Tapan K. Gupta

Effect of stress-induced phase transformation on the properties of polycrystalline zirconia containing metastable tetragonal phase

Polycrystalline zirconia containing a high content of metastable tetragonal phase shows high strength (∼ 700 MPa), high fracture toughness (Kc = 6 to 9 MN m−3/2) and small grain size (<0.3jμm). The strength and grain size remain nearly constant over a wide range of tetragonal phase content (100 to 30%). At a low concentration of tetragonal phase <30%, there is a rapid decrease in strength accompanied by a rapid increase in grain size. These results are explained by means of a stress-induced phase transformation in the metastable tetragonal phase.

Stabilization of tetragonal phase in polycrystalline zirconia

It is shown that the tetragonal phase can be stabilized in the sintered body of a partially stabilized zirconia (PSZ) containing low concentrations of yttria. Such sintered body containing the metastable phase undergoes stress-induced phase transformation by the absorption of thermal or mechanical stress and exhibits strengths in excess of 690 MPa (100ksi).

A grain-boundary defect model for instability/stability of a ZnO varistor

A defect model for the grain-boundary barrier has been proposed to explain the phenomena of voltage instability/stability of the ZnO varistor. The key element of the proposed model is the zinc interstitials which are present in the depletion layer as excess zinc, arising from the non-stoichiometric nature of ZnO. Both instability and stability have been described in terms of diffusion of these interstitials in the depletion layer, followed by chemical interactions with defects at the grain-boundary interface. Finally, a large body of experimental data is presented to indirectly validate the proposed defect model.

Improved varistor nonlinearity via donor impurity doping

Zinc oxide (ZnO) varistors exhibit a voltage upturn at higher current densities which reduces their effectiveness as over‐voltage protection devices. This effect has been attributed to a limiting series impedance offered by the ZnO grains. Based on literature data, the varistors were made for this investigation wherein the grain impedance has been reduced by doping with Al2O3 or Ga2O3. The high current surge and high frequency impedance data combined with microstructural investigation indicated an improved varistor nonlinearity via donor doping. A simple defect model developed for a pure ZnO crystal has been used to explain the doping behavior of the varistor grain.

Current instability phenomena in ZnO varistors under a continuous ac stress

A zinc oxide (ZnO) varistor subjected to a 60‐Hz voltage below the characteristic turn‐on voltage exhibits an increase in the resistive component of current with time. The rate of current increase with time can be increased with an increase in applied voltage and/or temperature. An analysis of current rise with an applied 60‐Hz voltage stress and current decay with no voltage stress data obtained for ZnO varistors indicates that the current instability phenomena is associated with the diffusion of interstitial zinc in the depletion layer regions adjacent to the ZnO‐ZnO grain boundary.

Barrier voltage and its effect on stability of ZnO varistor

The major voltage drop in ZnO varistors occurs across the grain boundaries, which behave generally as Schottky barriers supporting a barrier voltage Vg. This voltage is proportional to the device voltage. Experimental evidence shows that there is a time‐dependent reduction in the barrier voltage combined with an increase in resistive current iR when the varistor is subjected to a continuous ac voltage stress. The phenomena is reversed when the applied voltage is removed, showing nearly complete recovery. Instabilities of the resistive current and of the barrier voltage are shown to be manifestations of the same phenomenon and are attributed to a metastable component in the Schottky barriers. It is proposed that this metastable component is due to interstitial zinc ions that are capable of migration under thermal and electrical driving forces. When these ions are removed or stabilized by a suitable heat treatment, the instability of the device is reduced. This paper presents experimental data and analysis ...

Low voltage ZnO varistor: Device process and defect model

A new process is described for achieving the current‐voltage characteristics of a ZnO varistor with a breakdown voltage less than 20 V. The structure has been made by using Ag contacts containing oxides of Bi and Pb which serve both as a diffusion source for grain and/or grain boundary doping, and as metallic contacts on polycrystalline ZnO substrate. The structure was also made by sputtering a layer of various metal oxides such as Bi2O3, Sb2O3, Co3O4, etc. and then evaporating contact metals. The device shows a nonlinear current‐voltage characteristic when annealed at 800–900 °C in air. Annealing in N2 ambient however, results in linear I‐V characteristics. The presence of certain critical additives such as Bi2O3 was found essential for the development of nonlinearity. Based on these observations and the known defect structure of pure ZnO, a defect model which takes into account the role of oxygen in developing the nonlinear behavior of the proposed device is presented.

Design of low dielectric glass+ceramics for multilayer ceramic substrate

Compositional design and properties of a low dielectric constant glass+ceramics, containing borosilicate glass, high silica glass and alumina, for multilayer ceramic substrates are described. The new low dielectric system can be densified at below 1000/spl deg/C in air, allowing high electrical conductivity metallization including Au and Ag-Pd. Compositions with tailor-made properties are designed according to a working model based upon mixing rule, and validated with experimental results. Compositions with a thermal expansion coefficient compatible with Si and GaAs, and a dielectric constant in the range of 4-4.5 and 5-6, respectively, are developed. >

Devitrification inhibitors in borosilicate glass and binary borosilicate glass composite

Cristobalite is known to precipitate out of borosilicate glass (Corning 7740) and a binary glass mixture of borosilicate glass and high silica glass when these glasses are heated to elevated temperatures. To prevent cristobalite from forming in these glass systems, a devitrification inhibitor needs to be found. Among oxides selected for testing, both Al 2 O 3 and Ga 2 O 3 are found to prevent cristobalite from forming in these glass systems.

DENSIFICATION KINETICS OF BINARY BOROSILICATE GLASS COMPOSITE

Densification kinetics and mechanism of a binary borosilicate glass composite, containing low-softening borosilicate (BSG) and high-softening high silica (HSG) glasses, have been studied. Apparent activation energy of densification varies from 200 to 400 kJ/mol, and decreases with increasing BSG content at a given densification factor. At a given BSG content, the activation energy of densification initially remains relatively unchanged with increasing densification factor (DF), but increases with densification when DF reaches a critical value (DF ∗ ). Moreover, the value of DF ∗ increases with increasing BSG content. From the activation energy estimates of densification, it is concluded that the predominant densification mechanism for BSG ≥ 30 vol. % with DF ∗ is viscous flow of low-softening BSG. For BSG ≥ 30 vol. % with DF ∗ and BSG ⋚ 20 vol. % with all DF investigated, the activation energies are within the range governed by viscous flow of both BSG and HSG, indicating that the densification is controlled by viscous flow of a new glass with a composition between BSG and HSG. The latter evidence stems from the microstructural observation that as sintering proceeds, the HSG particle undergoes an extensive dissolution process.