Showing papers on "Power density published in 1996"

Status of silicon carbide (SiC) as a wide-bandgap semiconductor for high-temperature applications: A review

Abstract: Silicon carbide (SiC), a material long known with potential for high-temperature, high-power, high-frequency, and radiation hardened applications, has emerged as the most mature of the wide-bandgap (2.0 eV ≲ Eg ≲ 7.0 eV) semiconductors since the release of commercial 6HSiC bulk substrates in 1991 and 4HSiC substrates in 1994. Following a brief introduction to SiC material properties, the status of SiC in terms of bulk crystal growth, unit device fabrication processes, device performance, circuits and sensors is discussed. Emphasis is placed upon demonstrated high-temperature applications, such as power transistors and rectifiers, turbine engine combustion monitoring, temperature sensors, analog and digital circuitry, flame detectors, and accelerometers. While individual device performances have been impressive (e.g. 4HSiC MESFETs with fmax of 42 GHz and over 2.8 W mm−1 power density; 4HSiC static induction transistors with 225 W power output at 600 MHz, 47% power added efficiency (PAE), and 200 V forward blocking voltage), material defects in SiC, in particular micropipe defects, remain the primary impediment to wide-spread application in commercial markets. Micropipe defect densities have been reduced from near the 1000 cm−2 order of magnitude in 1992 to 3.5 cm−2 at the research level in 1995.

High performance direct methanol polymer electrolyte fuel cells

Abstract: Direct methanol fuel cells (DMFCs) using Pt‐Ru electrocatalysts and perfluorosulfonic acid membranes provide high performances if operated above 100°C with optimized catalyst layers. A decal transfer method is used to apply thin‐film catalyst/ionomer composite layers to Nation® membranes. A Nation 112 membrane/electrode assembly operating on 5 atm oxygen at 130°C yields a current of 670 mA/cm2 at 0.5 V cell voltage. Peak power density is 400 mW/cm2. The same cell operating on 3 atm air at 110°C yields 370 mA/cm2 at 0.5 V and provides a maximum power density of 250 mW/cm2.

Silicon carbide high-power devices

Abstract: In recent years, silicon carbide has received increased attention because of its potential for high-power devices. The unique material properties of SiC, high electric breakdown field, high saturated electron drift velocity, and high thermal conductivity are what give this material its tremendous potential in the power device arena. 4H-SiC Schottky barrier diodes (1400 V) with forward current densities over 700 A/cm/sup 2/ at 2 V have been demonstrated. Packaged SITs have produced 57 W of output power at 500 MHz, SiC UMOSFETs (1200 V) are projected to have 15 times the current density of Si IGBTs (1200 V). Submicron gate length 4H-SiC MESFETs have achieved f/sub max/=32 GHz, f/sub T/=14.0 GHz, and power density=2.8 W/mm @ 1.8 GHz. The performances of a wide variety of SiC devices are compared to that of similar Si and GaAs devices and to theoretically expected results.

Method employing UV laser pulses of varied energy density to form depthwise self-limiting blind vias in multilayered targets

Abstract: The output of a continuously pumped, Q-switched, Nd:YAG laser (10) is frequency converted to provide ultraviolet light (62) for forming vias (72, 74) in targets (40) having metallic layers (64,68) and a dielectric layer (66) The invention employs a first laser output of high power density to ablate the metallic layer and a second laser output of a lower power density to ablate the dielectric layer The parameters of the output pulses (62) are selected to facilitate substantially clean, sequential drilling or via formation These parameters typically include at least two of the following criteria: power density first above and then below the ablation threshold of the conductor, wavelength less than 400 nm, a temporal pulse width shorter than about 100 nanoseconds, and a repetition rate of greater than about one kilohertz The ability to generate ultraviolet light output pulses at two power densities facilitates the formation of depthwise self-limiting blind vias in multilayer targets, such as a target composed of a layer dielectric material covered on either surface by a layer of metal

Measured microwave power performance of AlGaN/GaN MODFET

Abstract: We report the first microwave power measurement on GaN FET's. At 2 GHz, a class A output power density of 1.1 W/mm with a power added efficiency of 18.6% was obtained on a 1 /spl mu/m gate-length AlGaN/GaN MODFET. Mathematical simulation estimated that the transistor was operating at a channel temperature of 360/spl deg/C as a result of the poor thermal conductivity of the sapphire substrate. Despite this serious heating problem, the power output density still rivals GaAs MESFET's.

Invited paper Observation of plasma shielding by measuring transmitted and reflected laser pulse temporal profiles

Abstract: Mass ablation rate increases with laser power density following a power law dependence and a significant change occurs at 0.3 GW/cm $^2$ . A reflected laser temporal profile was measured from a brass sample. When the power density is greater than 0.3 GW/cm $^2$ +, the temporal profile changes. The transmitted laser-pulse temporal profile through a glass sample also was measured. When the power density is greater than 0.3 GW/cm $^2$ , the later part of laser pulse becomes truncated. The power density at which the laser temporal profile changes for each case is same as the power density that the mass ablation rate coefficient changes. The ablated mass can absorb incoming laser radiation through inverse Bremsstrahlung. The mass becomes thermally ionized and opaque to the incident radiation, preventing laser light from reaching the surface. A model based on thermal evaporation and inverse Bremsstrahlung absorption was developed. Calculations show that plasma shielding occurs at approximately 0.3 GW/cm $^2$ . The experiments and model suggest that the significant change observed in mass ablation rate coefficient is caused by plasma shielding.

Tin sulphide films deposited by plasma-enhanced chemical vapour deposition

Abstract: Tin sulphide thin films have been prepared by the plasma enhanced chemical vapour deposition technique, using and as source materials. For given values of the deposition pressure, relative gas flow rates and deposition time, the plasma power density and the substrate temperature were varied in the ranges from 0.023 to 0.080 W and from 100 to 300 respectively. The deposited films show an orthorhombic crystalline structure for the entire plasma power range and for substrates temperatures higher than 150. From the studied optical properties, considering that this is an indirect energy bandgap material, the energy bandgap is calculated to have a value of 1.16 eV; the phonon involved in the electronic transition has an energy of 0.18 eV. From the measurements of electrical conductivity as a function of temperature an activation energy of 0.3 eV was determined with a p-type electrical conductivity.

Energy efficiency of NO removal by pulsed corona discharges

Abstract: Pulsed positive corona discharges are used to remove NO from the flue gas of a methane burner. At low power input this leads to an increase in NO2, which shows that the process is oxidative. Removal efficiency is greatest when discharges are produced with high-voltage pulses, which are shorter in duration than the time required by the primary streamers to cross the discharge gap, in combination with a dc bias. Other important parameters are input power density and residence time. The best result obtained so far is an energy consumption of 20 eV per NO molecule removed, at 50% deNOx i.e., a removal of 150 ppm NOx, using a residence time of 15 s and an input power density, of 3.5 Wh/Nm3. [Wh/Nm3 stands for watt-hour per normal cubic meter, i.e., at normal conditions (273 K and 1 bar). This implies that 1 Nm3 contains 2.505 1025 molecules.] There appears to be room for improvement by the addition of gaseous and particulate chemicals or the use of multiple corona treatment. It is argued front comparison between results from models and experiments that the direct production of OH by the discharge is only the initiation of the cleaning process.

Maximum power density analysis of an irreversible Joule - Brayton engine

Abstract: A performance analysis based on a power density criterion has been carried out for an irreversible Joule - Brayton (JB) heat engine. The results obtained were compared with those of a power performance criterion. It is shown that design parameters at maximum power density lead to smaller and more efficient JB engines than an engine working at maximum power output conditions. Due to irreversibilities in the heat engine, the power and thermal efficiency will reduce by a certain amount, however the maximum power density conditions still give a better performance than at the maximum power output conditions. The analysis demonstrated in this paper may provide a basis for the determination of optimal operating conditions and the design parameters for real JB heat engines.

Long-pulse and CW tests of a 110-GHz gyrotron with an internal, quasi-optical converter

Abstract: A high-power gyrotron, employing an internal converter that produces a Gaussian-like output mode, has been designed and tested. The tube employed a TE/sub 22.6.1/-mode interaction cavity that was designed for operation at a frequency of 110 GHz. An internal converter, consisting of an advanced launcher design and four mirrors, produced a Gaussian mode that had a relatively uniform profile at the tube output window to minimize the peak power density. Tests on the tube resulted in output power levels of 680, 530, and 350 kW for pulse durations of 0.5, 2.0, and 10.0 s, respectively. Measurements of the temperature of the output window were made during the long-pulse tests. Output power levels of 1 MW were achieved under short-pulse (1 ms) operation and the tube was operated at CW power levels in excess of 100 kW.

Regenerative gas turbines at maximum power density conditions

Abstract: A new kind of power analysis has recently been presented which is based on the maximization of the power density and predicts smaller and more efficient non-regenerative Joule - Brayton engines than those designed at maximum power. In this paper we apply the power density maximization method to regenerative gas turbines using a theoretical framework where the optimal operating conditions of the heat engine are expressed in terms of the isentropic efficiencies of the compressor and turbine and of the heat exchanger efficiency. It is shown that, unlike non-regenerative results, real regenerative gas turbines are less efficient at maximum power density conditions than at maximum power conditions.

Measured performance of a high-power-density microfabricated transformer in a DC-DC converter

Abstract: Experimental measurements are reported on microfabricated power conversion transformers. A device fabricated using a simplified 'sandwich' process confirms predicted performance in an 8 MHz DC-DC power converter, where it achieves a power-handling density of 22.4 W/cm/sup 2/ at 61% efficiency. Preliminary test results are reported for transformers based on a closed-core design, which is expected to achieve higher efficiency.

Method and device for vehicle fuel cell dynamic power control

Abstract: A method and an apparatus are provided for controlling the power of an electric drive unit in a vehicle. The drive unit is supplied with electrical energy by a fuel cell in the vehicle. On the basis of a power request which is determined from the accelerator pedal position, the air flow rate required to provide the set power from the fuel cell is calculated and set by controlling rotational speed of a compressor arranged in the air intake line to the fuel cell. To prevent the fuel cell from producing more electrical power than the drive unit can absorb, the drive unit acts limits the power request by emitting appropriate error messages. The set value for the power is fed to the drive unit and can be corrected such that the drive unit never demands more power than the amount of power instantaneously produced by the fuel cell to prevent fuel cell collapse.

D-band Si-IMPATT diodes with 300 mW CW output power at 140 GHz

Abstract: Silicon flat profile double drift (DD) and double Read (DLHL) IMPATT diodes have been designed and fabricated, using a novel beamlead process. The goal for the design of the active layer is a maximum negative resistance of the device for a given DC power density. Experimental results are 225 mW of RF power with DD diodes and 300 mW for DLHL diodes, both in the 140 GHz range.

Applying distributed power modules in telecom systems

Abstract: The design of modern decentralized power systems in telecommunication applications are increasingly realized by using distributed DC/DC power modules. Power modules are usually characterized by high switching frequencies, which enable the use of small ferrite magnetics and ceramic capacitors to enhance the reliability, the power density, and the possibility of live insertion, all desired features in new power system designs. However, to utilize the advantages of power modules there are a number of considerations to be made in the design. These considerations are addressed focusing on telecom systems with an average power dissipation of <10 W/board, in free convection, i.e., nonforced convection, and <80 W/board, in forced convection cooled cabinets of commonly used mechanical sizes and designs for telecommunication equipment.

The H-mode in COMPASS-D

Abstract: H-mode confinement is observed in COMPASS-D in ohmic and ECR heated single null divertor configuration discharges over a wide parameter range. At low densities the L - H transition power threshold shows a sharp increase, in contrast to scaling law behaviour. Transition to ELM-free H-mode is better correlated with a critical value of plasma pressure than edge power density. In experiments influencing H-mode dynamics, edge current density is found to be destabilizing, suggesting the possible role of the peeling mode. ELM characteristics are discussed, and impurity rotation observations and derived radial electric fields presented.

Method of repairing metallic articles by energy beam deposition with reduced power density

Abstract: A filler material is melted into a metallic substrate having a defect under conditions chosen to preclude cracking. In a preferred embodiment of the invention, a laser beam is operated at a relatively low power density and at a relatively large diameter for an extended length of time to produce a molten pool with a low aspect ratio.

Power supply method for electric motor car and its equipment

Abstract: PROBLEM TO BE SOLVED: To reduce the size and weight of a power supply equipment and moreover increase the discharge capacity and output by installing 2 kinds of storage batteries to electric motor cars and supplying power by selecting one of the batteries depending on the value of a discharge current requested. SOLUTION: A power supply equipment 1 for electric motor car has two kinds of storage batteries: a battery A with a large energy density, and a battery B with a large output density, and power can be supplied by selecting one of the batteries depending on the value of the discharge current requested. That is, if the detected information from discharge current detection means 10 exceeds a predetermined value, then a change-over switch SWa opens and a change-over switch Swb closes, power is supplied from the battery B to a load 4 and, if it is lower than the predetermined value, then the change-over switch SWb is opened, charge-over switch SWa is closed, and power is supplied from the battery A to the load 4. The switching of the batteries A and B is indicated on a power supply switching means 23. As started above, the compact, lightweight power source having large discharge power and output can be obtained for the electric motor car.

Polar distribution of ablated atomic material during the pulsed laser deposition of Cu in vacuum: Dependence on focused laser spot size and power density

Abstract: Experiments have been carried out to investigate the polar distribution of atomic material ablated during the pulsed laser deposition of Cu in vacuum. Data were obtained as functions of focused laser spot size and power density. Thin films were deposited onto flat glass substrates and thickness profiles were transformed into polar atomic flux distributions of the form f(θ)=cosn θ. At constant focused laser power density on target, I=4.7±0.3×108 W/cm2, polar distributions were found to broaden with a reduction in the focused laser spot size. The polar distribution exponent n varied from 15±2 to 7±1 for focused laser spot diameter variation from 2.5 to 1.4 mm, respectively, with the laser beam exhibiting a circular aspect on target. With the focused laser spot size held constant at φ=1.8 mm, polar distributions were observed to broaden with a reduction in the focused laser power density on target, with the associated polar distribution exponent n varying from 13±1.5 to 8±1 for focused laser power density va...

Containerless method of producing crack free metallic articles by energy beam deposition with reduced power density

Abstract: A containerless method of producing a crack free metallic article of near-net shape includes melting a filler material into a metallic substrate or seed under conditions chosen to preclude cracking In a preferred embodiment of the invention, a laser beam is operated at a relatively low power density and at a relatively large beam diameter at the substrate surface for an extended length of time to produce a molten pool with a low aspect ratio Near-net shape is achieved by applying the process in a closed-loop, multi-axis material deposition system

5 mm high-power-density dual-delta-doped power HEMT's for 3 V L-band applications

Abstract: A high-power-density dual-/spl delta/-doped AlGaAs-InGaAs-GaAs high electron mobility transistor (HEMT) for personal communication applications has been developed. A 5.0 mm gate-width device operating at a drain bias of 3.0 V gave an output power over 1 W. The 1 /spl mu/m gate-length HEMT exhibited a current density of 425 mA/mm at V/sub gs/=0.5 V. The maximum transconductance of the device was 270 mS/mm. The effective knee voltage was as low as 0.3 V. At the class AB operation, the HEMT demonstrated an output power density of 200 mW/mm, 64% power-added efficiency and 18.2 dB linear gain at 900 MHz. This is the highest power density of a dual-/spl delta/-doped AlGaAs-InGaAs-GaAs HEMT reported to date for low voltage (3 V) wireless applications.

H-modes on Alcator C-Mod

Abstract: H-modes exhibiting improved confinement above the L-mode are achieved in Alcator C-Mod with ICRF and with ohmic heating alone without boronization. Both ELM-free and ELMy H-modes are obtained with total input power from 0.75 to 4.2 MW over a range of densities (0.8 to ) and toroidal fields (3 to 8 T). Type III ELMs are often observed to have coherent, high m and n precursor oscillations with frequencies of 100 - 160 kHz. The threshold power required to achieve the H-mode increases with density and toroidal field, in rough agreement with scalings derived from other tokamaks. The power densities and density times toroidal field products are an order of magnitude larger than in other tokamaks, in the range of values expected for ITER. The L - H and H - L transitions occur at approximately the same edge electron temperature. A low density limit to the H-mode is found at about . A high midplane neutral pressure limit of about 0.6 mTorr is also observed.

High-Power High-Temperature Heterobipolar TransistorWith Gallium Nitride Emitter

Abstract: A new heterobipolar transistor was made with the wide bandgap semicon-ductors gallium nitride (GaN) and silicon carbide (SiC). The heterojunction allows high injection efficiency, even at elevated temperatures. A record current gain of ten million was obtained at room temperature, decreasing to 100 at 535°C. An Arrhenius plot of current gain vs 1/T yields an activation energy of 0.43 eV that corresponds to the valence band barrier blocking the escape of holes from the base to the emitter. This activation energy is approximately equal to the difference of energy gaps between emitter and base. This Transistor can operate at high power without cooling. A power density of 30 KW/cm2 was sustained.

Ion production by lasers using high–power densities in a near infrared region

Abstract: Results are presented of experiments on ion production from Ta targets using a short pulse (350-600 ps in focus) illumination with focal power densities exceeding 10 14 Wcm -2 at the wavelength of an iodine photodissociation laser (1.315 μm) and its harmonics. Strong evidence of the existence of tantalum ions with the charge state +45 near the target surface was obtained by X-ray spectroscopy methods. The particle diagnostics point to the existence of frozen high charge states ( 4 MeV) for the highest observed charge states. A tentative theoretical explanation of the observed anomalous charge state freezing phenomenon in the expanding plasma produced by a subnanosecond laser pulse is given.