Seamless Joining of Silicon Nitride Ceramics

High-strength joining of Si₃N₄ ceramics has been achieved by developing a process that effectively eliminates the seam, and may allow for fabrication of large or complex silicon nitride bodies. This approach to joining is based on the concept that when sintering aids are effective in bonding individual grains, they could be equally effective in joining bulk pieces of Si₃N₄. Optimization of the process led to Si₃N₄/Si₃N₄ joints with room-temperature bend strengths as high as 950 MPa, corresponding to more than 90% of the bulk strength of the Si₃N₄. At elevated temperatures of 1000° and 1200°C joint strengths of 666 and 330 MPa, respectively, were obtained, which are the highest values reported to date for these temperatures. These bend strengths are also more that 90% of the strength of bulk Si₃N₄ measured at these temperatures.     

Cu-YSZ cermet solid oxide fuel cell anode prepared by high-temperature sintering

Porous YSZ-Cu alloy cermet structures are prepared by sintering above the metal melting point in reducing atmosphere. Unexpectedly good wetting of the molten metal within the YSZ network is obtained, resulting in cermets with fine structure and excellent electronic conductivity. Anode-supported solid oxide fuel cells are prepared with YSZ-Cu alloy cermet as the anode. Addition of infiltrated ceria catalyst improved the initial performance. Maximum power density of about 275 mA cm⁻² and operation for about 110 h was achieved in the 700-800 °C range. After operation, AC impedance revealed that the high-frequency impedance was unchanged, whereas the low-frequency impedance increased. It was concluded that the Cu alloy network conductivity remains high, but catalyst stability needs improvement.     

Hydrogen reduction of cobalt ferrite

The kinetics of reduction of cobalt ferrite by hydrogen as a function of reduction temperature and pressure have been measured by thermogravimetric analysis. A minimum in the rate as a function of temperature has been observed and its cause attributed to the formation of a cobalt-wiistite subscale at higher reduction temperatures. A mathematical model, based on one derived by Spitzer, Manning, and Philbrook,¹ has been used to interpret the results in terms of the rate constants for the individual steps in the reaction. Optical microscopy has been used to characterize the morphology of the reduction product and, additionally, partially reduced single crystals of cobalt ferrite have been examined by transmission electron microscopy to characterize the microstructure of the reaction interface. A fine network of pores in the reduced scale was shown to allow the reducing and product gases to reach the immediate vicinity of the chemical reaction. The structure of the porosity and consequently the effective gaseous diffusion coefficient in the scale were both shown to be functions of the reduction temperature and pressure. The interface reaction was shown to follow Langmuir-Hinshelwood kinetics. A model was developed to explain such kinetics by incorporating a solid-state diffusion step. Such a step was considered necessary to explain the development of the observed microstructures. An ‘incubation time’ for the development of a continuous cobalt-wüstite subscale at higher reduction temperatures was attributed to the different growth kinetics for the spinel-metal and spinel-wüstite interfaces.     

ML-oriented NDA carrier synchronization for general rotationallysymmetric signal constellations

We point out that the nondecision-aided (NDA) carrier synchronizer, maximizing the low E_s/N_o limit of the likelihood function averaged over a general 2π/N-rotationally symmetric signal constellation, reduces to the familiar timing-aided Nth power synchronizer; this extends a result of D'Andrea, Mengali and Reggiannini (1988) where only M-PSK constellations have been considered. Whereas in the case of M-PSK the tracking error variance of this NDA ML synchronizer is known to converge to the Cramer-Rao bound (CRB) with increasing E_s/N_o. We show that for other rotationally symmetric constellations (such as QAM) the tracking error variance is substantially larger than the CRB     

Initiation of mode I degradation in sodium-beta alumina electrolytes

The extension of initial surface cracks by the focusing of the ionic current in beta alumina electrolytes (Mode I degradation) is discussed in terms of existing models. Focusing for an ion current impinging on an elliptic-cylindrical flaw is calculated by solving for the electric potential with suitable boundary conditions. The current density distribution along the crack is used to calculate the sodium flow velocity and Poiseuille pressure inside the flaw. Calculated critical current densities using aK _lc criterion are several orders of magnitude higher than measured average critical current densities. This implies a lower effectiveK _lc for electrolytic degradation than for mechanical testing. Current density enhancement around insulating barriers, such as non-wetted surface areas, is also calculated using elliptic-cylindrical coordinates. Significant current density enhancements are found, but they are localized in very small regions. Crack growth would occur within these regions, but should be arrested once the flaw extends past the high current density zone. A plausible mechanism for decreasingK _lc in the electrolytic case is discussed.     

Zirconia-toughened sodium beta″-alumina solid electrolyte

The effect of a 15 vol% zirconia dispersion on the critical current density for failure initiation of beta-alumina solid electrolyte was examined. Single phase and composite electrolytes were tested in standard sodium-sodium test cells and subjected to increasing ionic currents. Onset of degradation in the electrolyte was detected by monitoring acoustic emissions from the cell. Preliminary examination of the electrolyte material showed that the problem of producing a uniform dispersion of zirconia in pure beta-alumina had not been solved. However, the electrolytes were of sufficient quality to draw important conclusions about the potential of transformation toughening for improving electrolyte performance.     

Low-temperature sintering of zinc oxide varistors

High-field varistors in the system ZnO-CoO-MnO-Bi₂O₃ were fabricated using powders prepared by a previously developed coprecipitation process. Following calcination, the powders were compacted and densified by conventional pressureless sintering at temperatures below 750° C in air, The effects of sample green density, sintering temperature, and grain-growth inhibitor on densification and microstructure development were investigated. Addition of aluminium at the 125 p.p.m level was used to inhibit grain growth. Samples with densities >0.98 theoretical and grain sizes <1m were fabricated by high-pressure cold-isostatic pressing followed by sintering at 730° C. For comparison, typical commercial varistor devices have grain sizes of about 20 m and switching fields of approximately 2 kV cm^–1 after sintering at 1200 to 1400° C. As a result of the fine grain size, our high-field varistors had switching fields of 45 kV cm^–1 at a current density of 10 A cm^–2. Consistent with earlier work on extremely high-density varistors (>0.98 theoretical) prepared from similar powders, nonlinearity coefficients of about 10 were measured for current densities between 2.5 and 10 A cm^–2.     

Pre-eutectic densification in MgF2---CaF2

Increased densification rates were found as much as 200°C below the eutectic temperature (980°C) for MgF₂ containing small amounts of CaF₂. Constant heating rate and constant temperature sintering data, as well as microstructural developments indicated that solid state grain boundary transport rates had been enhanced by the eutectic forming additive. The effect saturated at about 1 wt% CaF₂. The results suggest that densification of ceramic powders could be favorably affected without a substantial increase in the grain growth rate, by the addition of small amounts of eutectic forming additives, and sintering below the eutectic temperature.