vacancy along a 〈111〉 of the original fluorite. No other fluorite related ordered compound could be detected in these systems.