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Decomposing Electronic and Lattice Contributions in Optical Pump–X-ray Probe Transient Inner-Shell Absorption Spectroscopy of CuO
摘要： Electronic and lattice contributions to picosecond time-resolved X-ray absorption spectra (trXAS) of CuO at the oxygen K-edge are analyzed by comparing trXAS spectra, recorded using excitation wavelengths of 355 nm and 532 nm, to steady-state, temperature-dependent XAS measurements. The trXAS spectra at pump-probe time-delays ≥150 ps are dominated by lattice heating effects. Insight into the temporal evolution of lattice temperature profiles on timescales up to 100’s of nanoseconds after laser excitation are reported, on an absolute temperature scale, with a temporal sensitivity and a spatial selectivity on the order of 10’s of picoseconds and 10’s of nanometers, respectively, effectively establishing an 'ultrafast thermometer'. In particular, for the 532 nm experiment at ~5 mJ/cm2 fluence, both the initial sample temperature and its dynamic evolution are well captured by a one-dimensional thermal energy deposition and diffusion model. The thermal conductivity k = (1.3±0.4) W m–1 K–1 derived from this model is in good agreement with the literature value for CuO powder kpowder = 1.013 W m–1 K–1. For 355 nm excitation, a quantitative analysis of the experiments is hampered by the large temperature gradients within the probed sample volume owing to the small UV penetration depth. The impact of the findings on mitigating or utilizing photoinduced lattice temperature changes in future X-ray free electron laser (XFEL) experiments is discussed.
关键词： X-ray absorption spectroscopy,Ultrafast X-ray spectroscopy,CuO,lattice heating,thermal conductivity
更新于2025-09-19 17:15:36
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Phonon scattering by dislocations in GaN
摘要： Crystal imperfections such as dislocations strongly influence the performance and thermal transport behavior of GaN-based devices. We show that the experimental data used to parameterize the effect of dislocations on the thermal conductivity can be explained using only the reported film thickness and point defect concentrations. The analysis highlights the boundary-scattering-governed reduction of thermal conductivity in GaN, which had been underestimated in earlier models. To quantify the influence of dislocations on the thermal transport in GaN, we adopt a Green’s function approach based on accurate ab-initio interatomic force constants. While calculations at the level of density functional theory are necessary for 3-phonon and point defect scattering, we show that scattering due to dislocations can be satisfactorily approximated using semiempirical potentials. This makes the Green’s function approach to dislocation scattering a quantitatively predictive, yet computationally practical, method for obtaining detailed phonon scattering rates.
关键词： Dislocation,Phonon Scattering,Density Functional Theory,GaN,Thermal Conductivity
更新于2025-09-19 17:15:36
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Bulk polycrystalline ceriaa??doped Al <sub/>2</sub> O <sub/>3</sub> and YAG ceramics for high-power density laser-driven solid-state white lighting: Effects of crystallinity and extreme temperatures
摘要： Here, we develop and characterize high thermal conductivity/high thermal shock-resistant bulk Ce-doped Al2O3 and propose it as a new phosphor converting capping layer for high-powered/high-brightness solid-state white lighting (SSWL). The bulk, dense Ce:Al2O3 ceramics have a 0.5 at.% Ce:Al concentration (significantly higher than the equilibrium solubility limit) and were produced using a simultaneous solid-state reactive current activated pressure-assisted densification (CAPAD) approach. Ce:Al2O3 exhibits a broadband emission from 400 to 600 nm, which encompasses the entire blue and green portions of the visible spectrum when pumped with ultraviolet (UV) light that is now commercially available in UV light–emitting devices and laser diodes (LD). These broadband phosphors can be used in the commonly used scheme of mixing with other UV-converting capping layers that emit red light to produce white light. Alternatively, they can be used in a novel composite down-converter approach that ensures improved thermal–mechanical properties of the converting phosphor capping layer. In this configuration, Ce:Al2O3 is used with proven phosphor conversion materials such as Ce:YAG as an active encapsulant or as a capping layer to produce SSWL with an improved bandwidth in the blue portion of the visible spectrum. To study the effect of crystallinity on the Ce photoluminescent (PL) emission, we synthesize Ce:YAG ceramics using high-pressure CAPAD at moderate temperatures to obtain varying crystallinity (amorphous through fully crystalline). We investigate the PL characteristics of Ce:Al2O3 and Ce:YAG from 295 to 4 K, revealing unique crystal field effects from the matrix on the Ce dopants. The unique PL properties in conjunction with the superior thermal–mechanical properties of Ce:Al2O3 can be used in high-powered/high-brightness–integrated devices based on high-efficiency UV-LD that do not suffer efficiency droop at high drive currents to pump the solid-state capping phosphors.
关键词： thermal conductivity,photoluminescent emission,high-power density,Ce-doped Al2O3,thermal shock-resistant,crystal field effects,solid-state white lighting,laser-driven,Ce:YAG
更新于2025-09-19 17:13:59
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Thermal Analysis and Operational Characteristics of an AlGaN/GaN High Electron Mobility Transistor with Copper-Filled Structures: A Simulation Study
摘要： In this study, we investigated the operational characteristics of AlGaN/GaN high electron mobility transistors (HEMTs) by applying the copper-filled trench and via structures for improved heat dissipation. Therefore, we used a basic T-gate HEMT device to construct the thermal structures. To identify the heat flow across the device structure, a thermal conductivity model and the heat transfer properties corresponding to the GaN, SiC, and Cu materials were applied. Initially, we simulated the direct current (DC) characteristics of a basic GaN on SiC HEMT to confirm the self-heating effect on AlGaN/GaN HEMT. Then, to verify the heat sink effect of the copper-filled thermal structures, we compared the DC characteristics such as the threshold voltage, transconductance, saturation current, and breakdown voltage. Finally, we estimated and compared the lattice temperature of a two-dimensional electron gas channel, the vertical lattice temperature near the drain-side gate head edge, and the transient thermal analysis for the copper-filled thermal trench and via structures. Through this study, we could optimize the operational characteristics of the device by applying an effective heat dissipation structure to the AlGaN/GaN HEMT.
关键词： thermal conductivity,high electron mobility transistor,GaN,self-heating effect,copper-filled structure
更新于2025-09-16 10:30:52
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Perspective: Ga <sub/>2</sub> O <sub/>3</sub> for ultra-high power rectifiers and MOSFETS
摘要： Gallium oxide (Ga2O3) is emerging as a viable candidate for certain classes of power electronics with capabilities beyond existing technologies due to its large bandgap, controllable doping, and the availability of large diameter, relatively inexpensive substrates. These applications include power conditioning systems, including pulsed power for avionics and electric ships, solid-state drivers for heavy electric motors, and advanced power management and control electronics. Wide bandgap (WBG) power devices offer potential savings in both energy and cost. However, converters powered by WBG devices require innovation at all levels, entailing changes to system design, circuit architecture, qualification metrics, and even market models. The performance of high voltage rectifiers and enhancement-mode metal-oxide field effect transistors benefits from the larger critical electric field of β-Ga2O3 relative to either SiC or GaN. Reverse breakdown voltages of over 2 kV for β-Ga2O3 have been reported, either with or without edge termination and over 3 kV for a lateral field-plated Ga2O3 Schottky diode on sapphire. The metal-oxide-semiconductor field-effect transistors fabricated on Ga2O3 to date have predominantly been depletion (d-mode) devices, with a few demonstrations of enhancement (e-mode) operation. While these results are promising, what are the limitations of this technology and what needs to occur for it to play a role alongside the more mature SiC and GaN power device technologies? The low thermal conductivity might be mitigated by transferring devices to another substrate or thinning down the substrate and using a heatsink as well as top-side heat extraction. We give a perspective on the materials’ properties and physics of transport, thermal conduction, doping capabilities, and device design that summarizes the current limitations and future areas of development. A key requirement is continued interest from military electronics development agencies. The history of the power electronics device field has shown that new technologies appear roughly every 10-12 years, with a cycle of performance evolution and optimization. The older technologies, however, survive long into the marketplace, for various reasons. Ga2O3 may supplement SiC and GaN, but is not expected to replace them.
关键词： MOSFETs,β-Ga2O3,rectifiers,power electronics,thermal conductivity,Gallium oxide,Ga2O3,doping,wide bandgap semiconductors,military electronics
更新于2025-09-16 10:30:52
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zT = 1.1 in CuInTe <sub/>2</sub> Solid Solutions Enabled by Rational Defect Engineering
摘要： In this study, the synthesis and thermoelectric performance of CuInTe2?In2Te3 and Cu0.85Ag0.15InTe2?In2Te3 solid solutions are reported. The experimental results associated with model fitting reveal that the cation vacancies generated by creating solid solutions with a compound with a smaller cation-to-anion ratio can strongly scatter phonons with high frequency, which remarkably decrease the total as well as lattice thermal conductivity of the CuInTe2 system and finally realize an enhanced thermoelectric properties compared to the pristine sample. Furthermore, substitutional Ag/Cu defects integrated with vacancies lead to a further reduction in lattice thermal conductivity. With the benefit from the rational defect design, a high figure of merit of 1.1 is gained at 840 K for the sample (Cu0.85Ag0.15InTe2)0.98?(In2Te3)0.02. In addition, a 188% improvement on average zT is obtained. This work provides an effective method for boosting thermoelectric performance of chalcopyrite compounds by defect engineering on multiple types of defects.
关键词： defect engineering,thermoelectric performance,vacancy phonon scattering,thermal conductivity,CuInTe2
更新于2025-09-12 10:27:22
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Role on nanofluids in cooling solar photovoltaic cell to enhance overall efficiency
摘要： The electrical conversion efficiency of Solar photovoltaic module strongly depends on the operating temperature. Only up to 20% of the incident solar irradiation is converted to electricity and rest energy either reflected back to atmosphere or converted into heat and thereby increasing the cell temperature. As the cell temperature rises, the electrical efficiency decreases. There is need to cool down temperature of the solar photovoltaic panel to get maximum electrical conversion efficiency. Researchers have implemented many techniques for cooling solar PV by active and passive cooling technique. Active cooling uses air, water, nanofluids etc for cooling where as in passive cooling uses of phase change materials like paraffin wax, eutectics, organic materials, cotton wick etc for the cooling purpose. Nanofluids are used to bring down the temperature of the panel by increasing the thermal conductivity of base fluids. In this paper, emphasis has been given to present the effect of nanofluids as coolant in solar photovoltaic panel.
关键词： Nanofluids,Electrical conversion efficiency,Solar Photovoltaics,Thermal conductivity,Overall efficiency
更新于2025-09-12 10:27:22
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Effect of Laser Ablation on Microwave Attenuation Properties of Diamond Films
摘要： Thermal conductivity is required for developing high-power microwave technology. Diamond has the highest thermal conductivity in nature. In this study, a diamond ?lm was synthesized by microwave plasma chemical deposition, and then long and short conductive graphite ?bers were introduced to the diamond ?lms by laser ablation. The permittivity of the samples in the K-band was measured using the transmission/re?ection method. The permittivity of diamond ?lms with short graphite ?bers increased. The increase in real part of permittivity can be attributed to electron polarization, and the increase in the imaginary part can be ascribed to both polarization and electrical conductivity. The diamond ?lms with long graphite ?bers exhibited a highly pronounced anisotropy for microwave. The calculation of microwave absorption shows that re?ection loss values exceeding ?10 dB can be obtained in the frequency range of 21.3–23.5 GHz when the graphite ?ber length is 0.7 mm and the sample thickness is 2.5 mm. Therefore, diamond ?lms can be developed into a microwave attenuation material with extremely high thermal conductivity.
关键词： high thermal conductivity,laser ablation,diamond ?lms,microwave attenuation,permittivity
更新于2025-09-12 10:27:22
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Life span and overall performance enhancement of Solar Photovoltaic cell using water as coolant: A recent review
摘要： In today’s world, as electricity consumption is increasing, people are more dependent on electricity. Solar Photovoltaic system is one method to generate electricity. The conversion efficiency of solar photovoltaic panel depends on atmospheric condition and reflection. The operating temperature of photovoltaic module plays an important role in performance of PV system as efficiency of PV system decreases when temperature module increases. The operating photovoltaic cells at high temperature degrades the material of it in long time. Operating solar photovoltaic at lower temperature will increase its lifespan. This will reduce module surface area by increasing overall output power. Researchers have proposed and tested several cooling techniques for the panel. One of the most common and effective way to cool PV module is used of water as coolant. In this paper, efforts made by various researchers to cool down solar photovoltaic module to increase the efficiency using water application have been discussed. The application of water on front surface, rear surface, both front and rear surface have been reviewed extensively. The performance of module by immersion in water also have been reported.
关键词： Nanofluids,Electrical conversion efficiency,Solar Photovoltaics,Thermal conductivity,Overall efficiency
更新于2025-09-12 10:27:22
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A Review on Improving the Quality of Perovskite Films in Perovskite Solar Cells via the Weak Forces Induced by Additives
摘要： It is a challenge to design intelligent thermal metamaterials due to the lack of suitable theories. Here we propose a kind of intelligent thermal metamaterials by investigating a core-shell structure, where both the core and shell have an anisotropic thermal conductivity. We solve Laplace’s equation for deriving the equivalent thermal conductivity of the core-shell structure. Amazingly, the solution gives two coupling relations of conductivity tensors between the core and the shell, which cause the whole core-shell structure to counterintuitively self-?x a constant isotropic conductivity even when the area or volume fraction of the core changes within the full range in two or three dimensions. The theoretical ?ndings on fraction-independent properties are in sharp contrast with those predicted by the well-known e?ective medium theories, and they are further con?rmed by our laboratory experiments and computer simulations. This work o?ers two coupling relations for designing intelligent thermal metamaterials, and they are not only helpful for thermal stabilization or camou?age/illusion, but they also o?er hints on how to achieve similar metamaterials in other ?elds.
关键词： Laplace’s equation,intelligent thermal metamaterials,core-shell structure,anisotropic thermal conductivity,self-fixing behavior
更新于2025-09-11 14:15:04