Grain Boundary Scattering Research Articles

Enhancement of Thermoelectric Performance for InTe by Selective Substitution and Grain Size Modulation

The different masses, ionic radii, and chemical valences of the nonequivalent crystallographic sites of thermoelectric (TE) compounds provide an effective way to modulate the thermoelectric performance by selective substitution. In this work, the selective substitution of In+ by Pb for the binary InTe material monotonically reduces the carrier concentration, which is greatly beneficial to the mechanism investigation of serious grain boundary scattering (GBS). This is the first time this point has been mentioned with regard to InTe material. As a result, we found that GBS was dominated by the grain size when the carrier concentration was higher than 0.7 × 1019 cm−3 but was inversely governed by the carrier concentration when the carrier was situated at a lower density. In particular, the occupation of Pb on the targeted In+ site could further reduce the lattice thermal conductivity. Finally, In0.9999Pb0.0001Te achieved the improved power factor and average zT value, which could contribute to high-power generation below a medium temperature. This effect of increasing the carrier concentration on the suppression of GBS sheds light on the possibility of improving electron mobility by increasing the carrier concentration.

Low-Threshold pentacene OTFT by using NdTaON gate dielectric and ITO gate electrode

Ta-doped Nd oxynitrides (NdxTa(1-x)ON) are adopted as gate dielectrics in pentacene organic thin-film transistors on ITO glass for transparent applications. The device with Nd0.76Ta0.24ON can achieve the optimal performance with a high carrier mobility of 2.32 cm2/V·s and a small threshold voltage of −0.21 V. The capacitance–voltage results indicate that besides producing a high permittivity (high-k) for the gate dielectric, Ta incorporation in NdON can generate negative fixed charges in the oxide to assist the external gate voltage to produce a small threshold voltage. The threshold voltage is further reduced by the ITO gate electrode because ITO with higher work function can decrease the energy barrier for the charge-carrier injection at the source/channel interface. XPS and AFM reveal that the moisture resistance of pure Nd oxynitride can be enhanced by Ta doping to improve the smoothness of the dielectric surface, which results in weaker surface-roughness and grain-boundary scatterings, and consequently higher carrier mobility. The carrier mobility is further enhanced by the ITO gate electrode because similar to metal-gate/high-k stack, metallic ITO gate with higher electron concentration than Si gate can offer stronger screening effect on the carrier scattering induced by the severe atomic oscillation in the high-k NdTaON gate dielectric.

Microscopic mechanism of unusual lattice thermal transport in TlInTe2

We investigate the microscopic mechanism of ultralow lattice thermal conductivity (κl) of TlInTe2 and its weak temperature dependence using a unified theory of lattice heat transport, that considers contributions arising from the particle-like propagation as well as wave-like tunneling of phonons. While we use the Peierls–Boltzmann transport equation (PBTE) to calculate the particle-like contributions (κl(PBTE)), we explicitly calculate the off-diagonal (OD) components of the heat-flux operator within a first-principles density functional theory framework to determine the contributions (κl(OD)) arising from the wave-like tunneling of phonons. At each temperature, T, we anharmonically renormalize the phonon frequencies using the self-consistent phonon theory including quartic anharmonicity, and utilize them to calculate κl(PBTE) and κl(OD). With the combined inclusion of κl(PBTE), κl(OD), and additional grain-boundary scatterings, our calculations successfully reproduce the experimental results. Our analysis shows that large quartic anharmonicity of TlInTe2 (a) strongly hardens the low-energy phonon branches, (b) diminishes the three-phonon scattering processes at finite T, and (c) recovers the weaker than T−1 decay of the measured κl.

Strong electron-phonon coupling induced anomalous phonon transport in ultrahigh temperature ceramics ZrB2 and TiB2

Ultrahigh temperature ZrB2- and TiB2-based ceramics are widely used in extreme thermal environment. Yet, open questions remain pertaining to their lattice thermal conductivity (кph). In this work we investigate the phonon transport of ZrB2 and TiB2 by systematically evaluating the phonon-phonon interaction (PPI), electron-phonon interaction (EPI) and grain boundary scattering (GBS) from the atomistic level using first-principles. Upon including EPI, the room-temperature кph of ZrB2 and TiB2 is significantly reduced by 38.16% and 52.34%, respectively, and agrees excellently with experimental measurement. Such giant reduction arises from the strong EPI for the heat-carrying acoustic phonons due to phonon anomaly and the existence of Fermi nesting vectors along high symmetry line in the Brillouin zone. Following the Casimir model, the GBS further decreases кph of ZrB2 even by 49.27% for small grain boundary spacing of 50 nm and theoretical calculations agree well with experiments. Thus, GBS crucially influences phonon transport, which explains the large deviation of previous experimental measurements on кph for ZrB2-based ceramics. Moreover, the combined influence of EPI and GBS results in the anomalous phonon transport where кph is almost temperature-independent over a large temperature range, consistent with experimental observations. This work directly reveals the phonon transport mechanism for high temperature ceramics ZrB2 and TiB2, gains deep insight into the large variation in the previously reported кph and provides guidance to engineer it for practical applications.

Ultrashort pulse-induced elastodynamics in polycrystalline materials. Part I: Model validation

The coupled thermal-mechanical response of a polycrystalline metallic film in response to ultrafast optical impingement is investigated. The thermo-elastodynamics formulated in the article considers laser absorption along the axial direction and thermal diffusion along the radial direction to account for the normal and shear stresses initiated by the rapid heating. The generalized formulation incorporates a two-step hyperbolic temperature model that characterizes the energy transport of electrons and lattices as finite in velocity. The coupling of thermal and mechanical fields is established through considering the energy dissipated in the form of propagating thermo-mechanical disturbances. The thermo-elastodynamical response of the polycrystalline film is found to be a strong function of the electron heat capacity that is also temperature-dependent. In addition, grain size effects due to film surface and grain-boundary scatterings are found to impact several thermophysical properties of the material. The impact of the energy transport of electrons is particularly prominent when the thickness becomes comparable with the electron mean-free-path. A staggered-grid finite difference scheme is followed to simultaneously resolve the coupled thermo-elastodynamical responses using an axisymmetric model. The time variation of the normalized electron temperature of a single crystalline gold film derived from the generalized model is favorably examined against published physical data, thus demonstrating the feasibility of the formulation in depicting the electron transport dynamics in response to non-ablating ultrafast irradiation.

Angle dependent magnetotransport in transfer-free amorphous carbon thin films

Amorphous carbon thin films were fabricated on a SiO2 substrate using a chemical vapor deposition (CVD) technique. The structural and surface morphology of the films were analyzed by Raman spectrometry and high resolution transmission electron microscopy (HRTEM), respectively. The atomic ratio % of C(sp2) and C(sp3) bonds was estimated using x-ray photoelectron spectroscopy (XPS). The film shows angular magnetoresistance (MR) of 18% and 1.6% at 2 K and room temperature respectively. The mechanism of this angular MR was discussed by use of a grain boundary scattering (GBS) model.

Lattice thermal conductivity of UO2 using ab-initio and classical molecular dynamics

We applied the non-equilibrium ab-initio molecular dynamics and predict the lattice thermal conductivity of the pristine uranium dioxide for up to 2000 K. We also use the equilibrium classical molecular dynamics and heat-current autocorrelation decay theory to decompose the lattice thermal conductivity into acoustic and optical components. The predicted optical phonon transport is temperature independent and small, while the acoustic component follows the Slack relation and is in good agreement with the limited single-crystal experimental results. Considering the phonon grain-boundary and pore scatterings, the effective lattice thermal conductivity is reduced, and we show it is in general agreement with the sintered-powder experimental results. The charge and photon thermal conductivities are also addressed, and we find small roles for electron, surface polaron, and photon in the defect-free structures and for temperatures below 1500 K.

Tensoresistive effect in thin metal films in the range of elastic and plastic strain

The tensoresistive effect in single-layer Cr, Fe, Mo, Ni, and Pd films and double-layer Fe/Cr, Cu/Cr, Ni/Cr, and Pd/Fe films is studied in the range of elastic and plastic strain. The influence of the surface and grain-boundary scatterings of electrons on strain sensitivity Sρ of the single-layer films is analyzed. Sensitivities Sρ of the single- and double-layer films to elastic and plastic strains are compared. It is found that the volume and grain-boundary scatterings of electrons are main contributors to Sρ in the Fe/Cr film system. Grain-boundary scattering is equally effective under elastic and plastic strain, as judged by a high coefficient of electron grain-boundary scattering (R ≌ 0.36–0.41).

Thin Metal Film Sensors

During the last decade some progress have been made in the field of sensors using thin film techniques. In particular thin metal film strain gauges and thin film temperature sensors based on the temperature dependent resistivity of metal are now commonly used. But changes in other transport parameters with various measurands are also useful for the design of metal film sensors. Difficulty arises in thin film techniques when structural defects are frozen in films.Intensive theoretical investigations are carried out to explain the effect of grain-boundary and external surface scatterings on transport parameters. Accordingly the main results are presented to specify the influence of film structure on the sensor performance. The grain-boundary effects are discussed according to applications of metal film sensors. Theoretical predictions are analyzed in terms of sensitivity, thermal stability and long term behavior. But other problems induced by the presence of grain boundaries or point defects are also discussed, in particular problems associated with bulk diffusion, electromigration induced failures or intrinsic stresses.