The effect of macroscopic polarization on intrinsic and extrinsic thermal conductivities of AlN

The effect of macroscopic polarization on thermal conductivity of bulk wurtzite AlN has been theoretically investigated. Our results show that macroscopic polarization modifies the phonon group velocity, Debye frequency and Debye temperature of the AlN. Using revised phonon velocity and Debye temperature, various phonon scattering rates and combined scattering rate are calculated as functions of the phonon frequency at room temperature. The intrinsic and extrinsic thermal conductivities of AlN have been estimated using these modified parameters. The theoretical analysis shows that up to a certain temperature the polarization effect acts as negative effect and reduces the intrinsic and extrinsic thermal conductivities. However, after this temperature both thermal conductivities are significantly enhanced. High phonon velocity and Debye temperature are the reason of this enhancement which happens due to the polarization effect. The revised thermal conductivities at room temperature are found to be increased by more than 20% in AlN due to macroscopic polarization phenomenon. The method we have developed can be taken into account during the simulation of heat transport in optoelectronic nitride devices to minimize the self heating processes.     

The effect of phonon anisotropic scattering on the thermal conductivity of silicon thin films at 300K and 400K

Previous studies have shown that anisotropy in phonon transport exist because of the difference in phonon dispersion relation due to different lattice directions, as observed by a difference in in-plane and cross-plane thermal conductivities. Our current work intends to study the effect of anisotropy scattering on silicon thermal conductivity at 300 K and 400 K. We adopt the Henyey and Greenstein probability density function in our phonon Monte Carlo simulation to investigate the effect of highly forward and backward scattering events. The impact of applying the anisotropy scattering using this approach is discussed in detail. While the forward and backward scattering will increase and decrease thermal conductivity respectively, the extent of the effect is non-linear such that forward scattering has a more obvious effect on thermal conductivity than backward scattering.     

Dependence of thermal conductivity on electrical resistivity in Bismuth-Tellurides

The p-type (Bi_0.25Sb_0.75)₂Te₃ doped with 3-12 wt% excess Te alone and n-type Bi₂(Te_0.94Se_0.06)₃ codoped with 0.017-0.026 wt% Te and 0.068-0.102 wt% I were prepared by the Bridgman method, to produce intentionally polycrystalline. Some of the as-grown specimens were annealed, in order to prepare specimens with much different ρ. These polycrystalline specimens have almost the same degree of alignment of the c plane parallel to the freezing direction. The electrical rersistivity ρ and thermal conductivity κ were measured at 298 K along the freezing direction and κ was plotted as a function of ρ. As a result, the lattice components κ_ph obtained by subtracting the electronic component κ_el from the observed κ were found to decrease almost linearly with a decrease of ρ in both p- and n-type specimens, where κ_el was calculated using Wiedemann-Franz law. This tendency is consistent with the conventional result that κ_ph becomes negligible small in metals. The significant decrease in κ_ph with decrease in ρ is considered to be caused predominantly by the phonon scattering due to dopants. The relationship between κ_ph and ρ was first clarified in the intermediate region between the metal and insulator.     

Built-in-polarization and thermal conductivity of InxGa1-xN/GaN heterostructures

The effect of built-in-polarization (BIP) field on thermal properties of In_xGa_1−xN/GaN heterostructure has been investigated. The thermal conductivity k of In_xGa_1−xN alloy has been estimated using Callaway's formula including the BIP field for In content x = 0, 0.1, 0.3, 0.5 and 0.9. This study reports that irrespective of In content, the room temperature k of In_xGa_1−xN/GaN heterostructure is enhanced by BIP field. The result predicts the existence of a characteristic temperature T_p at which both thermal conductivities (including and excluding BIP field) show a crossover. This gives signature of pyroelectric nature of In_xGa_1−xN alloy which arises due to variation of polarization with temperature indicating that thermal conductivity measurement can reveal pyroelectric nature. The pyroelectric transition temperature of In_xGa_1−xN alloy has been predicted for various x. The composition dependent nature of room temperature k for x = 0.1 and 0.5 are in line with prior experimental studies. The result can be used to minimize the self heating effect in In_xGa_1−xN/GaN heterostructures.     

Thermal conductivity for single-walled carbon nanotubes from Einstein relation in molecular dynamics

Equilibrium molecular dynamics based Einstein relation with an appropriate definition for integrated heat current (i.e., with modified energy moment) are combined to quantify the thermal conductivity of individual single-walled carbon nanotubes, armchair, zigzag and chiral tubes. The thermal conductivity has been investigated as a function of three parameters, tube radius, length and chirality at and near room temperature with Brenner potential model. Thermal conductivity is found to have unusually high value and varies with radius, length and chirality of tubes. Also the thermal conductivity at temperature range from 50 to 100 K is found to have a maximum value. For 12.1 nm tube length, the thermal conductivity has converging trend which its value dependents on the tube radius and chirality. Tubes with large radius have lower values of thermal conductivity. Furthermore, the results show that armchair tubes have large values of the thermal conductivity comparing with zigzag and chiral tubes. It seems possible to uncover carbon nanotubes thermal properties based on measurements having heat dependence by adding another methods for calculations.     

Thermal conductivity of hydrogenated amorphous silicon

The thermal conductivity of amorphous silicon thin films is measured in one dimension steady state condition. The experimental method is based on heating the sample front surface and monitoring the temperatures at its two sides. The experiments were carried out in conditions ensuring one-direction heat flow from top to bottom throughout the sample thickness. Sputtered a-Si:H films prepared with different conditions are used in order to investigate the dependence of thermal conductivity on material properties (i.e. hydrogen content and microstructure). The results show that, firstly, amorphous silicon is a very bad thermal conductor material. Secondly, the disorder in the film network plays an important role in thermal conduction. The highly disordered film exhibits the lowest thermal conductivity.     

Effects of non-equilibrium polar optic phonons and the band non-parabolicity on small signal high-frequency hot electrons ac mobility in narrow gap semiconductors in high magnetic fields

The small signal high-frequency ac mobility of hot electrons in n-HgCdTe in the extreme quantum limit at low and high temperatures have been calculated considering the non-equilibrium phonon distribution as well as the thermal phonon distribution .The energy loss rate has been calculated considering only optical phonon scattering while the momentum loss rate has been calculated considering acoustic phonon scattering and piezoelectric scattering together with polar optical phonon scattering and separately considering only the polar optical scattering. The results have been discussed and compared. It has been observed that at 20 K, the normalized mobility considering all the three scattering mechanisms differs appreciably from that considering only the polar optical phonon scattering. However, at 77 K, there is no difference in the normalized mobility. This establishes the fact that at higher temperature, the effect of acoustic phonon scattering and piezoelectric coupling is negligible, compared to the polar optical phonon scattering. So the ac mobility considering only polar optical phonon scattering has been studied at 77 and 20 K. The ac mobility is found to remain constant up to 100 GHz and thereafter it started decreasing at higher frequencies at 77 K whereas the ac mobility reduces at much lower frequencies at lower temperature at lower field. The non-parabolicity of the band structure enhances the normalized mobility.     

Heat conduction in extended X-junctions of single-walled carbon nanotubes

Non-equilibrium molecular dynamics (NEMD) simulations are employed to investigate the longitudinal thermal conductivity of non-orthogonal extended X-junction (EX-junction) of single-walled carbon nanotubes (SWCNTs). Different from standard junctions of SWCNTs, two distinct jumps in the temperature profile around the EX-junction are observed, which are responsible for the larger temperature gradient and reduction in thermal conductivity when compared to standard X-junction. Quantum corrected results show that the longitudinal thermal resistance of the X-junction and EX-junction decreases monotonically with increasing temperature which makes the longitudinal thermal conductivity of the tube with junction less sensitive to temperature above 400 K comparing with the individual pristine tube. The origin of the significant decrease of thermal conductivity of EX-junction is discussed through phonon spectra analysis.     

Screening effect of Ti ions on the electrical conductivity and thermoelectric power of Mg ferrite

The samples Mg_1+xTi_xFe_2−2xO₄ were prepared in a single phase spinel structure as indicated from X-ray analysis. The preference of Mg²⁺ ions to the octahedral site decreases the ratio of the normal spinel in the investigated ferrite where the Mg²⁺ increases on the expense of the Fe³⁺ ions on the same site. The increase in the conductivity was found to be due to thermally activated mobility of charge carriers. The mobility data enhances the use of Verway model of conductivity which depends on the electron exchange between iron ions of different valences located on the same crystallographic sites. The existence of Ti⁴⁺ ions on the octahedral site screens the polarization and decreases the conductivity of the samples. Peculiar behavior was obtained for Ti content of 0.7 and 0.8 due to the presence of secondary phases.     

Thermal conductivity of silver doped lanthanum manganites between 10 and 300 K

Thermal conductivity (λ) of nanocrystalline La_1−xAg_xMnO₃ (x=0.05, 0.15, 0.25, 0.3) pellets prepared by pyrophoric method is reported between 10 and 300 K. Magnitude of thermal conductivity has been found to be strongly influenced by monovalent (Ag) substitution at the La site. Silver doping in LaMnO₃ enhances T_C of the system to ∼299 K. Qualitative nature of the temperature variation of thermal conductivity of the silver substituted lanthanum manganites remains closely similar to that for divalent doped systems. Our analysis demonstrates that in La_1−xAg_xMnO₃ also, the mechanism of heat conduction is predominantly by phonons. The contribution of the electronic part is only ∼1% of the total λ. The spin wave contribution is also estimated close to T_C, which for all the samples lies within ∼2%. At temperatures below ∼100 K, the measured data have been analyzed using phonon relaxation time method and the strengths of the various phonon scattering processes have been estimated. Our analysis further suggests strong influence of phonon scattering by 2D like defects in the thermal conductivity of monovalent doped lanthanum manganites at low temperatures (<70 K) in the ferromagnetic region.     

Solid state electrical conductivity and thermal degradation studies on some phenothiazine derivatives

Electrical conductivity and thermal degradation studies of promethazine hydrochloride (PH); 2-chlorophenothiazine (CP); diethazine hydrochloride (DH) and trifluoperazine dihydrochloride (TFP) are reported. The activation energies are evaluated based on their electrical conductivity study conducted over the temperature range 30-150 °C. These energies for PH, CP, DH and TFP are found to be 0.86, 1.02, 0.68 and 1.08 eV, respectively. The materials are analyzed for the kinetic parameters like the activation energies for decomposition and the Arrhenious pre-exponential factors in their pyrolysis region using Broido's, Coats-Redfern and Horowitz-Metzger methods. Using these factors and the standard equations thermodynamic parameters such as enthalpy, entropy and free energies are calculated. Thermogravimetric study on these phenothiazine derivatives in air indicated that their stabilities are in the order CP>TFP>PH >DH.     

Thermoelectric properties of Ge-doped Bi85Sb15 alloys at low temperatures

Bulk polycrystalline Bi₈₅Sb_15−xGe_x (x=0, 0.5, 1, 1.5, 2) composites were prepared by mechanical alloying followed by pressureless sintering. The thermoelectric properties were studied in the temperature range of 77–300 K. The results indicate that increasing the Ge concentration causes the Seebeck coefficient to change sign from negative to positive. Moreover, it is found that the maximum value of the Seebeck coefficient can be precisely controlled with the Ge concentration. The maximum dimensionless figure of merit reaches 0.07 at 140 K. These results suggest that the preparation of p-type Bi–Sb alloys is possible by using the Ge-doping approach.     

Thermal conductivity, dislocation density and GaN device design

The performance of high power transistor devices is intimately connected to the substrate thermal conductivity. In this study, the relationship between thermal conductivity and dislocation density is examined using the 3 omega technique and free standing HVPE GaN substrates. Dislocation density is measured using imaging cathodoluminescence. In a low dislocation density regime below 10⁵ cm⁻², the thermal conductivity appears to plateau out near 230 W/K m and can be altered by the presence of isotopic defects and point defects. For high dislocation densities the thermal conductivity is severely degraded due to phonon scattering from dislocations. These results are applied to the design of homoepitaxially and heteroepitaxially grown HEMT devices and the efficiency of heat extraction and the influence of lateral heat spreading on device performance are compared.     

Flatness-dependent thermal conductivity of graphene-based composites

Since the graphene nanoplates (GNPs) are usually folded and wrinkled, we propose a factor, flatness ratio, to theoretical analyze the thermal conductivity of GNP composites. An analytical model for the thermal conductivity of GNP composites is presented, which shows an excellent agreement with the experimental data. Theoretical analysis reveals that flatness ratio acts as a dominant role in determining the influence of other factors. We further show that the two-dimensional geometry is the primary factor for GNP outperforming one-dimensional carbon nanotubes as thermal conductive filler, rather than the other factors of thickness, length and interfacial thermal resistance.     

Metal-insulator transition and variable-range hopping conductivity of n-CdSb in magnetic field

Resistivity, ρ, of a II-V group semiconductor n-CdSb doped with In is investigated in pulsed magnetic fields up to and at temperatures . The low-temperature resistivity ρ(T) increasing with T in the range of B<4 T is found to have an upturn around B∼4 T and strong activated behavior at further increase of B. These observations give evidence for magnetic-field-induced metal-insulator transition (MIT). In the insulating side of the MIT, Mott variable-range hopping (VRH) conductivity with two types of asymptotic behavior, ln ρ (T, B)∼T^−3/4B² and ln ρ (T, B)∼(B/T)^1/3, is established in low and high magnetic fields, respectively. The VRH conductivity is analyzed using a model of the near-edge electron energy spectrum established by investigations of the Hall effect. The VRH conductivity is shown to take place over the band tail states of one out of two impurity bands, which for T=0 and B=0 lie above the conduction band edge.     

Microstructure and thermoelectric properties of Zn1−xAlxO ceramics fabricated by spark plasma sintering

Al-doped ZnO powders were synthesized via solid reaction between Zn(OH)₂ and Al(OH)₃ and consolidated by spark plasma sintering (SPS) to fabricate fine-grained Zn_1−xAl_xO ceramics as a thermoelectric material. X-ray diffraction and spectrophotometer experiments revealed that Al doping into ZnO is enhanced by the present process, and consequently the SPS-processed Zn_1−xAl_xO samples show significantly improved electrical conductivity as compared with those prepared via mixing ZnO and Al₂O₃ oxide powders. Because of the combined effect of Al doping and grain refinement, the present Zn_1−xAl_xO ceramics show much lower thermal conductivity, which also results in an enhanced dimensionless figure of merit (ZT), than un-doped ZnO oxides prepared also by SPS.     

Hopping conductivity of Ni-doped p-CdSb in strong magnetic fields

Magnetoresistance (MR) of oriented single crystals of the anisotropic semiconductor p-CdSb doped with 2 at% of Ni is investigated between T=1.5 and 300 K in transversal pulsed magnetic fields up to B=30 T. In fields B∼4-15 T at T below 4.2 K, the resistivity obeys the law ln ρ∼η[B?(B)]^1/2 with ?(B)=a(0)/a(B), where a is the carrier localization radius and parameter η depends on a(0), on the acceptor concentration N_A and on the direction of the magnetic field with respect to the crystallographic axes, but does not depend on T. Such behavior gives evidence for MR realized by hopping charge transfer over the nearest-neighbor sites in strong magnetic field. The analysis of the experimental data yields the values of η, agreeing with calculated ones within an error of 10%, taking into account the effects of the anisotropy of the acceptor states and of the explicit dependence of a(B) due to the increase in the activation energy of shallow acceptors in magnetic field and the sensitivity of the metal-insulator transition to B.     

Low temperature specific heat and thermal conductivity of bulk metallic glass (Cu50Zr50)94Al6

The low temperature specific heat and thermal conductivity of (Cu₅₀Zr₅₀)₉₄Al₆ bulk metallic glass have been studied experimentally. A low temperature anomaly in the specific heat is observed in this alloy. It is also found that in addition to Debye oscillators, the localized vibration modes whose vibration density of state has a Gaussian distribution should be considered to explain the low temperature phonon specific heat anomaly. The phonon thermal conductivity dependence on temperature for the sample does not show apparent plateau characteristics as other glass materials do; however, the influence of the resonant scattering from the localized modes on the lattice thermal conductivity is prominent in the bulk metallic glass at low temperatures.     

A first-principles study of vibrational modes in Cu2O and Ag2O crystals

A first-principles investigation of cuprite crystals (Cu₂O and Ag₂O) has been performed. For Cu₂O, the calculated frequencies at the Γ point of the Brillouin zone are in very good agreement with the experimental frequencies. For Ag₂O, the presence of E_u and F_2u vibrational modes with negative frequencies indicates a low temperature phase transition, in agreement with recent high resolution X-ray and neutron diffraction measurements. The energy scanning along these two modes shows a double-well potential, within which only the Ag atoms vibrate. As a result, the origin of the phase transition can be attributed to displacive disorder of the Ag atoms.