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At least 613 records · Page 34
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Giant Isotope Effect of Thermal Conductivity in Silicon Nanowires

Isotopically purified semiconductors potentially dissipate heat better than their natural, isotopically mixed counterparts as they have higher thermal conductivity (κ). But the benefit is low for Si at room temperature, amounting to only ~10% higher κ for bulk 28 Si than for bulk natural Si ( nat Si). Here we show that in stark contrast to this bulk behavior, 28 Si (99.92% enriched) nanowires have up to 150% higher κ than nat Si nanowires with similar diameters and surface morphology. Using a first-principles phonon dispersion model, this giant isotope effect is attributed to a mutual enhancement of isotope scattering and surface scattering of phonons in nat Si nanowires, correlated via transmission of phonons to the native amorphous SiO 2 shell. The Letter discovers the strongest isotope effect of κ at room temperature among all materials reported to date and inspires potential applications of isotopically enriched semiconductors in microelectronics.

36 MATERIALS SCIENCE↗
NASA NTRS1947

Cooling of Gas Turbines I - Effects of Addition of Fins to Blade Tips and Rotor, Admission of Cooling Air Through Part of Nozzles, and Change in Thermal Conductivity of Turbine Components

An analysis was developed for calculating the radial temperature distribution in a gas turbine with only the temperatures of the gas and the cooling air and the surface heat-transfer coefficient known. This analysis was applied to determine the temperatures of a complete wheel of a conventional single-stage impulse exhaust-gas turbine. The temperatures were first calculated for the case of the turbine operating at design conditions of speed, gas flow, etc. and with only the customary cooling arising from exposure of the outer blade flange and one face of the rotor to the air. Calculations were next made for the case of fins applied to the outer blade flange and the rotor. Finally the effects of using part of the nozzles (from 0 to 40 percent) for supplying cooling air and the effects of varying the metal thermal conductivity from 12 to 260 Btu per hour per foot per degree Farenheit on the wheel temperatures were determined. The gas temperatures at the nozzle box used in the calculations ranged from 1600F to 2000F. The results showed that if more than a few hundred degrees of cooling of turbine blades are required other means than indirect cooling with fins on the rotor and outer blade flange would be necessary. The amount of cooling indicated for the type of finning used could produce some improvement in efficiency and a large increase in durability of the wheel. The results also showed that if a large difference is to exist between the effective temperature of the exhaust gas and that of the blade material, as must be the case with present turbine materials and the high exhaust-gas temperatures desired (2000F and above), two alternatives are suggested: (a) If metal with a thermal conductivity comparable with copper is used, then the blade temperature can be reduced by strong cooling at both the blade tip and root. The center of the blade will be less than 2000F hotter than the ends; (b) With low conductivity materials some method of direct cooling other than partial admission of cooling air is essential. From this study, it can be deduced that indirect cooling of turbine blades will not make possible large increases in gas temperature.

Brown, Byron↗
DOE OSTI2025

Modulating thermal conductance at ligand/nanocrystal interfaces via oxygen-coordinated ligands

Here, the interfacial thermal conductance (h lig–NC ) between a cadmium selenide (CdSe) nanocrystal (NC) and three related organic ligands—olealdehyde, oleyl alcohol, and oleic acid—was investigated computationally. These ligands have the same carbon backbone but differ in the number and type of oxygen-coordinated headgroups (carbonyl and/or hydroxyl), leading to distinct bonding geometries involving monodentate and bidentate bonds. For a fully encapsulated NC, h lig–NC increases in the order of olealdehyde, oleic acid, and oleyl alcohol ligands. To isolate the contributions of h lig–NC from each headgroup type, the distinct bonding geometries were analyzed. Aldehyde and alcohol ligands, each featuring a single oxygen headgroup (carbonyl or hydroxyl), exhibit similar O–Cd separations and nearly identical h lig–NC per ligand at full surface coverage. However, the hydroxyl group in the alcohol ligand enables a higher ligand grafting density on the NC surface, resulting in a greater overall h lig–NC than the aldehyde-grafted NCs. In contrast, the oleic acid ligand forms multidentate bonds with the NC, leading to a shorter average O–Cd separation and a higher h lig–NC per ligand compared to monodentate bonds. Nevertheless, steric hindrance from the acid ligand's larger headgroup reduces its grafting density relative to alcohol ligands, ultimately resulting in a lower overall h lig–NC .

Wong, Kae-Lin [Zhejiang Univ., Hangzhou (China)] (↗
NASA NTRS1986

Ideal condensations due to perpendicular thermal conduction in a sheared magnetic field

Cool condensations generated by a radiative thermal instability in a sheared magnetic field have previously been the bases of solar filament formation models. Through the assumption of fully anisotropic heat flow, a new set of condensation modes are here obtained which become singular in the limit of vanishing perpendicular thermal conductivity. The growth rates are noted to typically be greater than those reported previously for sheared field condensations. The fastest growth is exhibited by modes possessing the fewest oscillations.

Van Hoven, G.↗
DOE OSTI2024

Thermal conductance of interfaces between titanium nitride and group IV semiconductors at high temperatures

Measuring the temperature dependence of material properties is a standard method for better understanding the microscopic origins for that property. Surprisingly, only a few experimental studies of thermal boundary conductance at high temperatures exist. This lack of high temperature data makes it difficult to evaluate competing theories for how inelastic processes contribute to thermal conductance. To address this, we report time domain thermoreflectance measurements of the thermal boundary conductance for TiN on diamond, silicon-carbide, silicon, and germanium between 120 and 1000 K. In all systems, the interface conductance increases monotonically without stagnating at higher temperatures. For TiN/SiC interfaces, G ranges from 330 to 1000 MW/m2-K, with a room temperature conductance of 750 MW/m2-K. The interface conductance for TiN/diamond ranges from 140 to 950 MW/m2-K. Notably, for all four interfacial systems, the conductance continues to increase with temperature even after all phonon modes in the vibrationally soft material are thermally excited. This observation suggests that inelastic processes are significant contributors to the thermal conductance in all four interfacial systems, regardless of whether the materials forming the interface are vibrationally similar or dissimilar. Our study fills a notable gap in the literature for how interfacial conductance evolves at high temperatures and tests burgeoning theories for the role of inelastic processes in interfacial thermal transport.

Physics↗
NASA NTRS1991

Low-Thermal-Conduction Links For Silicon Sensors

Simple method of texturing surface of silicon reduces thermal conductivities of links in silicon x-ray calorimeters and infrared bolometers. Gives links high density of phonon scattering sites reducing conduction of heat. Links made shorter and more robust. Used in making x-ray calorimeters and infrared bolometers. Applicable to any microelectronic device in which high degree of thermal isolation needed.

Mott, D. Brent↗