Thermoelectric Power Curves Research Articles

Secondary hardening in Co-Ni-Cr super-alloy investigated by Mechanical Spectroscopy

The effects of thermal treatments on a Co-Ni-Cr super-alloy in the temperature range from 250°C to 900°C, resulting in secondary hardening, have been studied by Mechanical Spectroscopy. The simultaneous evolution of the internal friction spectra and of the dynamic modulus suggests that precipitates are involved in complex mechanisms, which control the dislocation mobility increasing the strength of the material. Precipitation induces pinning of dislocations and depinning is induced by a threshold stress. Such a precipitation process occurs in the temperature range from 400°C to 600°C and it is confirmed by reversible changes on thermoelectric power (TEP) due to the depletion of alloying elements from solid solution and by hardness measurements showing a hardness increase at the same annealing temperature as the maximum in TEP curve. The amplitude-dependent internal friction spectra as well as the interaction between dislocations and precipitates are interpreted by proposing a phenomenological model based on the Granato-Lücke theory.

Effects of Precipitation Hardedning in Co-Ni-Cr Alloy on Dissipative Motion of Dislocations by Amplitude-Dependent Internal Friction Measurements / Wpływ Utwardzenia Wydzieleniowego Stopu Co-Ni-Cr Na Rozpraszający Ruch Dyslokacji Badany Przez Zależne Od Amplitudy Pomiary Tarcia Wewnętrznego

The effects of precipitation hardening occurring in a Co-Ni-Cr alloy after annealing treatments have been studied by using mechanical spectroscopy. The amplitude-dependent internal friction (ADIF) due to the dissipative motion of dislocations reveals the presence of a threshold strain for weakly pinned dislocations. The change of ADIF curves and the increase of the elastic modulus after thermal cycles producing precipitates suggest that dislocations motion is hindered leading to increasing strength of the material. Precipitation is confirmed by the changes of thermoelectric power (TEP) and by hardness measurements showing a hardness increase at the same temperature as the maximum in TEP curve. The ADIF spectra as well as the interaction between dislocations and precipitates are interpreted by proposing a phenomenological model based on the Granato-Lücke theory.

Structural and transport properties of thermally evaporated nickel phthalocyanine thin films

X-ray diffraction patterns of NiPc in powder form show that it has a β-form with monoclinic structure. The thermal evaporation of NiPc led to α-polycrystalline films orientated preferentially to the (001) plane with an amorphous background. After annealing at 573 K for 2 h, a mixture of α- and β-phases was formed, while the conversion into β-NiPc is completed at 623 K. The dark electrical resistivity has been measured on NiPc films in the temperature range 298–423 K. Two activation energies ΔE1 = 0.12 eV and ΔE2 = 0.76 eV were obtained. The thermal activation energy ΔE1 is associated with impurity conduction and ΔE2 is associated with intrinsic conduction. Thermoelectric power measurement proved that NiPc films are p-type. The thermoelectric power curves also exhibit two different regions corresponding to extrinsic and intrinsic conductions. Room temperature J–V measurements show a linear ohmic dependence at low voltages, followed by SCLC at higher voltage levels, dominated by an exponential distribution of traps with total trap concentration of 2.79 × 1022 m−3.

Fabrication and thermoelectric properties of highly textured NaCo 2O 4 ceramic

Highly textured NaCo 2O 4 polycrystalline sample was fabricated by means of the cold high-pressure compacting followed by the solid-state reaction. X-ray diffraction and scanning electron microscope were employed to show that the plate-like grains within the sample are aligned along the pressing direction. The resistivity ρ and thermoelectric power S along the preferred {0 0 1} plane were measured in the whole temperature range from 15 to 973 K in air and the correlation between thermoelectric properties and texture was investigated. It was found that both ρ and S exhibit metallic behavior in the whole temperature range and the above sample exhibits lower ρ and higher S due to high texture and density. The power factor exhibits a steep rise above 400 K and reaches 761 μW m −1 K −2 at 973 K, suggesting a promising candidate for thermoelectric application at higher temperature. The change of slope in both resistivity and thermoelectric power curves at about 450 K might arise from the spin-state transition of Co ions in the CoO 2 blocks.

Sol–gel preparation, characterization and studies on electrical and thermoelectrical properties of gallium doped zinc oxide films

ZnO films doped with different concentrations of gallium have been fabricated using sol–gel method. The films are polycrystalline in nature having wurtzite structure and a tendency of growth of (101) reflection with doping. The crystallite size becomes larger with doping. The electrical resistivity curve with doping shows a minimum around 2 at.% doped specimen. The thermoelectric power (TEP) and Hall measurements agree with regards to the carrier concentrations and the sign of carrier, which is negative. The TEP curves show a peak around 210 K for all the samples, which might be due to interaction between charge carrier and anisotropic lattice vibrations.

The thermoelectric power of Kondo cerium and ytterbium compounds

The temperature dependence of the thermoelectric power (TEP) of Kondo Ce and Yb compounds is theoretically studied by a new approach: for T⪢TK, we start from the Coqblin–Schrieffer Hamiltonian including the crystalline field effect and we use a renormalized perturbation expansion with the coupling constants calculated by the “poor man's scaling” method; for T⪡TK, we use an effective spin-12 periodic Anderson model. This treatment can give a negative peak at low temperatures and a positive one at high temperatures, in good agreement with the TEP curves of many Ce compounds.

Correlation of the size effect with the thermoelectric power for the Pr-based manganites Pr0.7Ca0.3-xSrxMnO3.

The thermoelectric power (TEP) S versus temperature has been investigated for the manganite ${\mathrm{Pr}}_{0.7}$${\mathrm{Ca}}_{0.3\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{MnO}}_{3}$ series. Although these materials exhibit the same hole concentration considering their Mn valency of 3.3, their S(T) curves are dramatically affected when x varies from 0 to 0.3, i.e., when the mean size of the interpolated cation increases. Particularly the ${\mathit{S}}_{290\phantom{\rule{0ex}{0ex}}\mathrm{K}}$ values go from positive to negative when x increases. The remarkable feature is that TEP data for samples which exhibit colossal magnetoresistive (CMR) properties are found to be similar and can be well described by the paramagnetic (P)-antiferromagnetic (AF) and AF-ferromagnetic (F) transitions which exist in these samples. On the other hand, for the samples with higher x values, the CMR effect is smaller and the S(T) curves change shape. Clearly, the electronic properties of these materials are very sensitive to the atomic orbitals overlapping which can be varied via cationic size variations as demonstrated by the present TEP measurements. Moreover, the signatures of both P\ensuremath{\rightarrow}AF and AF\ensuremath{\rightarrow}F transitions on the TEP curves seem to be a good indicator to detect samples with CMR properties. \textcopyright{} 1996 The American Physical Society.

Thermoelectric power of the Kondo-lattice system: CePdSn.

Thermoelectric power (TEP) of the antiferromagnetic compound CePdSn has been measured in the temperature range 20--300 K. It is positive at 300 K, exhibits a broad maximum at 100 K, and becomes negative at low temperatures. The maxima in TEP indicate the presence of Kondo and crystalline electric-field effects. These results are analyzed using the Bhattacharjee-Coqblin model, which explains the gross features of the experimental TEP curve except the negative sign observed at low temperatures.

Transport properties of several series of high T c oxides

The main results of transport studies on several series of high T c oxides in the normal state are reported. The resistivity of YBa 2(Cu 1−xZn x) 3O 7 alloys obeys Matthiessen's rule. Oxygen depletion of YBa 2Cu 3O 7 (YBCO) or doping with Co or Fe decouples the CuO 2 planes and enhances superconducting fluctuations above T c. For three materials there is a change in sign in the thermoelectric power (TEP) near to the hole concentration corresponding to maximum T c. The TEP curves of four of the more anisotropic high T c oxides are remarkably similar and therefore are characteristic of weakly coupled CuO 2 layers in the superconducting region. For oxygen depleted YBCO and the related TlPb septenary compound the Hall number decreases strongly with doping while for YBCO the Hall conductivity initially shows a much weaker variation.

Thermoelectric power of YBa2Cu3O7- delta.

Results of thermoelectric power (TEP) measurements of the high-temperature superconductor ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ as a function of oxygen content are reported here. Removal of oxygen produces two distinct changes in TEP versus temperature curves: (1) It raises the TEP curves, and (2) it raises the temperature at which the slope of the curves changes from positive to negative. These changes result from (1) the decrease in the charge carrier density and (2) lattice distortions; both are due to increasing oxygen vacancy. Results of measurements on strontium-substituted compounds show only the second type of change in the TEP versus temperature curves.

Transport properties under magnetic fields of the heavy fermion system CeRu2Si2 and related compounds (Ce, La)Ru2Si2

Measurements of thermoelectric power (TEP) and thermal conductivity of single crystals CeRu{sub 2}Si{sub 2} and Ce{sub 1-x}La{sub x}Ru{sub 2}Si{sub 2} are reported with special emphasis on the effect of the magnetic field. The roles of the crystal electric field splitting and the Kondo effect on TEP curves are debated. The TEP results at low temperature are analyzed with a two-band model, and the parameters are compared with those extracted from the thermodynamic measurements. In CeRu{sub 2}Si{sub 2} the occurrence of a coherent regime at very low temperature is discussed. The influence of the magnetic field on the TEP and the thermal conductivity is investigated and related with the occurrence of the metamagnetic-like transition already reported in magnetization measurements.

Low-temperature thermoelectric power in the asymmetric Anderson model

For a sufficiently large correlation (u>or approximately=1), the authors show that the static susceptibility, specific heat, resistivity and thermoelectric power, calculated for the asymmetric Anderson model by means of the determinantal perturbation expansion of Yosida and Yamada (1975), all show universal behaviour in the low-temperature, low-energy region, characterised by a single scaling constant theta r= Delta / pi chi up arrow up arrow sin ( delta sigma ). As the asymmetry increases, the onset of the universality shifts towards larger u values. The thermoelectric power (TEP) takes the form S=SN((T/ theta r)-Ks(T/ theta r)3) where Ks is a parameter-independent constant and SN=(2 pi kB/3 mod e mod )cos( pi Zd/2(2l+1)) is the normalisation constant determined by the number Zd of electrons in the virtual bound state (VBS). Thus, the TEP changes sign as one moves across the 3d or 4f series. Comparison with the experimentally obtained universal TEP curve shows our results to be valid up to T approximately= 0.2 theta r. The Zd values calculated from SN as given by the TEP data are consistent with the values obtained by different experimental methods.

The transport properties of Heusler alloys: 'ideal' local moment ferromagnets

The authors have made resistivity, magnetoresistance, Hall effect and thermoelectric power measurements on the Heusler alloys Pd2MnSn, Ni2MnSn and Cu2MnAl. Results are characteristic of well-ordered spin-only local-moment ferromagnets. The thermoelectric power curves all show a unique behaviour which the authors suggest is that characteristic of any local-moment ferromagnet.

Thermoelectric and thermomagnetic properties of graphite—I: The cylindrical band model

The cylindrical band model of graphite is used to obtain the degeneracy parameters necessary for the analysis of the basal conductivity, thermoelectric power and Nernst-Ettingshausen coefficient (drift-diffusion terms). Densities and mean mobilities of electrons and holes are obtained from galvanomagnetic properties, then the thermoelectric power and Nernst-Ettingshausen coefficient are computed. Several features of the thermoelectric power curves with temperature can be qualitatively understood with this model, particularly differences in the behavior of pyrolytic and natural, or Kish graphites. A minimum in the Nernst-Ettinghausen coefficient depends strongly on the type of scattering law assumed, but the model cannot account realistically for the size of this effect near 300°K, where phonon-drag terms are thought to be very small.

Semiconducting properties of pure and Mn-doped chromium disilicides

The phase analysis was carried out for the system ( l− x) CrSi 2 + xMnSi 2 in the range 0⩽ x⩽0.5 by X-ray technique. The solid solution Cr 1− x Mn x Si 2 was identified in the composition range 0⩽ x⩽0.225, where the added Mn-atoms occupied substitutionally the Cr-atom sites in the CrSi 2 structure. Resistivity, Hall coefficient as well as thermoelectric power were measured as functions of temperature in the range, 80–1200 K and composition x in the single phase region, 0 ⩽ x⩽0.225. The pure CrSi 2 ( x = 0) was a p-type degenerate semiconductor, whose hole concentration was determined to be 7.7 × 10 20 cm −3 at room temperature. Mn-atoms introduced in the CrSi 2 crystal were found to act as donors. The forbidden energy gap was determined to be 0.30 eV from the Hall-data in the intrinsic region. With increasing x, a conversion from p- into n-type semiconductor took place in Cr 1− x Mn x Si y . From the analysis of Hall- as well as resistivity-data, the mobility ratio b was obtained as a function of composition x. It was revealed that b increased with increasing x from 0.01 for x = 0 to 0.12 for x = 0.182. The electron-hole concentration product could be expressed as np = 1.2 × 10 35T 35 exp (− 3480 T ) , and the hole mobility as μ p = 7.0 × 10 4T − 3 2 in the acoustic scattering region. The effective mass of hole was found out to be 3.2 m 0 and independent of x, whereas that of electron varied from 20.2 m 0 for x = 0 to 7.5 m 0 for x = 0.182. When these parameters are used, the theoretical temperature variation of the thermoelectric power curve was found out to be in good agreement with the measurement.

Sensitivity of the anomalous thermoelectric power to lattice defects in a dilute copper-iron alloy

The thermoelectric power (TEP) of a Cu-Fe alloy containing 115 at ppm iron in the temperature range 1.5–80 K is measured in its unannealed state as well as after annealing at 530–550 C under fore-vacuum for 16 and 66 h, respectively. The TEP curves present a giant negative peak, which increases numerically with annealing and also shifts to lower temperatures. This observed numerical increase of the peak TEP value is attributed to the removal of the lattice defects rather than the decrease of the effective isolated impurity concentration consequent upon a partial precipitation of the impurity during annealing. Such an explanation is quite consistent with our inference of a phase shift for ordinary scattering increasing with impurity concentration in dilute magnetic alloys characterized by a negatives-d exchange interaction. The shift of the TEP peak to lower temperature is caused by an increase of the phonon drag component due to the removal of lattice defects. Such a decrease of the temperature of the peak TEP,Tp, is reminiscent of a similar variation of its counterpart,TK in electrical resistivity. However,Tp seems to be even more sensitive to the physical environment of the magnetic impurity.

Low-Temperature Thermoelectric Power of the Rare-Earth Metals

The Seebeck coefficients (thermoelectric powers) of yttrium, lanthanum, and 11 of the rare earths have been measured for the temperature range 7° to 300°K. The transitions reported by other investigators on specific heat, magnetic moment, and resistivity at the magnetic ordering temperatures are, in many instances, visible also in the curves of thermoelectric power (TEP) vs temperature. The Néel temperature is evidenced by a change in slope in the TEP vs T curve. The ferro-antiferro transition temperature is obvious only in the case of dysprosium and appears as a sharp drop in the curve. With the exception of samarium and ytterbium, the TEP's of the metals are negative throughout most of the temperature range covered, and, with the same exceptions, the TEP curves have about the same slope near room temperature.

The kinetics of recovery and recrystallization of copper from hardness and thermoelectric-power measurements

Annealing of H.C. copper strip cold-rolled with reductions in thickness of 50 per cent and 96 per cent has been studied as a rate-process by parallel observations of changes in hardness and thermoelectric power. In accordance with earlier investigations, change in hardness is taken to be proportional to fraction of metal recrystallized, while thermoelectric power is assumed to be directly related to residual elastic lattice strain. While the hardness—isothermal annealing curve of strip rolled 50 per cent exhibits a definite discontinuity indicative of two processes, such as recovery (or polygonization) and recrystallization, the thermoelectric power curve is, in agreement with Brindley's earlier conclusion, pseudo first order. The rate of decrease of average lattice strain during both polygonization and recrystallization is apparently governed by similar processes of dislocation migration. The annealing of heavily rolled strip is more complex; hardness and thermoelectric power curves, however, are identical, and analysis of the curves, outlined in an appendix, indicates that the dominant process is one of second order in agreement with previous findings. The second-order character follows naturally from the concept that decrease in thermoelectric power and softening occurs by direct interaction and annihilation of stress fields associated with stable arrays of dislocations at grain interfaces. It is pointed out that this concept is also consistent with recrystallization as a process of nucleation and growth.

Theory of Thermoelectric Power in Semiconductors with Applications to Germanium

The thermoelectric power $Q$ of a semiconductor is found by calculating the Thomson coefficient ${\ensuremath{\sigma}}_{T}$ from electrical and thermal current density expressions and then integrating the relation ${\ensuremath{\sigma}}_{T}=\frac{T\mathrm{dQ}}{\mathrm{dT}}$. This procedure yields a general expression for $Q$ in terms of the Fermi level, forbidden band width, temperature, ratio of electron to hole mobility, and effective electron and hole masses. In the impurity range the general formula for $Q$ reduces to a simple dependence on the Hall coefficient and temperature if carrier scattering is largely due to the lattice of the semiconductor; the same expression may be used with the addition of a correction term when carrier scattering by impurity ions becomes important at the lower temperatures. When both holes and electrons must be considered as carriers, $Q$ can be evaluated at any temperature from the resistivity and Hall coefficient at that temperature. An expression is also obtained for the thermoelectric power of an intrinsic semiconductor in a form depending on the mobility ratio, forbidden band width at 0\ifmmode^\circ\else\textdegree\fi{}K, and the temperature rate of change of this band width. Hall and resistivity data measured for six polycrystalline germanium samples and two silicon samples have been inserted into the theoretical expressions derived in this paper. The thermoelectric power curves so calculated are found to give generally good agreement with the measured curves.

Measurement of the Thermoelectric Power of Germanium at Temperatures above 78°K

A systematic investigation of the thermoelectric power of germanium samples has been made in the temperature range between 78\ifmmode^\circ\else\textdegree\fi{}K and about 925\ifmmode^\circ\else\textdegree\fi{}K. The general method of measurement is the determination, through thermocouple readings, of the ratio $\frac{\ensuremath{\Delta}E}{\ensuremath{\Delta}T}$, where $\ensuremath{\Delta}E$ is the thermal emf (relative to chromel) measured between two points in the sample having a temperature difference $\ensuremath{\Delta}T$. At first, difficulty was experienced in obtaining reliable and reproducible thermoelectric power data at the lower temperatures (78\ifmmode^\circ\else\textdegree\fi{}K to about 350\ifmmode^\circ\else\textdegree\fi{}K). A detailed discussion is given of the steps taken to eliminate this difficulty by reducing or removing the effects of thermal conduction by the thermocouple wires, convection currents around the the sample, poor contacts between thermocouple junctions and the sample, and inhomogeneities in the thermocouple wires. In the impurity range the sign of the thermoelectric power is the sign of the Hall coefficient, and the magnitudes of thermoelectric power, resistivity, and Hall coefficient increase with decreasing impurity content. With rising temperature the thermoelectric power increases in magnitude and passes through an extremum at a temperature somewhat lower than that corresponding to the end of the exhaustion range or flat portion of the Hall curve. The thermoelectric power of a $p$-type sample passes through zero at a temperature higher than that at which the Hall coefficient of the same sample becomes zero. The thermoelectric power curves of both $p$-type and $n$-type samples approach a common curve, having negative values in the intrinsic range.