E. Sonder

Effect of oxidizing and reducing atmospheres at elevated temperatures on the electrical properties of zinc oxide varistors

Voltage–current characteristics have been measured for varistors that have been heated in reducing or oxidizing gases. Reducing environments produced large decreases in the leakage resistance. However, the original varistor characteristics could be restored by subsequent heating in oxidizing ambients. The oxygen partial pressure below which degradation occurred is 100 Pa at 800 °C. Degradation by reducing ambients occurred at much lower temperatures (150–500 °C) than did restoration of the varistor characteristics by oxidizing ambients (500–700 °C). An explanation of the observed behavior is proposed.

Hardening of KCl by Electron and Gamma Irradiation

Measurements of the flow stress and F‐band coloration have been made at room temperature on a number of different samples of KCl as a function of gamma and electron irradiation. The rate of increase of flow stress with irradiation is found to be greater (or smaller) in those samples and for those irradiation conditions for which the coloring rate is greater (or smaller). Moreover, the flow stress increase in any set of samples is proportional to the square root of the F‐band absorption. The implication is that hardening is caused either by F centers or other defects whose production is related to the production of F centers. Since bleaching of F centers does not produce any softening, and since the magnitude of the hardening is of the order of magnitude to be expected for interstitial type defects, the conclusion is drawn that the increase of flow stress due to irradiation of KCl is caused by interstitial Cl atoms or interstitial clusters.

Gamma Irradiation of Silicon. II. Levels in n‐Type Float‐Zone Material

Measurements of the electrical properties of gamma‐irradiated float‐zone silicon have been used to locate a number of net acceptor levels in the upper half of the forbidden gap. In material doped with less than 1016 donors, a level due to a vacancy‐oxygen atom pair located 0.17 eV below the conduction band is introduced at a rate comparable to its introduction in oxygen‐containing material. However, in addition there is introduced a level located 0.47 eV below the conduction band in phosphorus‐doped and 0.43 eV below the conduction band in antimony‐doped float‐zone silicon. This latter level is presumably due to vacancy‐donor complexes. Annealing above 150°C removes the deep level; further irradiation and annealing causes the reappearance of a deep net acceptor level at 0.47 eV. In irradiated and annealed silicon and additional net acceptor level at 0.21 eV below the conduction band was observed.

Aggregation of defects and thermal‐electric breakdown in MgO

Transmission electron microscopy has been used to study defects in as‐grown MgO single crystals and in crystals which had undergone high‐temperature electrical conduction for long periods of time. Besides dislocation loops of (a/2〈110〉) Burgers vectors and precipitates, a new type of defect, dislocation loops of α〈100〉 Burgers vectors lying in {100} planes, has been observed in electric‐field‐treated specimens. The nature of both (a/2) 〈110〉 and a〈100〉 loops was determined to be vacancy type. The relation of this microstructure to the observed increase in electrical conductivity, which ultimately results in thermal‐electric breakdown of the samples, is discussed.

The effect of electric current on the conductivity of MgO single crystals at temperatures above 1300 K

Changes in electrical conductivity over the temperature range 1000–1400 K due to treatment of MgO crystals with moderate (1000 V cm−1) electric fields at 1300 or 1400 K have been measured. Although different samples of MgO have different initial conductivities, changes produced by the electric fields are qualitatively similar. After treatments for times of the order to 20 h, the temperature dependence changes and the conductivity decreases by anywhere from a small amount to a factor of 4 at ∼1273 K. This decrease correlates with the electrolytic reduction of impurities in the MgO. More lengthy treatment (more than 100 h) causes the samples to become progressively more conducting until catastrophic failure of the electrical insulating properties occurs. The electric current passed as such failure is approached is more than two orders of magnitude greater than at the beginning of treatment. If the temperature is lowered just before failure with the 1000 V cm−1 still applied and the temperature dependence of...

GAMMA IRRADIATION OF SILICON. III. LEVELS IN p-TYPE MATERIAL

60Co gamma irradiation of p‐type silicon produces donors in the lower half of the forbidden gap. From the temperature dependence of the electrical properties, a level 0.35 eV above the valence band was located for the case of silicon pulled from silica crucibles. For the case of float‐zone silicon, two levels 0.21 and 0.28 eV above the valence band were found. The 0.35‐eV level is introduced at the rate of 10−4 net donors/cm3 per photons/cm2. The introduction rate for the 0.21‐eV level is lower than that for the 0.35 level, and that for the 0.27‐eV level is so low that it was difficult to measure.

Gamma Irradiation of Silicon. I. Levels in n‐Type Material Containing Oxygen

The resistivity and Hall coefficient of n‐type silicon containing oxygen have been measured as a function of temperature before and after a number of successive irradiations in a Co60 gamma‐ray source. A net acceptor level 0.17 ev below the conduction band was observed to result from the irradiation. Its rate of introduction was 7×10−4 traps/cm3 per photon/cm2 in 50‐ ohm‐cm material and was about twice that in more heavily doped material (∼2 ohm‐cm). Acceptor levels, lying deep within the forbidden gap, were also observed. Their total introduction rate was smaller than that of the 0.17‐ev level by a factor of 50. A lowering of the mobility below ∼100°K was also a result of the irradiations. In heavily irradiated samples this lowering of the mobility was much greater than could be explained on the basis of point‐charge scattering.

Determination of the Fe2+ and Fe3+ concentration in MgO

Crystals of MgO containing 140 ppm iron were reduced in CO or oxidized in air at 1150 °C and the iron concentration in different valences was determined by optical and magnetic techniques. Optical density, electron spin resonance, and magnetic circular dichroism measurements gave self‐consistent results that yield calibration constants for determining the Fe2+ and Fe3+ concentrations by optical means. Oscillator strengths of the absorption bands at 1000 and 285 nm were calculated from this knowledge of the iron concentration. At low concentrations more than 90% of the iron is unassociated, Fe2+ or Fe3+. Alternate oxidation and reduction converted 80% of this iron between these valence states, while the other 20% remained as Fe2+.

Role of short‐circuiting pathways in reduced ZnO varistors

Electrical measurements and secondary ion mass spectrometry (SIMS) observations have been made to elucidate the mechanisms involved in the degradation of ZnO varistors at elevated temperatures in reducing atmospheres. The electrical measurements indicate that the degradation sensitivity depends to some extent on varistor composition but can be highly variable for different varistors of the same formulation and even within a given varistor sample. The SIMS data indicate that oxygen ions are highly mobile when varistors are heated above 290 °C, and it is proposed that flaws, such as microcracks in the varistors, act as conduits for oxygen at elevated temperatures. A circuit model in which a reduced varistor is characterized by short‐circuiting regions around such flaws predicts an electric field versus current density that is similar to that observed in experiments.

Magnetic and Electrical Properties of Reactor‐Irradiated Silicon

Magnetic susceptibility measurements above 3°K and Hall effect and resistivity determinations between 50 and 300°K are reported for n‐type silicon samples irradiated with increasingly higher doses of fission neutrons. The paramagnetism due to electronic states in the forbidden gap shows an initial decrease after short irradiation but a reversal, increase, and final saturation at a value less than that originally contributed to the paramagnetism by the filled donors after longer irradiation.The Hall coefficient shows evidence of a distribution of irradiation‐produced energy levels in the neighborhood of 0.3 ev below the conduction band. The mobility goes through an initial sharp decrease with irradiation but recovers partially after longer irradiations. The results are discussed in terms of several models of radiation damage. It is concluded that a simple model based on uniformly dispersed interstitials and vacancies is not adequate to explain the results and that interactions between centers, and nonunifo...