The overall purpose of the present study is basically to understand the manifestation of the thermo-electrical properties of the matrix ZnO-P 2O 5 first, and of the ZnO-P 2O 5 composites loaded with different volume fractions of nickel (Ni) as conductive fillers. In the matrix ZnO-P 2O 5, the values of electrical conductivity varied between 1.14 × 10-8 and 7.8 × 10 -7 (S/cm), and the Seebeck coefficient value varied between minimal value 265 and maximal value 670 (μV/K) in the studied temperature. In composite ZnO-P 2O 5/Ni, it was shown that the Seebeck coefficient changed from high positive to negative values when the filler amount was increased, indicating a non-conducting to conducting phase transition. Such behavior exhibits that this transition is accompanied by the passing of carrier charge from p to n type. The study of thermoelectrically transport for high volume fraction of filler enabled the achievement, for the first time on this kind of composites, of an original transition called PTC transition. Thus, highest values of power factor (PF = S 2 ≈ 2 × 10 -3 W ·m -1 ·K -2 at 407 K) were obtained, giving a possibility of industrial applications.
The phosphate glasses are semiconducting materials, transforming it as conductive materials that can offer more opportunities of applications. And the zinc phosphate glasses are very studied for long time, but their use as composites is rare. For these reason we decide to prepare composites from this glass due to their special properties.
The study of material thermoelectric properties is among these objectives. The direct conversion between thermal energy and electric energy was discovered by Seebeck in 1882. Many applications were found to valorize this effect in thermoelectric power generation, microdevices, refrigeration etc. [
The present study is to understand the manifestation of the thermo-electrical property of the of the ZnO-P2O5 matrix first, and ZnO-P2O5 composite loaded with different volume fractions of nickel (Ni) as conductive filler. It well known that in this composite system, a transition from insulator to conductor as the volume fraction of the conducting phase is increased. Particular attention is paid to study the parameter of the two phases. For this reason, various models have been proposed to account for the overall composite electrical properties. Good reviews of these models have been presented by the percolation theory in our further work [
The 45 mol% ZnO-55 mol% P2O5 matrix, was prepared using the classical quenching technique [
Scanning Electron Microscopy (SEM) with Energy Dispersive X-ray (EDX) measurement has been used to identify and characterize the matrix. Scanning Electron Microscopy (SEM) studies have been undertaken to examine the morphology of matrix and homogeneity. Energy Dispersive X-ray (EDX) analysis was specially used to detect contaminant species: to obtain qualitative information on the full elemental composition of an unknown contaminant in the matrix.
The XRD measurements were carried by using a diffractometer BRAGG-BRENTANO equipped with a vertical goniometer θ-θ configuration with direct optical encoder for direct position feedback angular on the arms of the goniometer. The diffractometer is equipped with an X-ray tube with copper anticathode (λ = 1.54051 A˚). The data acquisition was carried out using a software-based data sheets ASTM (American Society for Testing and Materials).
The measurements of electrical conductivity (σdc) at room temperature were performed with a Keithley 224 current source and a Keithley 616 voltmeter, by four-point technique of Valdes [
where, ΔT and ΔV are the difference of temperature and the difference of potential respectively, between two points of the sample.
The volume expansion of the composites was obtained with a SETARAM TMA 92 thermo-mechanical analyzer, using a programmed heating rate of 2 K/min in a temperature range varying from 300 to 500 K.
・ Structural characterization
The binary phosphate glasses of molar composition 45 mol% ZnO-55 mol% P2O5 were synthesized. After cooling the melts, the glass was homogenous, transparent, colorless, and amorphous. In
The Energy Dispersive X-ray (EDX) analysis showed a good purity in the composition of matrix. Their amorphous nature was confirmed by X-ray diffraction in
400˚C and crystalline phase completely is obtained at temperature 500˚C. This could be understood by a thermodynamic approach [
・ Electrical conductivity of matrix
In this study, to get correlation between structural characteristic and physical properties, the electrical conductivity of amorphous ZnO-P2O5 matrix has measured in the temperature range (300 - 400 K).
where σo is the conductivity of the matrix at room temperature, E0 is the activation energy for hopping, k the Boltzmann constant and T absolute temperature. The linear fit of the conductivity data versus (1/T) (
・ Thermoelectric power characterization of matrix
In amorphous materials, the thermoelectric power is an important property to determine the charge carriers. The thermoelectric power is given by the general expression [
The above expression can be integrated as follow:
where K is Boltzman’s constant, Ec is the position of the band edge, Ev the valence band edge. Av and Ac are temperature independents. The sign of thermopower is depend of the mechanism of conduction above or below the Fermi level [
In our study, we have just determined electrical conductivity and thermopower measurements, to calculate a part of the figure of merit, i.e. power factor (PF), with the following formula:
The power factor (PF) can be used as an index for thermoelectricity. This value gives us information on the figure of merit ZT, which a high ZT indicates a high thermoelectric efficiency.
The zinc phosphate glass in this study is chosen because of its good glass forming ability and good chemical durability, but the PF obtained is about 2 × 10−13 W/m・K2. This value is not sufficient and very low for application
and needs improvement. For these reason we decide to prepare composites from this glass due to their special properties. The measurements of electrical conductivity and thermoelectric power of composites were achieved in this study. We have calculate the PF of composite as function of volume fraction of Nickel (from 3 to 40 vol.%) at room temperature, and as a function of temperature for composite 40 vol.% of Ni.
・ Power factor versus volume fraction measurements
The
・ Power factor versus temperature measurements
The power factor (PF) as function of temperature for the composite 40 vol.% of Ni, are showed in
In our study, the measurements of the volume expansion of composite 40 vol.% of Ni as function of temperature.
The result obtained in this study by conductivity σ and thermoelectric power S measurement shows as follows:
・ Structural characterization by SEM and XRD of binary phosphate glasses 45 mol% ZnO-55 mol% P2O5 shows homogenous and amorphous phases.
・ The conductivity of this amorphous phases increases with increase in temperature. In addition, the electrical conductivity increase and the activation energy decrease in the range studied of temperature. And the plot of both conductivity and Seebeck coefficient versus temperature follows Arrhenius relation.
・ For the composite ZnO-P2O5/Ni, the electrical conductivity measurements showed percolating transition from a non-conducting to conducting phase as a function of metal filled. And the change of S from high positive to negative values exhibits that this transition is accompanied by the passing of carrier charge from p-type to n-type. The values of Seebeck coefficient measured are large and confirm the conductivity results.
・ Highest value of power factor was obtained giving a possibility of industrial applications in thermoelectricity and solar cell devices.
This work was realized in the frame of the scientific projects (SPM10/10, 11) supported by the collaboration of Centre National pour la Recherche Scientifique et Technique (CNRST), Morocco and Centre National de la Recherche Scientifique (CNRS), France. These organisms are gratefully acknowledged for partial financial support.
Omar Oabi,Abdelkarim Maaroufi,Bruno Lucas,Aumeur El Amrani, (2016) Thermoelectric Properties of ZnO-P2O5/(Ni) Composites. New Journal of Glass and Ceramics,06,19-27. doi: 10.4236/njgc.2016.63003