摘要:

After many years of promise as a high temperature semiconductor, silicon carbide (SiC) is finally emerging as a useful electronic material. Recent significant progress that has led to this emergence has been in the areas of crystal growth and device fabrication technology. High quality single‐crystal SiC wafers, up to 25 mm in diameter, can now be produced routinely from boules grown by a high temperature (2700 K) sublimation process. Device fabrication processes, including chemical vapor deposition (CVD),in situdoping during CVD, reactive ion etching, oxidation, metallization, etc. have been used to fabricate p–n junction diodes and MOSFETs. The diode was operated to 870 K and the MOSFET to 770 K.

ISSN:0094-243X    

DOI:10.1063/1.39998

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The electrical and switching characteristics of high power semiconductor switches subjected to high levels of neutron fluences and gamma doses must be known by the designer of the power conditioning, control and transmission subsystem of space nuclear power systems. Location and the allowable shielding mass budget will determine the level of radiation tolerance required by the switches to meet performance and reliability requirements. Neutron and gamma ray interactions with semiconductor materials and how these interactions affect switch performance are briefly discussed. The effects of post‐irradiation thermal anneals at 300 K and up to 425 K for NPN Bipolar Junction Transistors (BJTs), N‐channel Metal‐Oxide‐Semiconductor Field Effect Transistors (MOSFETs), and N‐channel Static Induction Transistors (SITs) are discussed in terms of recovery of degraded electrical and switching parameters caused by either neutron or gamma irradiation. The important experimental results from these annealing tests show that BJTs have very good recovery to leakage current degradation but poor recovery to current gain degradation; MOSFETs show some recovery in gate‐source threshold voltage degradation but no significant recovery in drain‐source on‐resistance degradation; and likewise, SITs show no significant recovery in drain‐source on‐resistance degradation.

ISSN:0094-243X    

DOI:10.1063/1.40052

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The performance of the multilayer ceramic capacitors (MLC) in high frequency power applications has improved significantly over the last years. One of the possible applications of MLC capacitors is the automotive industry where repetitive discharging of capacitors is required. A 0.25‐&mgr;F capacitor using NPO dielectric subjected to repetitive discharging with the rate of 700 pulses per second, magnitude of 600‐V and 195‐A peak currents showed no degradation in performance at 298 K or 398 K even after 1 billion discharge cycles. Less than a 5‐K temperature rise was observed under these conditions. The most exciting, newly emerging utilization for MLC capacitors, however, might be the high temperature application (up to 473 K for underhood utilization), where ceramic capacitors with higher volumetric efficiency as compared to glass or polymer type capacitors prove very superior. Moreover ceramic capacitors, which next to glass capacitors exhibit the greatest radiation resistance among all insulating materials (Hanks and Hamman 1971), might also be best suited in the future for high temperature operation in space environment. The pulsed power performance of the 0.25‐&mgr;F NPO capacitor was evaluated under repetitive discharge conditions (200 V, 700 pps) at high temperature, 473 K, and the results are presented in this paper.

ISSN:0094-243X    

DOI:10.1063/1.40053

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Limited experimental data exists for the specific core loss and dynamic B‐H loops for soft magnetic materials for the combined conditions of high frequency and high temperature. This experimental study investigates the specific core loss and dynamic B‐H characteristics of a nickel‐iron crystalline magnetic alloy (Supermalloy) and two‐iron‐based amorphous magnetic materials (Metglas 2605S‐3A and Metglas 2605SC) over the frequency range of 1–50 kHz and temperature range of 23–300 C under sinusoidal voltage excitation. The effects of maximum magnetic flux density, frequency, and temperature on the specific core loss and on the size and shape of the B‐H loops are examined. The Supermalloy and Metglas 2605S‐3A and 2605SC data are used to compare the core loss of transformers with identical kVA and voltage ratings.

ISSN:0094-243X    

DOI:10.1063/1.40054

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Two arcjet Power Conditioning Units (PCU’s) were recently tested wtih off‐site arcjet thrusters. The arcjet PCU’s used in these in these tests were designed and built by SPI. The design of PCU I.1 was based on our previous work in arcjet power conditioning funded by AFAL. SDIO‐SBIR funded the development of PCU II, modification of PCU I.1 and PCU I.2, as well as the testing of these PCUs. The first of these recent tests was conducted at NASA Lewis Research Center with a hydrogen arcjet. The second one was conducted in Jet Propulsion Laboratory with an ammonia arcjet. The major thrust of the tests was on thruster performance, so the technical objectives and the interests of these tests were not limited to the PCU performance. SPI’s major objectives of these tests were to demonstrate the stable operation of the PCU’s with arcjet thrusters, to prove the capability of initiating the arc breakdown with its built‐in starter, and to demonstrate the endurance of the PCU. The personnel at NASA LeRC and JPL successfully operated the SPI PCUs with their arcjet and obtained valuable test data.

ISSN:0094-243X    

DOI:10.1063/1.40055

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Tests have been conducted on bench‐scale heat‐pipe solar receivers to explore methods of constructing wicks. The primary objective was to develop a wick structure that was easy to fabricate and was capable of transporting liquid sodium over long distances against hydrostatic and frictional pressure drops. One of the wicks tested in this program was able to lift sodium 45 cm by capillary pumping alone to cool a 3‐cm×10‐cm area subjected to an average flux of 50 W/cm2.

ISSN:0094-243X    

DOI:10.1063/1.40056

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The operation of a heat pipe with both thermal radiation and convection heat rejection has been experimentally examined. The thermal radiation heat rejection conditions are similar to those which would be experienced in a space environment. The experimental results show good agreement with the analytical model.

ISSN:0094-243X    

DOI:10.1063/1.40057

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

A survey of space‐power related liquid metal heat pipe work at Los Alamos National Laboratory is presented. Heat pipe development at Los Alamos has been on‐going since 1963. Heat pipes were initially developed for thermionic nuclear‐electrical power production in space. Since then Los Alamos has developed liquid metal heat pipes for numerous applications related to high temperature systems in both the space and terrestrial environments. Some of these applications include thermionic electrical generators, thermoelectric energy conversion (both in‐core and direct radiation), thermal energy storage, hypersonic vehicle leading edge cooling, and heat pipe vapor laser cells. Some of the work performed at Los Alamos has been documented in internal reports that are often little‐known. A representative description and summary of progress in space‐related liquid metal heat pipe technology is provided followed by a reference section citing sources where these works may be found.

ISSN:0094-243X    

DOI:10.1063/1.40058

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Thermacore, Inc. of Lancaster, Pennsylvania has recently completed Phase I of a development program to investigate the use of layered metal/ceramic composites in the design of low‐mass hardened radiators for space heat rejection systems. This effort evaluated the use of layered composites as a material to form thin‐walled, vacuum leaktight heat pipes. The heat pipes would be incorporated into a large heat pipe radiator for waste heat rejection from a space nuclear power source. This approach forms an attractive alternative to carbon/carbon, or silicon‐carbide fiber reinforced metal heat pipes by offering a combination of low mass and improved fabricability. Thermacore and United Technologies Research Center have jointly developed an approach for fabrication of layered composite thin‐walled heat pipes for use in hardened space radiators. Potassium heat pipes with wall thicknesses as low a 0.3 mm have been built and tested. Wall thicknesses as low as 0.13 mm are believed to be achievable with this approach.

ISSN:0094-243X    

DOI:10.1063/1.40111

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

A very extensive uranium‐loaded graphite fuel element fabrication technology evolved from the nuclear propulsion program started at the Los Alamos Scientific Laboratory (LASL) in 1955. The highly enriched uranium was incorporated in the fuel elements as UO2, pyrocarbon‐coated UC2beads, or as UO2+ZrC particles. Property and compositional studies of raw materials for the extrusion mix had to be coupled with heat treatment studies to obtain optimum fuel element properties.

ISSN:0094-243X    

DOI:10.1063/1.40060

出版商:AIP    

年代:1991

数据来源: AIP