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At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS1975

Finite difference methods for transient signal propagation in stratified dispersive media

Explicit difference equations are presented for the solution of a signal of arbitrary waveform propagating in an ohmic dielectric, a cold plasma, a Debye model dielectric, and a Lorentz model dielectric. These difference equations are derived from the governing time-dependent integro-differential equations for the electric fields by a finite difference method. A special difference equation is derived for the grid point at the boundary of two different media. Employing this difference equation, transient signal propagation in an inhomogeneous media can be solved provided that the medium is approximated in a step-wise fashion. The solutions are generated simply by marching on in time. It is concluded that while the classical transform methods will remain useful in certain cases, with the development of the finite difference methods described, an extensive class of problems of transient signal propagating in stratified dispersive media can be effectively solved by numerical methods.

Lam, D. H.↗
NASA NTRS1978

Quantum theory of laser-stimulated desorption

A quantum theory of laser-stimulated desorption (LSDE) is presented and critically analyzed. It is shown how LSDE depends on laser-pulse characteristics and surface-lattice dynamics. Predictions of the theory for a Debye model of the lattice dynamics are compared to recent experimental results.

Slutsky, M. S.↗
NASA NTRS1983

Low frequency dielectric relaxation in boracites

In order to elucidate the nature of the imperfections which adversely affect pyroelectric processes in boracites, the thermal and dispersive characteristics of the low frequency dielectric response in iron-iodide boracite (Fe3B7O13I) and copper-chloride boracite (Cu3B7O13Cl) have been investigated. These characteristics were measured as a function of crystallographic orientation and applied field in both the ferroelectric and paraelectric states. The low frequency dielectric relaxation of 100 line oriented multi-domain copper-chloride boracite clearly indicates the dipole nature of the lattice imperfections. The activation energies calculated from a noninteracting Debye model, are 0.53 eV in the ferroelectric phase and 0.10 eV in the paraelectric phase.

Kim, Q.↗
NASA NTRS1989

Pressure dependence of the melting temperature of metals

A new method for the analysis of the experimental data for the pressure dependence of the melting temperature of metals is presented. The method combines Lindemann's law, the Debye model, and a first-order equation of state with the experimental observation that the Grueneisen parameter divided by the volume is constant. It is observed that, based on these assumptions, in the absence of phase transitions, plots of the logarithm of the normalized melting temperature versus the logarithm of the normalized pressure are straight lines. It is found that the normalized-melting--temperature versus normalized-pressure curves accurately satisfy the linear relationship for Al, Ag, Au, Cs, Cu, K, Na, Pt, and Rb. In addition, this technique provides a sensitive tool for detecting phase transitions.

Schlosser, Herbert↗
NASA NTRS2011

Ultra-Sensitive Transition-Edge Sensors (TESs) for Far-IR/Submm Space-Borne Spectroscopy

We have built surface micromachined thin-film metallized Si(x)N(y) optical absorbers for transition-edge sensors (TESs) suitable for the Background - Limited far-IR/Submm Spectrograph (BLISS). BLISS is a broadband (38 micrometers - 433 micrometers), grating spectrometer consisting of five wavebands each with a modest resolution of R (is) approx. 1000. Because BLISS requires the effective noise equivalent power (NEP) of the TES to be below 10 (exp 19) W/Hz(exp 1/2), our TESs consist of four long (1000 micrometers), narrow (0.4 micrometers ), and thin (0.25 micrometers ) Si(x) N(y) support beams that reduce the thermal conductance G between the substrate and the optical absorber. To reduce the heat capacity of the absorber and make the response time tau fast enough for BLISS, our absorbers are patterned into a mesh geometry with a fill factor of less than 10%. We use a bilayer of Ti/Au to make the effective impedance of the absorber match the impedance of the incoming radiation for each band. Measurements of the response time of the metallized absorbers to heat pulses show that their heat capacity exceeds the predictions of the Debye model. Our results are suggestive that the surface of the low pressure chemical vapor deposition (LPCVD) Si(x)N(y) used in the absorbers' construction is the source of microstates that dominate the heat capacity.

far-IR spectrometer↗
NASA NTRS2021

Specific Heat of Epoxies and Mixtures Containing Silica, Carbon Lamp Black, and Graphite

We present specific heat measurements in the 2 – 20 K range of several commercially available epoxies (Stycast 1266, Stycast 2850FT, EPO-TEK 377, and EPO-TEK 377H) and mixtures of one of these epoxy binders and fillers such as silica, carbon lamp black, and graphite. The results are briefly described in the context of the Debye model.

Thermosetting polymers↗
NASA NTRS1993

Plasma-screening effects on the electron-impact excitation of hydrogenic ions in dense plasmas

Plasma-screening effects are investigated on electron-impact excitation of hydrogenic ions in dense plasmas. Scaled cross sections Z(exp 4) sigma for 1s yields 2s and 1s yields 2p are obtained for a Debye-Hueckel model of the screened Coulomb interaction. Ground and excited bound wave functions are modified in the screened Coulomb potential (Debye-Hueckel model) using the Ritz variation method. The resulting atomic wave functions and their eigenenergies agree well with the numerical and high-order perturbation theory calculations for the interesting domain of the Debye length not less than 10. The Born approximation is used to describe the continuum states of the projectile electron. Plasma screening effects on the atomic electrons cannot be neglected in the high-density cases. Including these effects, the cross sections are appreciably increased for 1s yields 2s transitions and decreased for 1s yields 2p transitions.

Jung, Young-Dae↗
DOE OSTI2020

Impact of impurity-based phonon resonant scattering on thermal conductivity of single crystalline GaN

The impact of impurities on the thermal conductivity of halide vapor phase epitaxy gallium nitride (GaN) was studied. Phonon resonances with impurities, modeled as Lorentz oscillators, were used to explain the much lower thermal conductivity than predicted by the Debye–Callaway model. The resonance energies for the oscillators were determined by Raman spectroscopy for Mn and by mass difference approximation for C and Fe. Employing the obtained resonance energies and proportionality factors extracted as fitting parameters, the modified model showed a good agreement with the experimental data. While the doping decreased thermal conductivity for all temperatures, the room temperature values started decreasing significantly once the doping levels approached ~10 19 cm -3 . Consequently, required doping levels to achieve certain GaN-based devices may reduce the thermal conductivity of GaN by as much as 1/3.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Ab Initio Study of the Electronic Structure, Elastic Properties, Magnetic Feature and Thermodynamic Properties of the Ba{sub 2}NiMoO{sub 6} Material

We report first-principles calculations of the elastic properties, electronic structure and magnetic behavior performed over the Ba{sub 2}NiMoO{sub 6} double perovskite. Calculations are carried out through the full-potential linear augmented plane-wave method within the framework of the Density Functional Theory (DFT) with exchange and correlation effects in the Generalized Gradient and Local Density Approximations, including spin polarization. The elastic properties calculated are bulk modulus (B), the elastic constants (C{sub 11}, C{sub 12} and C{sub 44}), the Zener anisotropy factor (A), the isotropic shear modulus (G), the Young modulus (Y) and the Poisson ratio (υ). Structural parameters, total energies and cohesive properties of the perovskite are studied by means of minimization of internal parameters with the Murnaghan equation, where the structural parameters are in good agreement with experimental data. Furthermore, we have explored different antiferromagnetic configurations in order to describe the magnetic ground state of this compound. The pressure and temperature dependence of specific heat, thermal expansion coefficient, Debye temperature and Grüneisen parameter were calculated by DFT from the state equation using the quasi-harmonic model of Debye. A specific heat behavior C{sub V} ≈ C{sub P} was found at temperatures below T = 400 K, with Dulong–Petit limit values, which is higher than those, reported for simple perovskites.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Phonon gas model for thermal conductivity of dense, strongly interacting liquids

Developing predictive thermal property models for liquids based on microscopic principles has been elusive. The difficulty is that liquids have gas-like and solid-like attributes that are at odds when considering the frameworks of microscopic models: Models for gases are simple due to randomness and low density, whereas models for crystalline solids rely on symmetry and long-range order for easier calculation. The short-range order in liquids does, however, provide structure to neighboring molecules similar to amorphous solids, and there have been recent advances indicating that collective vibrational modes store heat in liquids. Models combining Debye approximations from solid-state physics and Frenkel’s theory of liquids can accurately predict the heat capacity of liquids. Phonon-like dispersions in liquids have also been widely observed in neutron scattering experiments. These developments motivate us to propose a model where high-frequency vibrational modes, which travel at the speed of sound and have a mean free path on the order of the average intermolecular distance, conduct heat in liquids. We use this liquid phonon gas model to calculate the thermal conductivity of liquids with varying intermolecular interaction energies from strongest to weakest—Coulomb, hydrogen-bonding, Keesom, and London dispersion energy. Generally, the model is more accurate as the intermolecular interaction energy and density of liquids increase. The calculated thermal conductivity of Coulombic-bound molten sodium nitrate and hydrogen-bonded water is within 1.46% and 2.98% of the experimentally measured values, respectively, across their entire temperature ranges. Further modal analysis of the velocity and the mean free path of collective vibrations could establish the liquid phonon gas model as an accurate model for weakly interacting liquids as well.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Thermal Conductivity of β-Phase Ga 2 O 3 and (Al x Ga 1– x ) 2 O 3 Heteroepitaxial Thin Films

Heteroepitaxy of β-phase gallium oxide (β-Ga 2 O 3 ) thin films on foreign substrates shows promise for the development of next-generation deep ultraviolet solar blind photodetectors and power electronic devices. In this work, the influences of the film thickness and crystallinity on the thermal conductivity of ($\bar{2}01$)-oriented β-Ga 2 O 3 heteroepitaxial thin films were investigated. Unintentionally doped β-Ga 2 O 3 thin films were grown on c-plane sapphire substrates with off-axis angles of 0° and 6° toward $\langle$$11\bar{2}0$$\rangle$ via metal–organic vapor phase epitaxy (MOVPE) and low-pressure chemical vapor deposition. The surface morphology and crystal quality of the β-Ga 2 O 3 thin films were characterized using scanning electron microscopy, X-ray diffraction, and Raman spectroscopy. The thermal conductivities of the β-Ga 2 O 3 films were measured via time-domain thermoreflectance. The interface quality was studied using scanning transmission electron microscopy. The measured thermal conductivities of the submicron-thick β-Ga 2 O 3 thin films were relatively low as compared to the intrinsic bulk value. The measured thin film thermal conductivities were compared with the Debye–Callaway model incorporating phononic parameters derived from first-principles calculations. The comparison suggests that the reduction in the thin film thermal conductivity can be partially attributed to the enhanced phonon-boundary scattering when the film thickness decreases. They were found to be a strong function of not only the layer thickness but also the film quality, resulting from growth on substrates with different offcut angles. Growth of β-Ga 2 O 3 films on 6° offcut sapphire substrates was found to result in higher crystallinity and thermal conductivity than films grown on on-axis c-plane sapphire. However, the β-Ga 2 O 3 films grown on 6° offcut sapphire exhibit a lower thermal boundary conductance at the β-Ga 2 O 3 /sapphire heterointerface. In addition, the thermal conductivity of MOVPE-grown ($\bar{2}01$)-oriented β-(Al x Ga 1–x ) 2 O 3 thin films with Al compositions ranging from 2% to 43% was characterized. Because of phonon-alloy disorder scattering, the β-(Al x Ga 1–x ) 2 O 3 films exhibit lower thermal conductivities (2.8–4.7 W/m∙K) than the β-Ga 2 O 3 thin films. The dominance of the alloy disorder scattering in β-(Al x Ga 1–x ) 2 O 3 is further evidenced by the weak temperature dependence of the thermal conductivity. This work provides fundamental insight into the physical interactions that govern phonon transport within heteroepitaxially grown β-phase Ga 2 O 3 and (Al x Ga 1–x ) 2 O 3 thin films and lays the groundwork for the thermal modeling and design of β-Ga 2 O 3 electronic and optoelectronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2022

A study of electroactive polyvinyl chloride (PVC) gel actuators through the use of the electric modulus formalism and cyclic linear voltage sweeps

The conductivity and dielectric properties are integral to the function of polyvinyl chloride (PVC) gel actuators. The frequency-dependent properties of PVC gel actuators are investigated here in terms of their impedance, permittivity, and for the first time, electric modulus. The data shows that PVC gels’ conductive properties are just as, if not more, important as their dielectric properties in electromechanical transduction applications. The electrode polarization (EP) and its impact on the impedance and dielectric spectra of PVC gels, as well as the developed asymmetric space charge at the anode, are discussed. The electric modulus and tan δ spectra are used for the fitting of Cole-Cole (CC) and Debye relaxation models for gels of varying plasticizer content. The electrostatic adhesive force for PVC gels of varying plasticizer is also measured, indicating large electrostatic adhesion (>2 N cm -2 ). A cyclic linear voltage sweep is used to clarify the dynamics of space charge within the gels. The peak current (associated with space charge development) is seen to be concurrent with the onset of mechanical deformation, showing the asymmetric charge as the origin of electromechanical transduction. Additionally, the maximum charge transferred (as measured by the integration of current over time) before space charge development is found to correlate with the electrostatic adhesive force measured for the gels, pointing to a new method of characterizing PVC gels for actuation.

Instruments & Instrumentation↗
DOE OSTI2024

Computational design of high-entropy rare earth aluminum garnets for advanced thermal and environmental barrier coatings

To enhance the protection of Ni-based superalloys in gas turbine engines’ high-temperature environments, it’s crucial to develop advanced thermal/environmental barrier coating (T/EBC) materials with a balanced combination of thermal and mechanical properties. This optimization is essential to safeguard against chemical and thermal challenges. Here, in this study, we harness the power of density functional theory (DFT) in conjunction with combinatorial chemistry methodologies to engineer high-performance high-entropy rare earth disilicates of the RE 3 Al 5 O 12 family (where RE denotes Y, Gd, Er, and Yb). These materials are meticulously designed to exhibit superior phase stability, a targeted coefficient of thermal expansion (CTE), low lattice thermal conductivity, and robust mechanical properties. The determination of CTE values is accomplished through phonon calculations at various volume settings within the quasi-harmonic approximation, while lattice thermal conductivities are rigorously assessed employing the Debye-Callaway model, accounting for three distinct phonon processes. Our findings highlight the remarkable attributes of the solid solution (Y 1/4 Gd 1/4 Er 1/4 Yb 1/4 ) 3 Al 5 O 12 , which displays a reduction in lattice thermal conductivity compared to its individual constituents while maintaining a favorable range of CTE values. The novel T/EBC material, distinguished by their multifaceted functionalities, are poised to use in substantial enhancements in the performance of engines.

36 MATERIALS SCIENCE↗
DOE OSTI2023

A prediction of the thermodynamic, thermophysical, and mechanical properties of CrTaO 4 from first principles

It is reported that the self-forming CrTaO 4 oxide scale can protect refractory high-entropy alloys from oxidation, superior to Cr 2 O 3 . In this paper, the phase stability, mechanical, and thermal properties of three polymorphous phases of CrTaO 4 are systematically investigated from first-principles density functional theory calculations. The mechanical properties predicted using the strain–energy methods indicated that all three phases are mechanically stable. The temperature dependence of elastic constants and polycrystalline moduli of three phases demonstrated the thermal softening as temperature increase. The Helmholtz-free energies as a function of volume and temperature are derived from phonon dispersions within the quasi-harmonic approximation at six strained volumes. The calculated apparent bulk coefficients of thermal expansion of these three phases are evaluated, the highest value approximately 13.4× 10 –6 K –1 within a temperature range of 500–2000 K for the rutile I4 1 md phase. The lattice thermal conductivity calculated by the Debye–Callaway model suggested that the rutile type I4 1 md phase has the lowest value of approximately 2.1 W/m/K at 1800 K. The other two phases, C2/m and P2/c, exhibit higher values due to relatively lower Grüneisen parameters and larger phonon velocities. The melting point of CrTaO 4 is predicted to be between 1975 and 2449 K using ab initio molecular dynamics simulations. Finally, this work provides a comprehensive theoretical understanding of the thermodynamic, mechanical, and thermal properties for the new material CrTaO 4 and serves as an example of a viable computational design strategy for improved oxidation resistance of refractory alloys at high temperatures.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Ground-State and Thermodynamical Properties of Uranium Mononitride from Anharmonic First-Principles Theory

We report on an advanced density-functional theory (DFT) approach for investigating the ground-state and thermodynamical properties of uranium mononitride (UN). The electronic structure for UN at zero temperature is obtained from DFT that utilizes the generalized gradient approximation (GGA) for the electron exchange and correlation functional and includes spin-orbit interaction and an extension with orbital polarization. Thermodynamical properties are computed within the quasi-harmonic approximation in the Debye–Grüneisen model while anharmonicity is captured in the self-consistent ab initio lattice dynamics (SCAILD) scheme. Anharmonic phonons have heretofore never been modeled from first-principles for UN but they turn out to be important. The computed free energy compares well with that of a CALPHAD (CALculation of PHAse Diagrams) assessment of available experimental data.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Pressure effects on the electronic structure and superconductivity of ( TaNb ) 0.67 ( HfZrTi ) 0.33 high entropy alloy

Effects of pressure on the electronic structure, electron-phonon interaction and superconductivity of the high entropy alloy (TaNb) 0.67 (HfZrTi) 0.33 , is studied in the range from 0 to 100 GPa. Electronic structure is calculated using the Korringa-Kohn-Rostoker method and the coherent potential approximation. Effects of pressure on the lattice dynamics were simulated using the Debye- Gru ¨neisen model, and the Gruneisen parameter at ambient conditions, Debye temperature and Sommerfeld electronic heat capacity coefficient were additionally experimentally determined. The electron-phonon coupling parameter λ is calculated using the McMillan-Hopfield parameters, computed in the rigid muffin tin approximation. We find, that the system undergoes the Lifshitz transition, as one of the bands cross the Fermi level at elevated pressures. The electron-phonon coupling parameter ? decreases above 10 GPa, however, calculated superconducting Tc increases up to 40 - 50 GPa and later is stabilized at the larger value than for the ambient conditions, in agreement with the experimental findings. Our results show, that the evolution of Tc with pressure in (TaNb) 0.67 (HfZrTi) 0.33 can be very well explained by the classical electron-phonon mechanism.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗