Seebeck Coefficient Anisotropy Research Articles

Ca3Co4O9-based transverse thermoelectric heat flux sensors with high sensitivity and fast response time

High-temperature thin film heat flux sensors have been fabricated by growing c-axis tilted epitaxial Ca3Co4O9 (CCO) thin films on 5° vicinal cut LaAlO3 (001) single crystal substrates. The layered structure of Ca3Co4O9 yields the significant Seebeck coefficient anisotropy between the ab-plane and c-axis, which could generate a voltage of the heat flux sensor via the transverse thermoelectric (TTE) effect of thin films. A sensitivity of 27.7 μV/(kW/m2) has been determined in such 5° tilted Ca3Co4O9 thin films, which is much larger than other reported ones based on the TTE effect from various materials. After a thermal treatment at 900 °C in air for 10 min, the sensitivity of such heat flux sensors is almost non-variable, which indicates that the temperature resistance of the CCO-based TTE heat flux sensor is as high as 900 °C. In addition, a fast response time of 45 μs has been identified in such CCO-based TTE heat flux sensors. These results demonstrate that the CCO is a promising candidate to manufacture the TTE heat flux sensors with the superiorly comprehensive performance, including the high temperature resistance, high sensitivity, and fast response.

Response characteristics of (La,Ca)Mn1+δO3 films for heat fluxes based on the transverse thermoelectric effect

Inclined epitaxial (La,Ca)Mn1+δO3 films (-0.3 ≤ δ ≤ 0.1) have been prepared on the miscut 12° STO single crystal substrates. The conductivity gradually increases from 1.51 × 103 S/m for (La,Ca)Mn0.7O3 to 1.23 × 104 S/m for (La, Ca)MnO3, and then slightly decreases to 1.12 × 104 S/m for (La, Ca)Mn1.1O3 films. The enhanced conductivity results in the increase of electron thermal conductivity, so the sensitivity gradually decreases from 12.44 μV/(kW/m2) for (La,Ca)Mn0.7O3 to 7.69 μV/(kW/m2) for (La,Ca)MnO3. As the Mn content continues to increase, even if the conductivity of (La,Ca)Mn1.1O3 decreases, the sensitivity still decreases to 6.12 μV/(kW/m2) due to the deteriorated Seebeck coefficient anisotropy. Additionally, the response time decreases to 22 ns for (La,Ca)MnO3 with increased conductivity, and then slightly increases to 24 ns for (La,Ca)Mn1.1O3 due to the decrease in conductivity. The above results indicate that the sensitivity and response time show a similar trend with the conductivity, which could serve as a reference to fabricate heat flux sensors based on the TTE effect by designing materials.

The atomic layer thermopile heat flux sensor based on the inclined epitaxial YBa2Cu3O7-δ films

The atomic layer thermopile (ALTP) heat flux sensor based on highly c-axis oriented YBa2Cu3O7-δ (YBCO) films prepared on miscut 12° LaAlO3 substrates using magnetron sputtering is achieved. Benefiting from good biaxial texture, the YBCO film maintains a large Seebeck coefficient anisotropy, so the sensitivity of the ALTP heat flux sensor reaches 17.54 μV/(kW/m2). In addition, the response frequency of the sensor is higher than 1 MHz due to the high electrical conductivity of the prepared YBCO films, which is also higher than the current maximum (650 kHz) reported in the literature. These results indicate that our ALTP heat flux sensor can realize high-frequency heat flux detection above 1 MHz.

Enhanced light-induced transverse thermoelectric effect in c-axis inclined Ba-doped BiCuSeO thin films

A significant enhancement of the light-induced transverse thermoelectric (LITT) effect is obtained in c-axis inclined BiCuSeO films through Ba doping. Upon the irradiation of a 308 nm pulsed laser, the LITT voltage signal in Ba-doped film shows higher sensitivity R and faster response time. The maximum sensitivity R of 5.9 V/mJ is obtained in Bi0.94Ba0.06CuSeO film, which is 4.9 times that of the intrinsic film (1.2 V/mJ). The improvement of LITT performance can be attributed to the increased Seebeck coefficient anisotropy and the decreased film resistivity due to the increase in hole carrier concentration of the Ba-doped films. For the case of irradiation from a continuous laser with wavelength of 532, 1064 and 1550 nm, respectively, obvious LITT voltage signals can be detected in film of Bi0.94Ba0.06CuSeO, and the induced voltage amplitude Vp increases with increasing power or reducing wavelength λ of the irradiated laser. This work demonstrates the potential application of Ba-doped BiCuSeO film in self-powered photodetectors, especially in high sensitivity and fast response UV detectors.

Thermoelectric Properties of Sb-S System Compounds from DFT Calculations.

By combining density functional theory, quantum theory of atoms in molecules and transport properties calculations, we evaluated the thermoelectric properties of Sb-S system compounds and shed light on their relationships with electronic structures. The results show that, for Sb2S3, the large density of states (DOS) variation induces a large Seebeck coefficient. Taking into account the long-range weak bonds distribution, Sb2S3 should exhibit low lattice thermal conductivity. Therefore, Sb2S3 is promising for thermoelectric applications. The insertion of Be atoms into the Sb2S3 interstitial sites demonstrates the electrical properties and Seebeck coefficient anisotropy and sheds light on the understanding of the role of quasi-one-dimensional structure in the electron transport. The large interstitial sites existing in SbS2 are at the origin of phonons anharmonicity which counteracts the thermal transport. The introduction of Zn and Ga atoms into these interstitial sites could result in an enhancement of all the thermoelectric properties.

Highly sensitive heat flux sensor based on the transverse thermoelectric effect of YBa2Cu3O7− δ thin film

Thin film heat flux sensors with the size of 5 mm × 3 mm have been fabricated by growing c-axis tilted YBa2Cu3O7−δ thin films on miscut SrTiO3 (001) substrates. The Seebeck coefficient anisotropy between the ab-plane and the c-axis of the YBa2Cu3O7−δ thin film is utilized to generate the output voltage of the heat flux sensor via the transverse thermoelectric effect. The sensitivities of 104.9, 174.1, and 220.9 μV cm2/W have been achieved in nominal 5°, 10°, and 15° inclined thin films, respectively. Such a sensitivity of 220.9 μV cm2/W exceeds a sensitivity of 180.3 μV cm2/W from a standard and water-cooled Gardon gauge. In addition, the fast response time in the range of 15–40 ms has been identified in these sensors based on the transverse thermoelectric effect, which is significantly superior to 400 ms of the Gardon gauge. These results may provide an approach to manufacture highly sensitive heat flux sensors with a low cost, in contrast to the traditional thin film heat flux sensors with complicated micro-fabrication processes.

Apparatus design for measuring of the strain dependence of the Seebeck coefficient of single crystals.

We present the design and construction of an apparatus that measures the Seebeck coefficient of single crystals under in situ tunable strain at cryogenic temperatures. A home-built three piezostack apparatus applies uni-axial stress to a single crystalline sample and modulates anisotropic strain up to 0.7%. An alternating heater system and cernox sensor thermometry measure the Seebeck coefficient along the uniaxial stress direction. To demonstrate the efficacy of this apparatus, we applied uniaxial stress to detwin single crystals of BaFe2As2 in the orthorhombic phase. The obtained Seebeck coefficient anisotropy is in good agreement with previous measurements using a mechanical clamp.

Probing anisotropy of Seebeck coefficient and enhanced thermoelectric performance of Mg2Si0.35Sn0.65 single crystal

A single crystal of Sb-doped Mg2Si0.35Sn0.65 has been successfully prepared using high temperature-gradient directional solidification. The composition distribution of the crystal is uniform, and the preferred growth direction of the crystal is [111] orientation. The Seebeck coefficient (S) of the crystal is anisotropic in different orientations, and the electrical and thermal conductivity are isotropic properties. The S value in the [111] orientation can reach −265 μVK−1, which is much larger than the reported results. Also, the power factor in the [111] orientation has a value of 5.56 mWm−1K−2, which leads to a maximum ZT of 1.3 at 730 K. Results of Hall testing and first principles calculations agree well with the experiment results.

Laser-induced transverse voltage effect and thermopower anisotropy of c-axis inclined Ca3Co4O9 thin film

Ca3Co4O9+δ thin films were prepared on 5° vicinal cut LaAlO3 (001) substrates by pulsed laser deposition and subsequently annealed at various temperatures. Under the irradiation energy density of 8 mJ cm−2, a large laser-induced transverse voltage (LITV) signal with 6.4 V peak voltage and 33 ns rising edge was obtained in the 800 °C annealed film, which was nearly 11 times larger and 2 times faster than that in the as-grown film without annealing (0.54 V, 100 ns). The results suggested that the significant enhancement of LITV by annealing was mainly owing to the improved crystallization and electrical transport property. Based on the LITV results and heat flow model, a nearly intrinsic Seebeck coefficient anisotropy ΔS = |S ab–S c | ≈ 33.4 μV K−1 was obtained, demonstrating the c-axis inclined Ca3Co4O9+δ thin film with large thermopower anisotropy has great application potentials in high sensitive and fast response thermoelectric detectors.

Anisotropy of Seebeck coefficient in un-doped Mg2Sn single crystal

Un-doped Mg2Sn single crystal has been revealed to be anisotropic in Seebeck coefficient at different orientations and possesses orientation-dependent conduction sign reversal (OCSR). At room temperature, the [111] orientation of the crystal shows the n-type conduction with a Seececk coefficient value of −265μVK−1 and the p-type conduction behavior in the [11¯0] and [112¯] orientations with 148μVK−1 and 115μVK−1, respectively. First principle calculation on the anisotropic Seebeck coefficient of the crystal matches well with the experimental results, showing that the conducting effective mass plays an important role in determining the value of Seebeck coefficient. The single crystal Mg2Sn with the preferred [111] orientation is expected to have the best thermoelectric performance due to its large Seebeck coefficient value and the same electrical and thermal conductivities compared with other orientations of the crystal.

Effects of substrate-induced-strain on the electrical properties and laser induced voltages of tilted La0.67Ca0.33MnO3 thin films

La0.67Ca0.33MnO3 thin films have been prepared on vicinal cut LaAlO3, (LaAlO3)0.3-(SrAlTaO6)0.7, and SrTiO3 (001) substrates by pulsed laser deposition. The influence of the substrate on the electrical transport properties and laser induced voltage (LIV) effect of the films was investigated. The high insulator to metal transition temperature Tp (263.6 K) and large peak voltage of LIV signal (2.328 V) were observed in the film grown on LaAlO3 substrate. The compressive strain and large Seebeck coefficient anisotropy ΔS (3.62 μV/K) induced by LaAlO3 are thought to be responsible for this result.

Preparation of ZrNiSn half-Heusler compounds with crystalline alignment by unidirectional solidification in short-duration microgravity and their thermoelectric properties

A ZrNiSn half-Heusler compound was prepared by unidirectional solidification in short-duration microgravity. When a ZrNiSn half-Heusler pellet with Zr:Ni:Sn = 1:1:1(in mole) was solidified in microgravity (m-g) by two different cooling systems, the sample solidified in m-g had a different crystallographic alignment of ZrNiSn phase along the cooling direction. However, both sample solidified in normal gravity (1-g) had no crystalline alignment. The solidified sample in m-g had higher electrical conductivity along the cooling direction than perpendicular to the cooling direction, although these samples did not exhibit Seebeck coefficient anisotropy. This electrical conductivity anisotropy was found to be affected by crystalline alignment of the sample. The thermal conductivity was also affected by the structural difference of the sample.

Thermoelectric conversion via laser-induced voltage in highly textured polycrystalline NaxCoO2 ceramic

We have studied and analyzed the laser-induced voltage effect in highly c-axis-oriented polycrystalline NaxCoO2. The textured and layered stacking NaxCoO2 (x ∼ 0.7) bulks were prepared by a solid-state reaction process. Under the irradiation on Na0.67CoO2 bulk surface with pulsed laser (λ = 248 nm), the induced voltage signals were observed on the inclined surface with rise time 30 ns–43 ns and peak voltage 200 mV–500 mV; the voltage peak values show a linear dependence of laser energy densities. The crystal grains orientation plays a critical role in voltage peak value whether in film or texture bulk. The transverse voltage signal brings the information of thermoelectric anisotropy. In NaxCoO2 band structure, the Fermi surface is different in the ab plane and along the c axis, leading to anisotropy of Seebeck coefficient. Additionally, the artificial structure of the inclined surface for highly textured bulk enables us to obtain a transverse voltage on inclined surface. These results demonstrated the layered textured bulk has potential applications in waste-heat conversion via transverse thermoelectric effect.

Transverse laser induced thermoelectric voltage effect in tilted La 0.5Sr 0.5CoO 3 thin films

The transverse laser induced thermoelectric voltage effect has been investigated in tilted La 0.5Sr 0.5CoO 3 thin films grown on vicinal cut LaAlO 3 (1 0 0) substrates when films are irradiated by pulse laser at room temperature. The detected voltage signals are demonstrated to originate from the transverse Seebeck effect as the linear dependence of voltage on tilted angle in the range of small tilted angle. The Seebeck coefficient anisotropy Δ S of 0.03 μV/K at room temperature is calculated and its distorted cubic structure is thought to be responsible for this. Films grown on a series of substrates with different tilted angles show the optimum angle of 19.8° for the maximum voltage. Film thickness dependence of voltage has also been studied.

Statistical Method to Study Thermopower Anisotropy in Small Crystals

We propose a statistical method to measure the Seebeck coefficient anisotropy in small crystals. Usually, the anisotropy of the Seebeck coefficient can be measured in large single crystals in which the directions of the crystallographic axes have been determined. Sometimes, however, large single crystals can scarcely be produced, perhaps because of a very high melting point or a solid-state phase transition. In our proposed new method, the study of one large single crystal is replaced by the study of an ensemble of many small crystals that can be produced from a polycrystalline sample, with each small crystal being measured many times in several random directions. It can be shown that statistical analysis of the results allows one to determine the anisotropy of the Seebeck coefficient. We have studied the influence of a phase transition and dopants on the Seebeck coefficient anisotropy of the higher manganese silicide and measured the anisotropy of the Seebeck coefficient of β-FeSi2.

Epitaxial La 0.9Ca 0.1MnO 3 films grown on vicinal cut substrates for the investigation of resistivity and thermoelectric anisotropy

La 0.9Ca 0.1MnO 3 thin films have been grown on 5° vicinal cut SrTiO 3 (1 0 0) substrates by pulse laser deposition. The single phase and the epitaxial growth aligned to the c-axis of vicinal cut substrates have been studied by X-ray diffraction analysis. The resistivity versus temperature measurements along and perpendicular to the substrate tilted direction show that the resistivity anisotropy ρ c / ρ ab is 22.44 at room temperature, originating from the intrinsic Jahn–Teller distortion, strain and defects. The Seebeck coefficient anisotropy Δ S of 0.317 μV K −1 attained by laser-induced theroelectric voltages experiment has coincidence with the resistivity anisotropy, which is interpreted by the Mott band theory for Seebeck coefficient approximation.

Laser-induced voltage effect in Pb(Zr0.3Ti0.7)O3 ferroelectric thin film

Pb(Zr0.3Ti0.7)O3(PZT)thin films have been grown by pulsed laser deposition (PLD) on LaSrAlTaO3 (LSATO),LaAlO3(LAO) and SrTiO3(STO) single crystal vicinal cut substrates. Laser-induced voltage(LIV) effect was also found in PZT thin films grown on the three vicinal cut substrates for the first time. LIV signals increased with the single-pulse laser energy following the linear relation in case of c-axis orientation of thin film; but this linear relation was not followed obviously in case of a-axis orientation of thin film. This result shows that the LIV effect in PZT thin film is the atomic layer thermopile effect, in the which anisotropy of Seebeck coefficient plays an important role. Through realizing impedance-matching of thin films and transmission line, LITV signal response time has been optimized, the time of rising edge dropped from 60 ns to 26 ns, the full width at half maximum dropped from 260 ns to 38 ns.

Laser induced thermoelectric voltage effect in [formula omitted] ferroelectric thin film

Laser Induced Thermoelectric Voltage (LITV) effect was observed for the first time in Pb(Zr 0.3Ti 0.7)O 3(PZT) ferroelectric thin films grown on LaSrAlTaO 3(LSATO), LaAlO 3(LAO) and SrTiO 3(STO) single crystal vicinal cut substrates by pulsed laser deposition (PLD). LITV signals increase linearly with the single-pulse laser energy for c -axis orientated thin film. Such a linear relationship does not hold for a -axis orientated thin film. The result shows that the LITV effect in PZT thin film is originated from the atomic layer thermopile effect, in which anisotropy of Seebeck coefficient plays an important role.

Figure of Merit for Detectors Based on Laser-InducedThermoelectric Voltages in La1−xCaxMnO3 andYBa2Cu3O7−σ Thin Films

A figure of merit (FOM) Z = Up/τr, where Up is the peakvoltage, and τr is the rise time of the laser-inducedthermoelectric voltage (LITV) signal, is defined for photodetector basedon the LITV and the influence of the parameters on FOM is analysed basedon the time dependence of LITVs in La1−xCaxMnO3 (LCMO)and YBa2Cu3O7−σ (YBCO) thin films grown on vicinal-cutsubstrates. We find that the FOM increases as the photon penetrationdepth decreases, and linearly increases with the thermal diffusionconstant D. To achieve the highest FOM, the film thickness d has to becontrolled to an optimum value. We also find that the FOM is directlyproportional to the laser absorption coefficient α0, thelaser energy density per pulse E, the illuminated length of film l,sin(2α) [αis the vicinal-cut angle], the Seebeckcoefficient anisotropy (Sab−Sc), and is inverselyproportional to the mass density ρ and the specific heat c0.

High Temperature Transport Property of B- and P-Doped GeSi Single Crystals Prepared by a Czochralski Method

B-doped (p-type) and P-doped (n-type) Ge 0.07 Si 0.93 single crystals were prepared by a Czochralski (CZ) method. The anisotropy of Seebeck coefficient (a), electrical conductivity (σ) and thermal conductivity (κ) among the [001], [110] and [111] directions was determined and the values were compared with those of polycrystalline materials. The Seebeck coefficient of B-doped specimen in the [111] direction (α 111 ) was the same as that in the [110] direction (α 110 ), and was greater than that in the [001] direction (α 001 ) measured at all temperature. The α 111 , α 110 and α 100 of the P-doped specimens were in agreement above 600 K, but anisotropy of a values was observed below 500 K, (|α 001 I > |α 1 11 | > |α 110 |). No anisotropy of σ and K was observed in both B- and P-doped specimens. The thermal conductivity of B- and P-doped single crystal specimens were much greater than that of polycrystalline materials.