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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,423 records · Page 79
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Tailoring the dipole properties in dielectric polymers to realize high energy density with high breakdown strength and low dielectric loss

High energy density polymer materials are desirable for a broad range of modern power electronic systems. Here, we report the development of a new class of polymer dielectrics based on polyurea and polythiourea, which possess high thermal stability. By increasing the dipole density, the dielectric constant of meta-phenylene polyurea and methylene polythiourea can be increased to 5.7, compared with aromatic polyurea and aromatic polythiourea, which have a dielectric constant in the range of 4.1–4.3. The random dipoles with high dipolar moment and amorphous structure of these polyurea and polythiourea based polymers provide strong scattering to the charge carriers, resulting in low losses even at high electric fields. Consequently, this new class of polymers exhibit a linear dielectric response to the highest field measured (>700 MV/m) with a high breakdown strength, achieving high energy density (>13 J/cm{sup 3}) with high efficiency (>90%)

36 MATERIALS SCIENCE↗
DOE OSTI2015

Very low Schottky barrier height at carbon nanotube and silicon carbide interface

Electrical contacts to silicon carbide with low contact resistivity and high current durability are crucial for future SiC power devices, especially miniaturized vertical-type devices. A carbon nanotube (CNT) forest formed by silicon carbide (SiC) decomposition is a densely packed forest, and is ideal for use as a heat-dissipative ohmic contact in SiC power transistors. The contact resistivity and Schottky barrier height in a Ti/CNT/SiC system with various SiC dopant concentrations were evaluated in this study. Contact resistivity was evaluated in relation to contact area. The Schottky barrier height was calculated from the contact resistivity. As a result, the Ti/CNT/SiC contact resistivity at a dopant concentration of 3 × 10{sup 18 }cm{sup −3} was estimated to be ∼1.3 × 10{sup −4} Ω cm{sup 2} and the Schottky barrier height of the CNT/SiC contact was in the range of 0.40–0.45 eV. The resistivity is relatively low for SiC contacts, showing that CNTs have the potential to be a good ohmic contact material for SiC power electronic devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Time temperature transformation diagram for secondary crystal products of Co-based Co-Fe-B-Si-Nb-Mn soft magnetic nanocomposite

Secondary crystallization is the subject of much investigation in magnetic amorphous and nanocomposites (MANCs) as it limits the long term and thermal stability of their operation in device applications, including power electronics, sensors, and electric motors. Secondary crystal products [Blazquez et al., Philos. Mag. Lett. 82(7), 409–417 (2002); Ohodnicki et al., Phys. Rev. B 78, 144414 (2008); Willard et al., Metall. Mater. Trans. A 38, 725 (2007)], nanostructure and crystallization kinetics [Hsiao et al., IEEE Trans. Magn. 38(5), 3039 (2002); McHenry et al., Scr. Mater. 48(7), 881 (2003)], and onset temperatures and activation energies [Ohodnicki et al., Acta. Mater. 57, 87 (2009); Long et al., J. Appl. Phys. 101, 09N114 (2007)] at constant heating have been reported for similar alloys. However, a time-temperature-transformation (TTT) diagram for isothermal crystallization, more typical of application environments, has not been reported in literature. Here, a TTT diagram for the Co based, Co-Fe-Si-Nb-B-Mn MANC system is presented, along with a method for determining such. The method accounts for the presence of primary crystal phases and yields crystal fraction of secondary phase(s) by using a novel four stage heating profile. The diagram, affirmed by Kissinger activation energy analysis, reports thermal stability of the MANC for millennia at conventional device operating temperatures, and stability limits less than a minute at elevated temperatures. Both extremes are necessary to be able to avoid secondary crystalline products and establish operating limits for this mechanically attractive, high induction soft magnetic nanocomposite.

36 MATERIALS SCIENCE↗
DOE OSTI2015

In-situ NC-AFM measurements of high quality AlN(0001) layers grown at low growth rate on 4H-SiC(0001) and Si(111) substrates using ammonia molecular beam epitaxy

Nitride wide-band-gap semiconductors are used to make high power electronic devices or efficient light sources. The performance of GaN-based devices is directly linked to the initial AlN buffer layer. During the last twenty years of research on nitride growth, only few information on the AlN surface quality have been obtained, mainly by ex-situ characterization techniques. Thanks to a Non Contact Atomic Force Microscope (NC-AFM) connected under ultra high vacuum (UHV) to a dedicated molecular beam epitaxy (MBE) chamber, the surface of AlN(0001) thin films grown on Si(111) and 4H-SiC(0001) substrates has been characterized. These experiments give access to a quantitative determination of the density of screw and edge dislocations at the surface. The layers were also characterized by ex-situ SEM to observe the largest defects such as relaxation dislocations and hillocks. The influence of the growth parameters (substrate temperature, growth speed, III/V ratio) and of the initial substrate preparation on the dislocation density was also investigated. On Si(111), the large in-plane lattice mismatch with AlN(0001) (19%) induces a high dislocation density ranging from 6 to 12×10{sup 10}/cm{sup 2} depending on the growth conditions. On 4H-SiC(0001) (1% mismatch with AlN(0001)), the dislocation density decreases to less than 10{sup 10}/cm{sup 2}, but hillocks appear, depending on the initial SiC(0001) reconstruction. The use of a very low growth rate of 10 nm/h at the beginning of the growth process allows to decrease the dislocation density below 2 × 10{sup 9}/cm{sup 2}.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Crystallographic and magnetic properties of the spinel-type ferrites Zn{sub x}Co{sub 1-x}Fe{sub 2}O{sub 4} (0.0 ≤ x ≤ 0.75)

Ultrahigh frequencies (UHF) have applications in signal and power electronics to minimize product sizes, increase production quantity and lower manufacturing cost. In the UHF range of 300 MHz to 3 GHz, ferrimagnetic iron oxides (ferrites) are especially useful because they combine the properties of a magnetic material with that of an electrical insulator. Ferrites have much higher electrical resistivity than metallic ferromagnetic materials, resulting in minimization of the eddy current losses, and total penetration of the electromagnetic (EM) field. Hence ferrites are frequently applied as circuit elements, magnetic storage media like read/write heads, phase shifters and Faraday rotators. The electromagnetic properties of ferrites are affected by operating conditions such as field strength, temperature and frequency. The spinel system Zn{sub x}Co{sub 1-x}Fe{sub 2}O{sub 4} (x=0.0, 0.25, 0.50 and 0.75) has been prepared by the standard solid state sintering method. X-ray and neutron powder diffraction measurements were performed at room temperature. Neutron diffraction data analysis confirms the cubic symmetry corresponding to the space group Fd3m. The distribution of three cations Zn{sup 2+}, Co{sup 2+} and Fe{sup 3+} over the spinel lattice and other crystallographic parameters like lattice constant, oxygen position parameter, overall temperature factor and occupancies of different ions in different lattice sites for the samples have been determined from the analysis of neutron diffraction data. The lattice constant increases with increasing Zn content in the system. The magnetic structure was found to be ferrimagnetic for the samples with x≤0.50. Magnetization measurements show that with the increase of Zn content in the system the value of saturation magnetization first increases and then decreases. The variation of the magnetic moment with Zn substitution has been discussed in terms of the distribution of magnetic and non-magnetic ions over the A and B sub-lattices and their exchange coupling.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Excellent performances of energy harvester using cantilever driving double-clamped 0.7Pb(Mg{sub 1/3}Nb{sub 2/3})O{sub 3}-0.3PbTiO{sub 3} plates and symmetric middle-stops

We present a high performance nonlinear piezoelectric energy harvester constituted by a cantilever with symmetrically middle-stops and double-clamped piezoelectric plates based on piezoelectric single crystal 0.7Pb(Mg{sub 1/3}Nb{sub 2/3})O{sub 3}-0.3PbTiO{sub 3}. Electrical properties of the device under different excitation frequencies, accelerations, and load resistances are studied systematically. Under a low acceleration of 3 m/s{sup 2} (0.3 g), a peak voltage of 26.2 V and a maximum normalized power of 25.6 mW/g{sup 2} were obtained across a matching impedance of 600 kΩ with favorable bandwidths. The low excitation acceleration and excellent performances indicate that the device can be a promising candidate for energy harvesting in low-power electronics and wireless sensors.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

High-mobility BaSnO{sub 3} grown by oxide molecular beam epitaxy

High-mobility perovskite BaSnO{sub 3} films are of significant interest as new wide bandgap semiconductors for power electronics, transparent conductors, and as high mobility channels for epitaxial integration with functional perovskites. Despite promising results for single crystals, high-mobility BaSnO{sub 3} films have been challenging to grow. Here, we demonstrate a modified oxide molecular beam epitaxy (MBE) approach, which supplies pre-oxidized SnO{sub x}. This technique addresses issues in the MBE of ternary stannates related to volatile SnO formation and enables growth of epitaxial, stoichiometric BaSnO{sub 3}. We demonstrate room temperature electron mobilities of 150 cm{sup 2} V{sup −1} s{sup −1} in films grown on PrScO{sub 3}. The results open up a wide range of opportunities for future electronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Electrically controlled pinning of Dzyaloshinskii-Moriya domain walls

We propose a method to all-electrically control a domain-wall position in a ferromagnetic nanowire with Dzyaloshinskii-Moriya interaction. The strength of this interaction can be controlled by an external electric field, which in turn allows a fine tuning of the pinning potential of a spin-spiral domain wall. It allows to create more mobile pinning sites and can also be advantageous for ultra-low power electronics.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Formation of high electrical-resistivity thin surface layer on carbonyl-iron powder (CIP) and thermal stability of nanocrystalline structure and vortex magnetic structure of CIP

This study focuses on the carbonyl-iron powder (CIP) used in the metal composite bulk magnetic core for high-efficient/light-weight SiC/GaN power device MHz switching dc-dc converter, where the fine CIP with a mean diameter of 1.1 μm is used to suppress the MHz band eddy current inside the CIP body. When applying the CIP to composite core together with the resin matrix, high electrical resistivity layer must be formed on the CIP-surface in order to suppress the overlapped eddy current between adjacent CIPs. In this study, tens nm thick silica (SiO{sub 2}) was successfully deposited on the CIP-surface by using hydrolysis of TEOS (Si(OC{sub 2}H{sub 5}){sub 4}). Also tens nm thick oxidized layer of the CIP-surface was successfully formed by using CIP annealing in dry air. The SiC/GaN power device can operate at ambient temperature over 200 degree-C, and the composite magnetic core is required to operate at such ambient temperature. The as-made CIP had small coercivity below 800 A/m (10 Oe) due to its nanocrystalline-structure and had a single vortex magnetic structure. From the experimental results, both nanocrystalline and single vortex magnetic structure were maintained after heat-exposure of 250 degree-C, and the powder coercivity after same heat-exposure was nearly same as that of the as-made CIP. Therefore, the CIP with thermally stable nanocrystalline-structure and vortex magnetic state was considered to be heat-resistant magnetic powder used in the iron-based composite core for SiC/GaN power electronics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Enhancement-mode operation of multilayer MoS{sub 2} transistors with a fluoropolymer gate dielectric layer

Enhancement-mode multilayer molybdenum disulfide (MoS{sub 2}) field-effect transistors (FETs), which are an immensely important component toward low-power electronics based on a two-dimensional layered semiconductor, are demonstrated using the fluoropolymer CYTOP as a gate dielectric. The fabricated devices exhibit threshold voltage (V{sub TH}) of ∼5.7 V with field-effect mobility (μ{sub FE}) of up to 82.3 cm{sup 2}/V s, and the characteristics are compared with the depletion-mode characteristics of MoS{sub 2} FETs with the cross-linked Poly(4-vinylphenol) gate dielectric (V{sub TH} ∼ −7.8 V). UV photoelectron spectroscopy analysis indicates that increased surface potential due to the surface dipole effect of the fluorine group influences the positive V{sub TH} shift.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Adaptive smart simulator for characterization and MPPT construction of PV array

Partial shading conditions are among the most important problems in large photovoltaic array. Many works of literature are interested in modeling, control and optimization of photovoltaic conversion of solar energy under partial shading conditions, The aim of this study is to build a software simulator similar to hard simulator and to produce a shading pattern of the proposed photovoltaic array in order to use the delivered information to obtain an optimal configuration of the PV array and construct MPPT algorithm. Graphical user interfaces (Matlab GUI) are built using a developed script, this tool is easy to use, simple, and has a rapid of responsiveness, the simulator supports large array simulations that can be interfaced with MPPT and power electronic converters.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Sequential control of step-bunching during graphene growth on SiC (0001)

We have investigated the relation between the step-bunching and graphene growth phenomena on an SiC substrate. We found that only a minimum amount of step-bunching occurred during the graphene growth process with a high heating rate. On the other hand, a large amount of step-bunching occurred using a slow heating process. These results indicated that we can control the degree of step-bunching during graphene growth by controlling the heating rate. We also found that graphene coverage suppressed step bunching, which is an effective methodology not only in the graphene technology but also in the SiC-based power electronics.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Proton irradiation effects on minority carrier diffusion length and defect introduction in homoepitaxial and heteroepitaxial n-GaN [Proton irradiation effects on minority carrier diffusion length and defect introduction in homoepitaxial n-GaN]

Here, inherent advantages of wide bandgap materials make GaN-based devices attractive for power electronics and applications in radiation environments. Recent advances in the availability of wafer-scale, bulk GaN substrates have enabled the production of high quality, low defect density GaN devices, but fundamental studies of carrier transport and radiation hardness in such devices are lacking. Here, we report measurements of the hole diffusion length in low threading dislocation density (TDD), homoepitaxial n-GaN, and high TDD heteroepitaxial n-GaN Schottky diodes before and after irradiation with 2.5 MeV protons at fluences of 4–6 × 10 13 protons/cm 2 . We also characterize the specimens before and after irradiation using electron beam-induced-current (EBIC) imaging, cathodoluminescence, deep level optical spectroscopy (DLOS), steady-state photocapacitance, and lighted capacitance-voltage (LCV) techniques. We observe a substantial reduction in the hole diffusion length following irradiation (50%–55%) and the introduction of electrically active defects which could be attributed to gallium vacancies and associated complexes (V Ga -related), carbon impurities (C-related), and gallium interstitials (Ga i ). EBIC imaging suggests long-range migration and clustering of radiation-induced point defects over distances of ~500 nm, which suggests mobile Ga i . Following irradiation, DLOS and LCV reveal the introduction of a prominent optical energy level at 1.9 eV below the conduction band edge, consistent with the introduction of Ga i .

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2018

On compensation in Si-doped AlN

Controllable n-type doping over wide ranges of carrier concentrations in AlN, or Al-rich AlGaN, is critical to realizing next-generation applications in high-power electronics and deep UV light sources. Silicon is not a hydrogenic donor in AlN as it is in GaN; despite this, the carrier concentration should be controllable, albeit less efficiently, by increasing the donor concentration during growth. At low doping levels, an increase in the Si content leads to a commensurate increase in free electrons. Problematically, this trend does not persist to higher doping levels. In fact, a further increase in the Si concentration leads to a decrease in free electron concentration; this is commonly referred to as the compensation knee. While the nature of this decrease has been attributed to a variety of compensating defects, the mechanism and identity of the predominant defects associated with the knee have not been conclusively determined. Density functional theory calculations using hybrid exchange-correlation functionals have identified V Al + n Si Al complexes as central to mechanistically understanding compensation in the high Si limit in AlN, while secondary impurities and vacancies tend to dominate compensation in the low Si limit. The formation energies and optical signatures of these defects in AlN are calculated and utilized in a grand canonical charge balance solver to identify carrier concentrations as a function of Si content. The results were found to qualitatively reproduce the experimentally observed compensation knee. Furthermore, these calculations predict a shift in the optical emissions present in the high and low doping limits, which is confirmed with detailed photoluminescence measurements.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Nanoscale groove textured β -Ga 2 O 3 by room temperature inverse metal-assisted chemical etching and photodiodes with enhanced responsivity

β-Ga 2 O 3 is an emerging wide band-gap semiconductor that holds great promise for next generation power electronics and optoelectronics. β-Ga 2 O 3 based ultraviolet photodetectors have been the subject of active research for the last few years. However, no micro and nanostructure surface texturing has been demonstrated for efficient light management in β-Ga 2 O 3 optoelectronic applications yet. We hereby present nanoscale groove textured β-Ga 2 O 3 metal-semiconductor-metal photodiodes, enabled by the unique metal-assisted chemical etching (MacEtch) method at room temperature in liquid. Although the textured surface stoichiometry shows ~10% oxygen deficiency which results in a reduced Schottky barrier height and increased dark current, clear enhancement of the responsivity is demonstrated, compared to the planar untreated surface. Overall, the realization of MacEtch's applicability to β-Ga 2 O 3 opens the door for producing more sophisticated device structures for this material, without resorting to conventional dry etch and potential damage.

42 ENGINEERING↗
DOE OSTI2019

Deep acceptors and their diffusion in Ga 2 O 3

β–Ga2O 3 is a wide-bandgap material with promising applications in high-power electronics. While n-type doping is straightforward, p-type doping is elusive, with only deep acceptors available. We explore the properties of these acceptors, from the point of view of achieving stable semi-insulating layers, which are essential in many device structures. Using hybrid density functional theory, we obtain the comprehensive first-principles results for a variety of deep-acceptor impurities in Ga 2 O 3 . Among the impurities examined, nitrogen on an oxygen site and magnesium on a gallium site have particularly low formation energies, making them prime candidates for acceptor doping. Closer inspection of various configurations shows that Mg can incorporate not only on Ga sites (where it acts as a deep acceptor under n-type conditions) but also on O sites, where it acts as a deep donor. Mg interstitials adopt a split-interstitial configuration, sharing a site with a host Ga atom. Similarly, N substituting on an O site acts as a compensating center, but N can also incorporate on the Ga site. We evaluate the diffusivities of these species in the crystal by calculating migration barriers and considering which native defects assist in diffusion. We find that diffusion of N is dominantly assisted by O vacancies, while Mg diffusion is assisted by gallium interstitials. Diffusion of Mg proceeds with significantly lower activation energies than diffusion of N. Our results can be used to assess activation energies and diffusion mechanisms for other impurities in Ga 2 O 3 .

36 MATERIALS SCIENCE↗
DOE OSTI2019

Thermal conductivity of crystalline AlN and the influence of atomic-scale defects

Aluminum nitride (AlN) plays a key role in modern power electronics and deep-ultraviolet photonics, where an understanding of its thermal properties is essential. In this work, we measure the thermal conductivity of crystalline AlN by the 3 ω method, finding that it ranges from 674 ± 56 Wm –1 K –1 at 100 K to 186 ± 7 Wm –1 K –1 at 400 K, with a value of 237 ± 6 Wm –1 K –1 at room temperature. We compare these data with analytical models and first-principles calculations, taking into account atomic-scale defects (O, Si, C impurities, and Al vacancies). We find that Al vacancies play the greatest role in reducing thermal conductivity because of the largest mass-difference scattering. Modeling also reveals that 10% of heat conduction is contributed by phonons with long mean free paths (MFPs), over ~7 μ m at room temperature, and 50% by phonons with MFPs over ~0.3 μ m. Consequently, the effective thermal conductivity of AlN is strongly reduced in submicrometer thin films or devices due to phonon-boundary scattering.

42 ENGINEERING↗
DOE OSTI2019

Thermal expansion coefficients of high thermal conducting BAs and BP materials

Recently reported very high thermal conductivities in cubic boron arsenide (BAs) and boron phosphide (BP) crystals could potentially provide a revolutionary solution in the thermal management of high power density devices. To fully facilitate such an application, the compatible coefficient of thermal expansion (CTE) between the heat spreader and the device substrate, in order to minimize the thermal stress, needs to be considered. Here, we report our experimental CTE studies of BAs and BP in the temperature range from 100 K to 1150 K, through a combination of X-ray single crystal diffraction and neutron powder diffraction. Here, we demonstrated that the room temperature CTEs, 3.6 ± 0.15 × 10 –6 /K for BAs and 3.2 ± 0.2 × 10 –6 /K for BP, are more compatible with most of the semiconductors including Si and GaAs, in comparison with diamond, and thus could be better candidates for the future heat spreader materials in power electronic devices.

36 MATERIALS SCIENCE↗