Minoru M. Freund

Critical review of electrical conductivity measurements and charge distribution analysis of magnesium oxide

The electrical conductivity σ of MgO single crystals shows a sharp increase at 500–800°C, in particular of σsurface, generally attributed to surface contamination. Charge Distribution Analysis (CDA), a new technique providing information on fundamental properties that was previously unavailable, allows for the determination of surface charges, their sign and associated internal electric field. Data on 99.99% purity, arc-fusion grown MgO crystals show that mobile charge carriers start to appear in the bulk of the MgO crystals between 200 and 400°C when σ (measured by conventional techniques) is in the 10−14 to 10−16 Ω−1cm−1 range. Above 500°C, as σ increases to 10−6 to 10−7 Ω−1cm−1, more charges appear giving rise to a strong positive surface charge supported by a strong internal field. This indicates that charges are generated in the bulk and diffuse to the surface by an internally controlled process. On the basis of their positive sign they are identified as holes (defect electrons). Because of the low cation content of these very pure MgO crystals, these holes cannot be associated with transition metal impurities. Instead, they are associated with the O2- sublattice, e.g. consist of O− states or positive holes. This conclusion is supported by magnetic susceptibility data showing the appearance of 1000 ± 500 ppm paramagnetic species between 200–500°C. The magnetic data are consistent with strongly coupled, diamagnetic O− pairs below 200–500°C, chemically equivalent to peroxy anions, O2−2 and probably associated with cation vacancies in the MgO matrix. The formation of O2−2 in arc-fusion grown MgO crystals is very unexpected because of the highly reducing growth conditions. Their presence implies an internal redox reaction involving dissolved “water” by which OH− pairs convert to O2−2 plus H2 molecules. This redox conversion is supported by mass spectroscopic measurements of the H2 release from highly OH−- doped, finely divided MgO and by wet-chemical analysis of its oxidant concentration.

The Infrared Telescope in Space (IRTS)

The Infrared Telescope in Space (IRTS) is a cryogenically cooled small infrared telescope that will fly aboard the small space platform Space Flyer Unit. It will survey approximately 10% of the sky with a relatively wide beam during its 20 day emission. Four focal-plane instruments will make simultaneous observations of the sky at wavelengths ranging from 1 to 1000 microns. The IRTS will provide significant information on cosmology, interstellar matter, late-type stars, and interplanetary dust. This paper describes the instrumentation and mission.

Highly mobile oxygen hole-type charge carriers in fused silica.

Some peculiar positive charge carriers are thermally generated in fused silica above 500 degrees C. These charge carriers appear to be positive holes, chemically O- states, probably arising from dissociation of peroxy defects. The charge carriers give rise to a pronounced positive surface charge which disappears upon cooling but can be quenched by rapid quenching from approximately 800 degrees C. Reheating to approximately 200 degrees C remobilizes these charge carriers and causes them to anneal below 400 degrees C. The generation of positive holes charge carriers may be important to understand failure mechanisms of SiO2 insulators.

The Far-Infrared Photometer on the Infrared Telescope in Space

We describe the design and calibration of the Far-Infrared Photometer (FIRP), one of four focal plane instruments on the Infrared Telescope in Space (IRTS). The FIRP will provide absolute photometry in four bands centered at 150, 250, 400, and 700 microns with spectral resolution wavelength/wavelength spread is approximately 3 and spatial resolution delta theta = 0.5 degrees. High sensitivity is achieved by using bolometric detectors operated at 300 mK in an AC bridge circuit. The closed-cycle He-3 refrigerator can be recycled in orbit. A 2 K shutter provides a zero reference for each field of view. More than 10% of the sky will be surveyed during the 3 week mission lifetime with a sensitivity of less than 10(exp -13) W per sq cm per sr per 0.5 degree pixel.

Near Infrared Extragalactic Background

We searched for the near infrared extragalactic background light (IREBL) in the data from the Near Infrared Spectrometer (NIRS) on the Infrared Telescope in Space (IRTS). After subtracting the contribution of faint stars and zodiacal component based on the model, significant isotropic emission is detected whose in-band flux amounts to ∼ 30 nWm-2sr-1. This brightness is consistent with upper limits of COBE/DIRBE, but is much brighter than the integrated light of faint galaxies at the H and K bands. A significant fluctuation of the skybrigh tness was also detected that can not be explained bykno wn foreground emission components. 2- point correlation analysis indicates that the fluctuation has a characteristic spatial frequencyat 1 ∼ 2 × 102 arcmin. These results indicate that the detected isotropic emission is cosmological in origin, and is new observational evidence for the study of the formation and evolution of galaxies.

Electrical Conductivity of Rocks and Dominant Charge Carriers: The Paradox of Thermally Activated Positive Holes

In this paper we have focused on fundamental processes that are important for understanding the electrical properties of materials, both single crystal minerals and igneous rocks, both laboratory-grown and from natural environments. The prevailing view in the geophysics community is that the electrical conductivity structure of the Earth’s continental crust over the 5-35 km depth range can best be understood by assuming the presence of intergranular fluids and/or intergranular carbon films. Based on studies of melt-grown MgO, magma-derived sanidine and anorthosite feldspar and upper mantle olivine single crystal we present evidence for the presence of electronic charge carriers, the importance of which has been largely ignored. These charge carriers derive from peroxy defects, which are introduced during cooling, under non-equilibrium conditions, through a redox conversion of pairs of solute OH- arising from the solid state dissolution of H2O. It can be shown that, during reheating, the peroxy defects become thermally activated in a 2-step process. Step 2 leads to the release of defect electrons in the oxygen anion sub lattice. Known as positive holes and symbolized by h•, these electronic charge carriers are associated with energy states at the upper edge of the valence band. They are highly mobile. Chemically equivalent to O– in a matrix of O2– they are highly oxidizing. However, though metastable, the h• can exist in minerals, which crystallized in highly reduced environments. The h• appear to control the electrical conductivity of crustal rocks over much of the 5-35 km depth range. We make the extraordinary and seemingly paradoxial claim that MgO crystals, grown from the melt under the viciously reducing conditions of a carbon arc fusion furnace, contain peroxy defects in their crystal structure, hence oxygen in the valence state 1–. When the peroxy defects break up, they release positive hole charge carriers, formally defect electron in the oxygen anion sublattice, equivalent to O– in a matrix of O2–.These positive holes have two outstanding properties: they are highly mobile and highly oxidizing.

Flight performance of the Far-Infrared Photometer (FIRP)

We present the flight performance of the far infrared photometer (FIRP) onboard the infrared telescope in space (IRTS). The FIRP was designed to measure the absolute sky brightness in four submillimeter wavelength bands centered on 150, 250, 400, and 700 micrometer with a spectral resolution of lambda/(Delta) (lambda) equals 3, and with spatial resolution of 0.5 degrees. The bolometers were cooled to 300 mK by a 3He refrigerator, and an ac bridge readout circuit was used to achieve high sensitivity. The 3He refrigerator achieved a temperature of approximately 300 mK during each of three 3He condensations carried out in orbit. The hold time was confirmed to be at least 7 days per cycle. We observed approximately 7% of the sky during a mission time of 3 weeks. Some excess noise was observed in most of the channels. Adequate sensitivity was achieved in all channels for observations at low galactic latitudes, where emission from interstellar dust is relatively bright. The preliminary sensitivities (60 sec integration, 1 (sigma) ) are estimated to be 7.7 multiplied by 10-7, 3.1 multiplied by 10-8, 7.7 multiplied by 10-8, and 3.7 multiplied by 10-8 Wm-2sr-1 for the 150, 250, 400, and 700 micrometer channels, respectively.