Humidity Related Low Temperature Conductivity Hysteresis of Ce1–xZrxO2 (0 ≤ x ≤ 1) Ceramics. Structural Disorder Relationship

Ion‐electron mixed conductivity of the Ce_1–xZr_xO₂ (0 < x < 1) series, prepared by solid‐state reaction, has been measured as a function of temperature, pO₂ and pH₂O relative pressures. The structural analysis of this series shows the formation of monoclinic, tetragonal, and cubic phases as the Ce content increases. The temperature used in samples preparation, 1,923 K, favors at intermediate compositions the homogeneous Ce and Zr mixing in the metastable tetragonal t′ phase. The variation of conductivity with the relative oxygen pressure indicates that electronic contribution increases, becoming preponderant, as the Ce‐content increases. The modifications in conductivity observed in Ce‐rich samples have been ascribed to the enhancement of the electronic contribution at the grain‐boundary of ceramics. The differences observed at low temperatures in electronic dc‐conductivity of Ce‐rich samples during heating and cooling treatments, have been ascribed to the incorporation of water as hydroxide defects at the grain boundary and interior of the particles. The surface hydroxylation is maximal in samples with improved OSC capacity, decreasing drastically in samples heated at 740 K. Finally, the potential application of prepared materials in intermediate temperature solid oxide fuel cells (SOFC) is discussed.     

Thermoelectric Properties of <Emphasis Type="Italic">x</Emphasis>PbTe/Yb<Subscript>0.2</Subscript>Co<Subscript>4</Subscript>Sb<Subscript>12</Subscript> Hot-Pressed Samples

The xPbTe/Yb_0.2Co₄Sb₁₂ compounds were prepared by the ball-milling and hot-pressed process. Electrical conductivity of the composite samples are reduced with a increase in PbTe content; and, their temperature dependence coefficients show the positive values. The maximum electrical conductivity of composite materials is ~80000 Sm⁻¹ at 800 K. The Seebeck coefficient (absolute value) of the composite material is obviously improved with an increase in the dispersed phase (PbTe) content; the Seebeck coefficient (absolute value) of the 10PbTe sample is ~260 μVK⁻¹ at 700 K, which increases by 13.6% relative to that of the Yb_0.2Co₄Sb₁₂ sample. The thermal conductivity of the composite samples is improved due to introduction of PbTe, and the thermal conductivity of the 10PbTe sample is ~3 Wm⁻¹ K⁻¹ at 550 K. The maximum value of ZT is 0.78 at 700 K for the 2.5PbTe sample.     

Poole-Frenkel photoconductivity in amorphous Se<Subscript>75</Subscript>Te<Subscript>20</Subscript>Sb<Subscript>5</Subscript> thin films

In many amorphous and liquid semiconductors and many other class of materials, pre-exponential factor of electrical conductivity σ₀ is found to increase exponentially with activation energy ΔE according to Meyer-Neldel rule. This rule is generally verified by selecting different compositions of different ΔE in a given class of materials. In the present paper, we report the investigation of Meyer-Neldel rule (MN rule) for the pre-exponential factor (σ _ph )₀ and activation energy of (ΔE _ph ) photoconductivity in amorphous thin films of Se₇₅Te₂₀Sb₅ for high field conduction. The photoconduction was found to be ohmic at low fields and of Poole-Frenkel type at high fields. In the present work we have changed ΔE _ph by applying high electric fields.     

Photoluminescence,FTIR, and electrical characterization of samarium(III) chloride‐doped polyaniline

The synthesized polyaniline (PANI) is doped with different concentrations of Samarium(III) chloride (SmCl₃). The electrical conductivity of doped PANI samples has been measured in the temperature range (300–400K). It has been found that dc conductivity increases with the increase of dopant concentration. Different parameters, based on the conductivity, such as pre‐exponential factor (σ₀) and activation energy (ΔE) have also been calculated. These parameters exhibit information about the nature and suitability of the dopant. Doped samples are characterized by FTIR and photoluminescence studies, which show the interaction of dopant with PANI. Two sharp peaks of different intensities from PL spectra at 388 and 604nm have appeared in doped PANI, which might be due to the effect of SmCl₃. It has been observed that SmCl₃ (dopant) shows noticeable changes in the electrical and spectroscopic properties of doped PANI. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Thermoelectric properties of W1−xNbxSe2−ySy polycrystalline compounds

We investigate the thermoelectric properties of bulk polycrystalline samples of WSe₂-based compounds with partial substitutions in the cationic (W) and the anionic (Se) sublattices in the temperature range from 4.2 to 650 K. The substitution of W for Nb leads to a significant increase in the charge carrier concentration, however, deteriorates the charge carrier mobility. In contrast, the substitution of selenium for sulfur increases the charge carrier mobility, the thermal conductivity, and the Seebeck coefficient but conductivity changes non-monotonical. We show that the addition of sulfur in anionic sublattice affects the grain sizes in the polycrystalline material. Using substitutions in the anionic and cationic sublattices, we find the optimal ratio of the elements for better thermoelectric efficiency. The W_0.98Nb_0.02Se_1.7S_0.3 sample showed the best value of the figure of merit ZT = 0.26 (T = 650 K).     

Thermophysical properties of polyurethane foams and their melts

The thermal conductivity (λ) and the specific heat (c_p) of seven polyurethane foam formulations and their melts are obtained using a transient plane source technique called the Hot Disk experiment. In the experiment, the Hot Disk sensor is sandwiched by the samples and acts as both a heat source and a temperature sensor. The fundamental assumption is that throughout the experiment, the heat from the sensor does not penetrate beyond the boundaries of the sample. The suitable sample dimensions and sensor radius are determined from the preliminary calculations. Through sensitivity analysis, the appropriate measuring time and output power for the experiments are established. For polyurethane foams, λ ranges from 0.048 to 0.050 W/m K, and c_p ranges from 2359 to 2996 J/kg K. For melts, λ ranges from 0.186 to 0.200 W/m K, and c_p ranges from 1958 to 2076 J/kg K. When foam decomposes into melts, the changes in thermophysical properties shows λ increases by approximately 300%, whereas c_p decreases by approximately 20%. On the basis of these changes, the collapse of the foam structure into melt appears to improve the heat transfer through the material. Copyright © 2013 John Wiley & Sons, Ltd.     

Investigation of the Meyer-Neldel rule for AC conduction in glassy Se<Subscript>100 − <Emphasis Type="Italic">x</Emphasis></Subscript>Te<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> alloys

In many amorphous and liquid semiconductors and other class of materials, the Meyer-Neldel rule is observed in the dc conductivity, where the preexponential factor (σ₀) is found to increase exponentially with the activation energy (ΔE). In the present paper, we report on the observation of the Meyer-Neldel rule in case of ac conductivity at high temperatures (300–350 K) in bulk samples of glassy Se_{100 − x} Te _x (x = 10, 20, 30) alloys. In this temperature range, the approximate variation in ac conductivity with temperature is found to be exponential and the activation energy is found to vary with frequency. The observation of the Meyer-Neldel rule in the present study is explained in terms of the compensation effect in the relaxation time. The text was submitted by the authors in English.     

Acceptor formation in Mg-doped,indium-rich GaxIn1?xN: evidence for p-type conductivity

We report on the Mg-doped, indium-rich Ga_xIn_1−xN (x < 30). In the undoped material, the intrinsic electron density is very high and as a result there is no detectable photoconductivity (PC) signal within the range of temperatures of 30 <T < 300 K. In the Mg-doped material however, where the conductivity is reduced, there is a strong PC spectrum with two prominent low-energy peaks at 0.65 and 1.0 eV and one broad high-energy peak at around 1.35 eV. The temperature dependence of the spectral photoconductivity under constant illumination intensity, at T > 150 K, is determined by the longitudinal-optical phonon scattering together with the thermal regeneration of non-equilibrium minority carriers from traps with an average depth of 103 ± 15 meV. This value is close to the Mg binding energy in GaInN. The complementary measurements of transient photoluminescence at liquid He temperatures give the e-A⁰ binding energy of approximately 100 meV. Furthermore, Hall measurements in the Mg-doped material also indicate an activated behaviour with an acceptor binding energy of 108 ± 20 meV.     

Flow instabilities of epoxide prepolymers

Flow of molten epoxide prepolymer (T_g = 303 K, M?_n 889, M?_u/M?_n = 1.46) shows stress jumps. A hydrodynamic explanation is forwarded, and a check of the theoretical jump is carried out, with shear viscosites and shear moduli measured at 361 K. The increase in pressure produces a decrease in volume, which increases the rigidity of the polymer, which is then reduced by relaxation of the compressed chain segments.     

Influence of magnesium on dielectric properties of Ca9–xMgxBi(VO4)7 ceramics

Solid solution of Ca_9‐xMg_xBi(VO₄)₇ in powder and ceramic forms are obtained by solid‐state reactions. Details of their crystal structures are determined for x = 0.25 and x = 0.5 by synchrotron radiation diffraction and the Rietveld method. The refinement has confirmed that Mg²⁺ is replacing Ca²⁺ in M5 position of a polar (S.G. R3c) β‐Ca₃(PO₄)₂‐type structure. Thermal analysis, dielectric and second harmonic generation experiments in broad temperature regions have proved this polar structure is formed for 0 ≤ x ≤ 0.7. Magnesium for calcium substitution enhances optical nonlinear activity of Ca_9‐xMg_xBi(VO₄)₇ in 0 < x ≤ 0.5. Two phase transitions have been found, one of which from polar to centrosymmetric phase is accompanied by dielectric constant peak of ferroelectric type. The other is upper on temperature, marked with smaller dielectric anomaly, and goes between 2 centrosymmetric phases. Temperatures of the phase transition only slightly depend on x, the first being near 1050 K, the second near 1100 K. Electric conductivity quickly rises with temperature in the polar phase. At higher temperature it changes according to the Arrhenius law with small activation energy, E_a ~ 0.7 eV for bulk conductivity and E_gb ~ 2.0‐2.5 eV for grain boundary conductivity. The analysis of bulk and grain boundary conductivities agrees with Ca²⁺‐ion fast transport in ceramics. The bulk conductivity slowly decreases with magnesium content, the grain boundary conductivity does not notably depend on the composition.     

Photoconductivity of DLC film deposited by pulsed discharge plasma CVD

DLC films were deposited by a new pulsed DC discharge plasma chemical vapour deposition (CVD) using hydrogen and methane gas mixture. When methane concentration (Cm) i.e. CH₄/(H₂ + CH₄) was increased from 3 to 40%, the graphitization of the carbon film increases as evident from Raman study. When Cm was increased to 30%, DLC film shows photoconducting property. The white light photoconductivity (S = I_l/I_d, where I_l is light current and I_d is dark current) measured with solar simulator under AM 1.5 condition was approximately 20 at room temperature. The photoconductivity was not clear when Cm was lower than 20%. ESR measurements also show that the electron spin density was slightly decreased with decreasing concentration of methane. Thus we can conclude here that at higher concentrations of methane at 30%, Sp² content of the film increases and the DLC film becomes photoconducting.     

Effect of annealing temperature in carbon powder on thermoelectric properties of the SrTiO3−δ single crystal in the [111] crystal orientation

SrTiO₃ is a promising thermoelectric material for applications in harsh environments, owing to its excellent thermoelectric (TE) properties and high-temperature stability. The effect of annealing in carbon powders on the TE properties of SrTiO_3?δ in the [111] direction was experimentally investigated and analysed using first-principles calculation. The electrical conductivity of the SrTiO_3?δ single crystals in the [111] direction increases with an increase in annealing temperature in the range of 1573–1673 K, which is opposite to that in the [510] direction as shown in a previous experiment. This is attributed to the different variation trend of carrier concentration. First-principle calculation results show that the carrier concentration in the [111] direction increases with increasing oxygen vacancy concentration, but it is the opposite in the [510] direction. Strong anisotropy of carrier concentration should be caused by the difference in atomic arrangement, making difference in the amount of free electrons that maybe constrained at high oxygen vacancy in different directions. Although the SrTiO_3?δ single crystal annealed at 1573 K shows a lower electrical conductivity, it obtains higher power factor with the maximum value of 2.24 mW m^?1 K^?2 at 323 K, which is predominantly due to its higher effective mass. It also yields the highest ZT value of 0.18 at 873 K owing to the lower thermal conductivity. The results of this study are imperative for the design and development of perovskite TE materials.     

Degradation of radiation exposed polymethacrylamide (Rohacell® 31) foam used in composite hardware

Rohacell^® foam is a critical structural component used to fabricate composite hardware for space applications. The effect of radiation exposure on Rohacell^® 31 HFHT has been evaluated to determine the degree of chemical and mechanical degradation that may occur in a space environment. A ⁶⁰Co source was used for exposure levels ranging from 0 to 8 Mrad. Tensile tests showed significant decreases of 50% in tensile strength and 75% decreases in failure strain, after only a 4‐Mrad exposure. However, the rate of degradation sharply decreased with additional exposure since crosslink density was more affected during the early stages of radiation exposure. Size exclusion chromatography and multiangle light scattering of the extracted material also provided evidence showing an increase in the concentration of lower molecular weight (M_w) fragments after exposure. The average M_w of the extracted material dropped by two orders of magnitude from the control to the 5‐Mrad specimens, however changes in M_w between the 5‐ and 8‐Mrad specimens only varied by 30% similar in trend to mechanical property data. LS analysis showed a structural conformation change from a heavily branched network to a more linear conformation. Dynamic mechanical analysis exhibited increases in the peak height and breadth of the tan δ curve with radiation dose. These increases in damping are a result of chain scission which results in increased segmental motion caused by decreases in the degree of crosslinking within the polymer structure. Gas chromatography–mass spectrometry was used to probe the potential sites of radiation susceptibility within the PMI polymer structure and are discussed. The radiation susceptibility of this polymeric material should be properly evaluated when utilizing this material for space applications since end of life conditions may vary significantly from beginning of life mechanical and thermal behavior. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44670.     

Effect of Na2O on the High‐Temperature Thermal Conductivity and Structure of Na2O–B2O3 Melts

The effect of Na₂O and temperature on the thermal conductivity of the Na₂O–B₂O₃ binary system has been measured using the hot‐wire method to examine the relationship between the thermal conductivity and structure in high‐temperature melts. The thermal conductivity of the binary melt is measured from 1173 to 1473 K in the fully liquid state. The thermal conductivity slightly increases with Na₂O content up to 20 wt%. Above 20 wt% Na₂O, the thermal conductivity decreases with increasing Na₂O. The network structure of molten glass was analyzed using Fourier transform infrared (FTIR), Raman spectroscopy, and XPS. The FTIR analysis shows that 3‐D complex borate structures, such as tri‐, tetra‐, and pentaborate are made by [BO₄] tetrahedral units interconnected with 2‐D structure boroxol rings in the low Na₂O region. Above 20 wt% Na₂O content, nonbridged oxygen in [BO₂O^?] units and diborate groups increase with increase in Na₂O. The same tendency is shown by the Raman spectroscopy and XPS analyses. The Raman analysis shows that boroxol rings disappeared with large [BO₄] groups, such as tri‐, tetra‐, and pentaborate structures, which increase at low Na₂O content. Isolated diborate groups and nonbridged oxygen in [BO₂O^?] units increase at high Na₂O content. It can be inferred that single structure units, such as isolated diborate groups, interfere with conduction. The XPS analysis results show that free oxygen produced by the interconnection of Na₂O in the borate structure does not cause significant changes to O^2? in the low Na₂O region, but increases the O^o and decreases the O^?. Above 20 wt% Na₂O, O^? slightly increases and O^o shows a decreasing trend.     

An oxygen sensor based on the semiconducting properties of (U0.3Y0.7)O2−x

Urania-yttria fluorite solid solutions with a U/Y ratio below 0.33/0.67 and having n-type conductivity are suggested as oxygen sensing materials. The electrical properties of one composition, (U_0.3Y_0.7)O_2–x , have been studied in detail. The resistance of this composition follows a (pO₂) ⁿ relationship over an oxygen concentration range of 0.1 to 100% and shows long-term stability. The value ofn decreases with increase in temperature (0.23 at 400° to 0.15 at 800°C), but is reproducible for a large number of specimens. A method for determining oxygen partial pressure in hot gases using these urania-yttria solid solutions as the sensing material is described.     

New organic semiconductor materials: electrical conductivity,thermal properties and application of voltage‐controlled negative resistance device of poly[(2‐acrylamido‐2‐methyl‐1‐propanesulfonic acid)‐co‐(maleic acid)]

BACKGROUND: Electrical conductivity, photoconductivity, voltage‐controlled negative resistance and thermal properties of copolymers of 2‐acrylamido‐2‐methyl‐1‐propanesulfonic acid and maleic acid were investigated in order to obtain new organic semiconductors. RESULTS: The room temperature conductivity of three different copolymers was found to be in the range 1.28 × 10^?8 ? 1.20 × 10^?7 S cm^?1. The dark‐ and photo‐current‐voltage characteristics indicate that the copolymers exhibit voltage‐controlled differential negative resistance behaviour. The electrical conductivity of the polymers increases by photo‐illumination, suggesting that the polymers exhibit photoconductivity. The width of the exponential tail in the forbidden band gap of the three polymers was determined via the transient photocurrent technique and E₀ values were in the range 34.4–36.49 meV. CONCLUSION: The results suggest that the copolymers could be used as organic semiconductor materials. Copyright © 2007 Society of Chemical Industry     

Magnetic properties of La0.7Sr0.3MnO3 under the pressure and the transport property under the magnetic field

Polycrystalline La_0.7Sr_0.3MnO₃ sample (LSMO) was synthesized by the solid phase reaction; it exhibits the paramagnetic-ferromagnetic transition at T_c = 362 K at the ambient pressure; it is paramagnetic metallic state above T_c and the ferromagnetic metallic state below T_c. It was observed that the pressure effect depends on the temperature range: (a) In the paramagnetic region, the magnetization M hardly changes with the pressure P, that is, ΔM≈0. There exist the antiferromagnetic (AFM) coupled ferromagnetic clusters in the paramagnetic region, and the pressure enhances the AFM coupling. (b) In the temperature range around T_c, the pressure increases M, that is, ΔM > 0, with the concomitant increase in T_c; the average pressure coefficient dT_c/dP is 5.40 K/GPa at P = .74 GPa, much smaller than 15.47 and 15.90 K/GPa for La_0.7Ca_0.3MnO₃ and La_0.9Ca_0.1MnO₃, respectively, due to the different distortion degree of MnO₆ octahedra in Ca and Sr doped manganites. (c) In the temperature region below T_c, the pressure reduces M, that is, ΔM < 0. M is determined by the competition between the Mn³⁺-O-Mn⁴⁺ double exchange and the interparticle dipolar interaction. The pressure enhances the interparticle dipolar interaction, leading to a significant decrease in magnetization. The resistivity of LSMO exhibits the metallic behavior in the temperature range of 5 K~370 K; it decreases as the applied magnetic field H increases from 0 to 7 T, that is, the magnetoresistance effect which is more significant around T_c. The fitting to the low-temperature resistivity shows that the applied magnetic field reduces the scattering from the grain boundary, electron, phonon, and magnon, especially reduces the electron-electron scattering.     

Rapid preparation of high-performance S0.4Co4Sb11.2Te0.8 skutterudites with a highly porous structure

In this study, S_0.4Co₄Sb_11.2Te_0.8 skutterudites with a highly porous structure inside grains are prepared by a one-step hot-pressing (OS-HP) method. The effect of the pressure relief treatment at the heating stage on the micro-morphology and thermoelectric properties of materials is investigated. When the temperature corresponding to the pressure relief treatment is less than 723 K, the grain size dramatically increases from ～1 to ～50 μm, and a large number of pores are distributed inside these large grains. Compared with those samples prepared by the conventional method, the thermal conductivity of samples prepared by the pressure relief treatment is significantly reduced due to the high porosity. The ZT values of samples prepared by the pressure relief treatment are greater than 1.6 at 825 K. This newly developed OS-HP method can be employed for the rapid fabrication of highly porous structured skutterudites with low-melting-point compositions as well as of other material systems.     

Gamma-ray irradiation effect on microstructure and physical performances of porous silica

In this paper, the gamma irradiation effect on the microstructure and physical performances of porous silica, including mechanical, thermal, and optical performances, are systematically investigated by using molecular dynamics and density-functional theory-based methods. The study of bond angle distribution, pair distribution function, coordination number distribution, and average ring size distribution show that, after gamma-ray irradiation, the microstructure of porous silica is obviously modified. The tight packing of SiO₂ tetrahedrons in the porous silica network is broken by gamma-ray irradiation. Defects of three-coordinated Si and non-bridging oxygen are induced by gamma-ray irradiation. Moreover, we find that the defects concentrations rapidly grow as gamma-ray dose increases. The mechanical, thermal, and optical performances of porous silica are all seriously degenerated by gamma-ray irradiation. Our results show that, for mechanical performance, Young's modulus, Bulk modulus, and Shear modulus first decrease and then keep stable as gamma-ray dose increases, but the change of Poisson's ratio is slight. For thermal performance, the thermal conductivity decreases exponentially as gamma-ray dose increases. For optical performance, light absorption coefficients increase exponentially and light transmittance drops as gamma-ray dose increases in the working range (photon energy range around 3.5 eV) of inertial confinement fusion. Present work is expected to be valuable for studying the degradation mechanism of silicate materials under gamma radiation and developing gamma-ray irradiation protection technology.     

Stepwise shock compression of C70 fullerene

Shock-induced phase transitions of C₇₀ fullerene are experimentally studied up to a pressure of 36 GPa and a temperature of 1200 K. Pressure–temperature histories of C₇₀ specimens are estimated and overlaid on the tentative phase diagram of C₇₀. The crystalline phase of fullerene C₇₀ with a hexagonal close-packed structure remained practically unchanged under stepwise shock compression up to а pressure of 8 GPa. Contrariwise the crystalline phase of fullerene C₇₀ with a rhombohedral structure fully converted to the phase with a cubic structure under similar conditions. Shock-induced transformation of the hexagonal phase into the face-centered cubic phase was observed at pressures in the range of 9–19 GPa. The amount of transformed material increases with the shock intensity. Upon further increase in the shock pressure, the destruction of C₇₀ molecules begins. In the sample recovered from 26 GPa, we observed the traces of C₇₀ with a face centered cubic structure only. The destruction of C₇₀ is accompanied by formation of graphitic carbon.