Transient Thermoelectric Effect Research Articles

The transient behavior of Peltier junctions pulsed with supercooling

There exists the transient thermoelectric supercooling effect that can be enhanced by keeping on increasing the Peltier cooling effect to compensate for the Joule heating effect and Fourier heat conduction effect arriving at the cold junction, in which a transient cold spike can be produced by superimposing an additional shaped current pulse of a large magnitude on the original steady-state optimum value. Most previous work on the transient supercooling mainly focused on the minimum supercooling temperature achievable and separately analyzed the beneficial or detrimental effects on the transient supercooling performance, which was not clarified quantitatively to what extent the interactional effects were on the enhancement of the transient supercooling performance. In this work, we systematically investigate a numerical solution involving time-dependent imposed voltage pulse and time-dependent thermal boundary conditions on the transient supercooling behavior as well as the response of characteristic time and cold-junction temperature distribution to the pulse operation parameters during the periods of pulse start-up, pulse-on time, and pulse-off time, which is served as a theoretical basis for exploiting the coupling interaction of the thermoelectric effects on the heat diffusion from or to the cold junction interrelated with the amount of the availably electrical conversion in the short time scale. Additionally, the advantage of certain pulse forms over others is described. The results indicate that Peltier supercooling capacity shows a decreasing monotonic trend in proportion to the total amount of electrical conversion, and the maximum coefficient of performance for cooling state is about 0.5 to be achieved at steady state. Taking advantage of the temporary Peltier effect focused electrical conversion as the additional cooling for a period long enough against the earlier arrival of the excessively Joule heating dominated heat accumulation is the key parameter for the significant level of thermal enhancement on the pulse supercooling. The discussions may be attractive for compact thermal system coupled with pulsed Peltier supercooling to come up to the localized cooling level of high power packaging.

The Charge Transport Properties of a HWCVD a-Si:H Thin Film under Bending Pressure

The transient thermoelectric effects (TTEs) method is used to measure the ambipolar space charge built up in a low-pressure hot wire chemical vapor deposition (HWCVD) technique a-Si:H layer deposited on a glass substrate. The stage 2 TTE-transients yield the trap state density difference with and without bending pressure up to 9 bars. The a-Si:H sample shows a reduction of the negative storage peaks at 0.045 eV and 0.026 eV with increasing pressure, while the positive (hole trap) peak and the zero crossing practically do not change with the pressure. At the maximum bending pressure, the negative peaks are almost zero and shifted into the band gap or toward the conduction band. Our result shows that it is necessary to produce and mount hydrogenated thin film solar cell stress-free.

Heat capacity study of β-FeSi2 single crystals

Heat Capacity of needle-like [length=5mm, diameter=1 mm] $\beta$-FeSi$_{2}$ single crystal, grown by chemical vapor transport has been measured. Two anomalies are found, a broad deviation centered around 160 K and a clear deviation at a temperature of 255 K approximately. We have attempted to relate these to the anomalies previously reported in the case of the resistivity data. The Transient Thermoelectric Effect [TTE] results lead us to the inference that the system under goes from single carrier system to at least two carrier system at 220 K-our heat capacity results seem to provide further independent evidence for this transition in this system.

Dynamics of vortex motion in high-Tc superconductor La1.86Sr0.14CuO4

Transient thermoelectric effect (TTE) and transient Nernst effect (TNE) have been measured for La1.86Sr0.14CuO4 crystals. It was found that the TTE and TNE signals are observed only in the vortex liquid states. The former rises with two exponential components of τf∼1μs and τs∼1 ms, while the latter rises with single exponential component of τs. We suppose that the components of τf and τs result from the motions of single vortex and collective vortices, respectively.

Static and dynamic transport study of β-FeSi 2 single crystals

Static transport properties and transient thermoelectric effect (TTE) of β-FeSi 2 single crystals, which has received considerable attention as optical and thermoelectric devices, have been measured. The Hall and TTE data clearly showed the existence of multiple conduction carriers. From these results, the tentative band scheme is discussed.

Photo-induced transient thermoelectric effect in ferromagnetic BaIrO 3

BaIrO 3 causes simultaneously charge-density wave and ferromagnetic phase transitions. The transient thermoelectric effect (TTE) has been investigated for this material. We have found that TTE signals are anisotropic on the 〈1 1 0〉, 〈1 0 1〉 and 〈0 0 1〉 axes, and that the magnitude of the TTE signals shows anomalies at the characteristic temperatures T c1 =26 K, T c2 =80 K and T c3 =175 K (T c3 : Curie temperature). A brief discussion is given on the nesting of the Fermi surface.

Charge-Density-Wave Domain Originated Altshuler-Aronov-Spivak Effect in 1T-TaS2 Single Crystal

We have performed photo-induced transient thermoelectric effect (TTE) experiments for charge-density-wave (CDW) material 1T-TaS 2 single crystals in hexagonal and triclinic nearly commensurate phases up to 5 T. The TTE voltages decay with the characteristic relaxation times τ 1 , τ 2 , and τ 3 . τ 1 and τ 2 in the hexagonal phase oscillate with magnetic field B, while those in the triclinic phase are independent of B. The oscillatory phenomenon is interpreted by the Altshuler-Aronov-Spivak effect around the hexagonal arrays of commensurate CDW domains.

Heat conductivity of La 1− xSr xMnO 3 surface layers

Using the transient thermoelectric effect (TTE), we have measured the thermoelectric power S( T) and the heat conductivity κ( T) of La 1− x Sr x MnO 3 ceramic surface layers with nominal compositions x=0.075, 0.1, 0.15, 0.2 and 0.3 in the temperature range 50–340 K. Except for a shift in the characteristic temperatures, the surface layer S( T) is still similar to the bulk thermopower while the heat conductivity κ( T) is markedly different, even changing the slope at intermediate temperatures. As S is less structure sensitive than κ, we relate these differences to an increased number density of defects near the surface. In particular, a stoichiometry gradient close to the surface and oxygen vacancy related (two level tunneling-) modes are indicated. In addition, new S and κ data are presented for lower doping, i.e. x=0.075 and 0.1.

Small polaron effect on carrier recombination in perovskite manganite thin films

Photoinduced ‘transient thermoelectric effect (TTE)’ in perovskite manganite La 0.6Ca 0.4MnO 3 thin film has been measured under magnetic fields. The fast decay process of TTE signals is due to a recombination of photogenerated electron–hole pairs through Mn 4+ ions as capture centers, whose evaluated cross section σ obeys the power law σ∝ T − n ( n=0.75) in the ferromagnetic phase far below T c and in the paramagnetic phase. From the observed relaxation time τ 1, we evaluated the parameter α characterizing a small polaron effect and the effective mass m ∗; both are enhanced appreciably near T c. Such anomaly is attributed to the change in the thermal velocity of diffusing holes with downspin due to a small polaron effect.

Photo-induced transient thermoelectric effect in MnTe

Resistivity, Hall effect, and pulsed laser-induced ‘transient thermoelectric effect (TTE)’ measurements have been carried out for semiconductor MnTe polycrystals in the temperature range of 4.5–350 K. The resistivity is reduced appreciably in the antiferromagnetic (AF) phase that is due to a suppression of the magnetic scattering. The conduction carriers are electrons in the AF phase but holes in the paramagnetic phase. We have observed TTE decay processes with characteristic relaxation times τ 1 and τ 2 that have been attributed to electron–hole pair recombination via ionized donors as capture centers. Both relaxation times decrease abruptly near the Néel temperature, reflecting the magnetic ordering of the system. From the fast relaxation time τ 1, the capture cross sectional area σ 1 is evaluated in the AF phase. This behavior is interpreted based on an extended cascade model.

Time-resolved thermoelectrical effect in Sm 0.56Sr 0.44MnO 3 perovskite

Sm 0.56Sr 0.44MnO 3 CMR-manganite with perovskite structure has been studied using the time-resolved transient thermoelectric effect and heat capacity measurements. It was observed that the Seebeck voltage shows an electron-like behavior in the temperature range 50–300 K except for a narrow interval in the vicinity of the Curie point. The transition into the metallic-like ferromagnetic state is not accompanied by an appreciable anomaly of the heat capacity. It is argued that the electrical conductivity arises by the holes moving in a 2p-band of oxygen as well as by electrons in a 3d-impurity band. Arguments are given in favor of a short-range charge ordering in a wide temperature interval above T C.

Carrier generation/recombination processes and polaron effect in perovskite manganite thin films

Pulsed-laser-induced transient thermoelectric effect (TTE) for perovskite manganite films, La 0.6Ca 0.4MnO 3 (LCMO), Pr 0.67Sr 0.33MnO 3 (PSMO), La 0.7Sr 0.3MnO 3 (LSMO), has been measured under DC electric and magnetic fields. The photoinduced TTE voltage and its sign are strongly dependent on both temperature and magnetic field, which is due to the formation of a bipolar state of photogenerated electrons and holes. It is found that Jahn–Teller and polaron effects are appreciable near the Curie temperature for the carrier recombination, in particular, for LCMO with small ionic radius of Ca 2+ ions.

Thermoelectromechanical refrigeration based on transient thermoelectric effects

This letter introduces the concept of a thermoelectromechanical cooler (TEMC), which modifies a traditional thermoelectric cooler (TEC) by using intermittent contact of a mechanical element synchronized with an applied pulsed current. Using Bi2Te3 as the thermoelectric material, it is predicted that the maximum temperature drop across a TEMC operated under zero applied heat flux is about 35% higher than that of a TEC. This effectively increases the thermoelectric figure of merit for maximum temperature differential applications by a factor of 1.8.

Transport properties of molecular conductor (DMe-DCNQI) 2Cu and its deuterated crystals

Pulsed-laser induced transient thermoelectric effect and dc magnetotransport have been measured for molecular conductors (DMe-DCNQI) 2Cu( h salt) and its deuterated crystals ( d 7 salt) over the temperature range 4.2–300 K. These dynamic and static data are reasonably explained by a two-carrier model of the three-dimensional hole bands located around the Λ and P points in the Brillouin zone at low temperatures, together with a contribution from the one-dimensional electron band at high temperatures (dimensional crossover at around 150 K). Discussions are given for the effect of deuteration on the band structure and metal-insulator transition.

Photoinduced transient thermoelectric effect in a perovskite manganitePr0.67Sr0.33MnO3film

Pulsed laser-induced transient thermoelectric effect (TTE) for perovskite manganite ${\mathrm{Pr}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_{3}$ thin films has been measured under dc electric and magnetic fields. Without any external fields, a photoinduced TTE voltage is small (\ensuremath{\sim}0.5 mV). When an electric field is applied, its magnitude enhances appreciably and decays exponentially with two relaxation times, but its sign is changed by varying temperature. The TTE voltage and its sign are also varied by applying magnetic field, from which the Curie temperature is determined. Based on the available band model of this material, we have discussed a possible mechanism for the observed TTE signals. The TTE phenomenon is primarily due to the formation of a bipolar state of photogenerated electrons and holes, which reflect strongly on the characteristic band structure in the paramagnetic and ferromagnetic phases. Its decay process is a recombination of the electron-hole pairs through some recombination centers, in which the Jahn-Teller or polaron effect is found to be appreciable near the Curie temperature.

Static and Dynamic Transport Studies of Molecular Conductors (DMe-DCNQI)2Cu

Pulsed-laser induced transient thermoelectric effect (TTE) and magnetotransport measurements have been made for molecular conductors (DMe-DCNQI) 2 Cu ( h salt) and its deuterated crystals ( d 7 salt) along the c -axis over the temperature range 4.2–300 K. The photogenerated TTE voltage decays exponentially with two characteristic relaxation times τ 1 and τ 2 due to hole diffusions, followed by a thermal diffusion process of photogenerated phonons. From these relaxation times, we have estimated the corresponding hole mobilities µ 1 and µ 2 (especially µ 1 at low temperature exceeds as high as 6,000 cm 2 /Vs), which show an unusually strong temperature dependence at high temperatures (µ 1 , µ 2 ∝ T -3 for the h salt and µ 2 ∝ T -3.5 for the d 7 salt), suggesting the presence of any anomalous scattering mechanism in these salts, in addition to conventional neutral impurity and phonon scatterings. Furthermore, with special ohmic contacts to a tiny sample, we have successfully measured the positive Hall voltag...

Transient thermoelectric effect with tunable pulsed laser: Experiment and computer simulations for p-GaAs

A photoinduced “transient thermoelectric effect” (TTE) has been measured for a p-GaAs crystal using a tunable pulsed laser, over the laser energy range 0.93–1.80 eV, laser intensity 0.2–130 mJ/cm2, time range 1 ns–1 ms, and temperature range 4.2–50 K, with special attention to native defects of EL2 centers, whose ground state (EL20) and excited state (EL2ex) are located, respectively, at 0.76 and 1.80 eV above the top of the valence band (their energy difference σex=1.04 eV). After laser irradiation at one end of the sample, a TTE voltage is induced within a rising time τr (1.0–1.5 μs) due to hole diffusion, followed by exponential decay with multiple decay times τ1–τ5 that depend on the laser energy, its intensity, and the temperature. The decay time τ1 is assigned to relate to photoexcited electron diffusion in the conduction band and others τ2–τ5 with electron recombinations with photogenerated holes in the valence band via EL2 centers in p-GaAs, for which a rough evaluation of the capture cross section is made. Based on the experimental data, we have discussed the photoinduced carrier generation/recombination processes in three laser energy ranges with the two boundaries σex and the band-gap energy Eg (=1.50 eV); regions I (E<σex), II (σex⩽E<Eg), and III (E⩾Eg). For these three energy regions, we have carried out computer simulations for the photoinduced TTE voltage profiles by solving one-dimensional transport equations for photogenerated electrons and holes, in qualitative agreement with the observations.

A NEW TECHNIQUE FOR OBTAINING THE “MOBILITIES SPECTRUM” TRANSIENT THERMOELECTRIC EFFECT

We introduced a new technique called “Transient Thermoelectric Effect (TTE)”, which was based on carrier-diffusion as irradiated by a pulsed laser. The observed TTE voltages decayed exponentially with time, showing a multi-relaxation process with characteristic relaxation times τi(i=1,2,3,…) for thermal diffusions of photo-induced conduction carriers, analyses of these results could give valuable information about carrier mobilities, effective masses and Fermi surfaces. The principle and the main analysis method were also described in detail.

Anomalous oscillations in magnetic field of photogenerated carriers of quasi-two-dimensional γ-Mo4O11 crystal

Pulsed-laser induced “transient thermoelectric effect (TTE)” under electric and magnetic fields has been measured for the charge-density-wave (CDW) material γ-Mo4O11. The photoinduced TTE voltages decay with the characteristic relaxation times τi (i=2 and 3) for thermal diffusions of two types of holes. In the CDW state [T<Tc (=100 K)], τi is found to oscillate periodically with the magnetic field, whose periods ΔBi (=0.56–0.68 T) are independent of the electric fieldE. Such oscillatory behaviors may be due to any CDW-related quantum phenomenon of normal carriers; its origin is uncertain at the moment.

Computer Simulations of Pulsed-Laser Induced Coherent Plasma Oscillations in GaAs Crystals

One-dimensional transport equations for pulsed-laser induced space-charge distributions in GaAs crystals have been computed using a finite difference procedure by two models; (a) Model A for i -GaAs with an applied dc electric field under a uniform illumination over the crystal and (b) Model B for n - and i -GaAs without the electric field under a half illumination to simulate the “transient thermoelectric effect” (TTE). With Model A, the temporal and spatial evolutions of restored space-charge density of photoexcited electrons and holes are obtained, from which an induced electric field is found to oscillate with a single frequency of plasma oscillations (0.3–2 THz), in satisfactory agreement with reported experiments. Model B calculations have revealed that near the boundary between the illuminated and non-illuminated regions, the appreciable enhancement of space-charge densities and induced electric fields are observed, which diffuse or drift toward a positive x direction, showing characteristic dampin...