Henry C. Graham

Characterization and properties of controlled nucleation thermochemical deposition (CNTD)-silicon carbide

The present investigation was undertaken to characterize the microstructure of controlled nucleation thermochemical deposition (CNTD)-SiC material and to evaluate the room-temperature and high-temperature bend strength and oxidation resistance. Utilizing the CNTD process, ultrafine grained (0.01 to 0.1μm) SiC was deposited on W wires (0.5 mm diameter by 20cm long) as substrates. The deposited SiC rods had superior surface smoothness and were without any macrocolumnar growth commonly found in conventional CVD material. At both room and high temperature (1200 to 1380° C), the CNTD-SiC exhibited a bend strength of ~ 200 000 psi (1380 MPa), several times higher than that of hot-pressed, sintered, or CVD SiC. The excellent retention of strength at high temperature was attributed to the high purity and fine grain size of the SiC deposit and the apparent absence of grain growth at elevated temperatures. The rates of weight change for CNTD-SiC during oxidation were lower than for NC-203 (hot-pressed SiC), higher than for GEs CVD-SiC and CVD-Si3N4 but considerably below those for HS-130 (hot-pressed Si3N4). The high-purity fully dense and stable grain size CNTD-SiC material shows potential for high-temperature structural applications, however, problem areas might include scaling the process to make larger parts, deposition on removable substrates, and the possible residual tensile stress.

Oxidation of hot-pressed Si3N4 with Zr-based additives

To determine the effect of Zr compounds used as aids in the hot pressing of Si3N4upon the subsequent oxidation resistance, the oxidation kinetics of four materials were studied under isothermal conditions at a total pressure of 150 Torr O2over the temperature range 1400–1570°C. The materials were hot-pressed at the Naval Research Laboratory, either using AME85Si3N4powder plus 3 wt.% ZrO2or ZrC, or using Plessy-Frenchtown high-purity Si3N4powder plus 1 wt.% MgO or 1 wt.% MgO+2wt.% ZrO2.The addition of Zr compounds improved the oxidation resistance by about an order of magnitude as compared to a commercial grade Si3N4(HS-130, Norton Co.). Limited results from studies on materials containing Mg+Zr indicated that the presence of Zr may offset the deleterious effect of Mg-compound hot-pressing aids upon the oxidation resistance of Si3N4.

Origin of Apparent Negative Impedance in Three Terminal Measurements

A brief discussion is presented of the occurrence of spurious negative dissipation factors, which appear to correspond to negative resistances, in the measurement of three terminal low loss specimens. The origin of these negative values is traced to series resistance in the guard ring lead and a technique, involving the use of a four electrode specimen, is described which obviates the need for extremely low resistance electrodes and leads.

Oxidation of Na2SO4 coated Ni-20Cr-2ThO2

The Na2SO4-induced oxidation of Ni-20Cr-2ThO2 has been studied at 900° and 1000°C in 150 torr O2. A rapid weight-gain period, corresponding to NiO formation occurs after a short incubation period. The duration of the rapid weight gain stage persists for about 2 h and terminates with the development of a continuous Cr2O3 internal layer. Subsequent oxide formation is extremely slow, similar to that reported for normal oxidation of TD-NiCr.

Defect Structure and Electrical Properties of Some Refractory Metal Oxides

A brief description of the equilibrium thermodynamic approach to the characterization of defect concentrations in refractory metal oxides as a function of temperature, oxygen partial pressure, and impurity content is given. Techniques for the determination of ionic and electronic transport numbers by a blocking electrode polarization measurement and for the measurement of conductivity, thermoelectric power, and weight change are reviewed. The application of this approach and these measurements to the determination of the extent of deviation from stoichiometry, the nature and ionization state of the defects which predominate, and the mechanism of charge transport are illustrated by detailed consideration of several specific examples.

Apparatus for Guarded Polarization Measurements

A circuit is presented for making guarded polarization measurements in situ on samples which vary from 10 to 108 Ω over the temperature range of the experiment. The time dependence of the voltage at constant current (or the current at constant voltage) can be followed from less than 1 msec to several days. Combined leakage resistance 105 times smaller than the sample resistance results in less than 1% error in the measurements. The circuit should prove generally useful whenever automatic guarding is required over a wide frequency range (including dc).