Corresponding author: Alexander S. Fedotov (fedotov@bsu.by)
The search for new economically advantageous technologies of new zinc oxide based composite ceramic materials and the study of their structure and properties attract special attention today. These ceramics have a number of advantages as compared with materials prepared by more expensive technologies, due to the possibility to fabricate items having different shapes and sizes and particularly to vary their morphology, structure and phase composition. This allows controlling their functional properties by varying the powder particle size in charge, the temperatures, durations and atmospheres of synthesis and heat treatment, and the types of doping impurities in the ceramics. The structure and electrical properties of (Fe
The search for new economically advantageous technologies and the study of the structure and properties of new zinc oxide base composite ceramic materials currently draw special attention [
Zinc oxide can be used as the base material for the fabrication of conventional devices (e.g. variable resistors [
Zinc oxide base composite ceramics can be used for the fabrication of thermoelectric transducers [
Doping of ZnO base composite ceramic materials with transition elements is of special research interest [
There are numerous literary experimental data on the effect of doping with transition or other elements on the structure and properties of single crystals, polycrystalline films, nanostructured powders, nanowires or tetrapods [5–12, 21–23]. However, ZnO base compositions synthesized using ceramic technologies have not yet been studied completely (except ceramics for variable resistors and optical applications [
The aim of this work is to study the effect of synthesis method and type of doping agents (Fe
The ZnO base specimens were synthesized using the conventional open air powder sintering technique for ceramics [
Specimen synthesis methods.
Spec. No. | Specimen* | Synthesis method | Preliminary/Final Sintering | |
---|---|---|---|---|
Temperature, °C | Time, h | |||
1 | ZnO | Sintering | 1100 | 2 |
2 | (ZnO)90(FeO)10-1 | Single-Stage Synthesis | 1200 | 2 |
3 | (ZnO)90(Fe2O3)10-1 | |||
4 | (ZnO)90(Fe3O4)10-1 | |||
5 | (ZnO)90(FeO)10-2 | Two-Stage Synthesis | 900/1200 | 2/2 |
6 | (ZnO)90(Fe2O3)10-2 | |||
7 | (ZnO)90(Fe3O4)10-2 | |||
8 | (ZnO)90(Fe2O3 + FeO)10-2 | Two-Stage Synthesis | 4/48 |
* “1” and “2” in the specimen notations refer to single- and two-stage synthesis, respectively.
The powders were mixed in a stainless steel attritor with hard-alloy balls in the presence of a humidifier (alcohol). The initial oxide particle sizes (as-grinded for 12 h) were 10–50 μm. For charge preparation the source oxides were dried at 500–900 °C approaching constant weight, following which 3 wt.% PVA glue binder was added to the charge. Then the mixture was uniaxially compacted at 200 MPa to 10–18 mm diam. 2–5 mm thickness tablets. For single-stage synthesis the as-compacted tablets were sintered in air at 1200 °C for 2 h. For two-stage synthesis the tablets were preliminarily sintered at 900 °C for 2 h, then ground again to powder, mixed with the binder, compacted and annealed in open air at 1200 °C for 2 h. As can be seen from Table
The structure and phase composition of the ceramics were studied at room temperature using X-ray diffraction (
The element concentrations in the composites were measured using selected area electron probe X-ray analyzers for energy dispersive X-ray analysis (
The porosity of the specimens was studied under an Olimpus GX41 microscope (Japan) with the AutoScan 005 software.
Mössbauer spectra were taken in the 20–300 K range on an MS4 Mössbauer spectrometer (SEE Co., USA) for powdered specimens (57Fe isotope, transmission mode, 57Co/Rh source (20 mCi)). The spectra were approximated using the Rancourt method based MOSMOD software [
Raman spectra were taken on a Nanofinder High-End LOTIS TII confocal spectrometer (Belarus, Japan). The signal was excited with a solid state laser (532 nm, 20 mW). The laser radiation was focused on the specimen surface with a 50× objective (digital aperture 0.8+). The incident laser radiation power was reduced to 2 mW for the avoidance of heat damage. The backscattered light was dispersed by a 600 mm-1 diffraction grating for achieving a spectral resolution of not worse than 3 cm-1. Spectral calibration was based on gas discharge lamp lines for achieving an accuracy of not worse than 3 cm-1. The signal acquisition time was 30 s. The photodetector was a cooled silicon CCD matrix.
The temperature dependences of the electrical resistivity ρ(
The current passing through the specimen was controlled with a Keithley 6430 Sub-Femtoamp Remote SourceMeter which provided for specimen electrical resistivity measurements in the 100 μOhm – 20 GOhm range with an accuracy of not worse than 0.1%. The specimen temperature was controlled with LakeShore thermal diodes calibrated accurate to a 0.0005 K and having a reproducibility of 0.001 K thus allowing temperature stabilization and measurement with an accuracy of not worse than 0.005 K using a LakeShore 331 measurement controller. The relative error of the electrical resistivity measurements was within 5%, being mainly determined by the specimen dimension measurement error and the sizes of the electric potential contacts.
Table
Main structural parameters of the specimens.
No. | Specimen | Porosity | Average Fe concentration in ZnO, at.% | Fe(δ) concentration in Zn1-δFeδO, at.% | Lattice parameters, nm | |
---|---|---|---|---|---|---|
Wurtzite phase | Ferrite phase | |||||
1 | ZnO | 14.6 | 0 | 0 | ||
– | ||||||
2 | (ZnO)90(FeO)10-1 | 17.0 | 1.2 | 0.77 | ||
– | ||||||
3 | (ZnO)90(Fe2O3)10-1 | 15.5 | 2.3 | 0.82 | ||
– | ||||||
4 | (ZnO)90(Fe3O4)10-1 | 18.7 | 2.87 | 0.69 | ||
– | ||||||
5 | (ZnO)90(FeO)10-2 | 18.3 | 1.0 | 0.81 | 0.83892 | |
6 | (ZnO)90(Fe2O3)10-2 | 17.1 | 2.81 | 0.66 | 0.84074 | |
7 | (ZnO)90(Fe3O4)10-2 | 16.5 | 2.98 | 0.79 | ||
0.84322 | ||||||
8 | (ZnO)90(Fe2O3 + FeO)10-2 | 17.0 | 5.04 | 0.87 | ||
0.84077 |
Figure
Typical SEM images of the grain structure at the cleavage surfaces of (Fe
According to the XRD data the (Fe
Typical XRD spectra for undoped ZnO and (Fe
Based on X-ray pattern fitting we can attribute the additional peaks (marked with arrows in Fig.
The presence of additional phases in the (Fe
Example of EDXA iron distribution in (ZnO)90(FeO)10-2 composite ceramic (Table
The Raman spectra of the specimens also suggest the presence of not only the wurtzite phase (Fig.
Examples of Raman spectra for the wurtzite phase in (
Examples of Raman spectra for phase inclusions in (
The Mössbauer data confirm the XRD, EDXA and Raman data indicating the presence of additional phases with high iron content in the experimental ceramic specimens. To analyze the phase transformations occurring during the synthesis of the composite ceramics studied we measured the Mössbauer spectra both for the initial powder mixtures before compacting and for the as-synthesized ceramic specimens with different doping impurities for single- and two-stage synthesis methods (Fig.
Mössbauer spectra of (
As can be seen from Fig.
Below we present the electrical properties of the (Fe
Main electrical parameters of the specimens.
No. | Specimen | Average Fe concentration in (Fe |
Fe(δ) concentration in Zn1-δFeδO, at.% | ρ at 300 K, Ohm · m | Δ |
---|---|---|---|---|---|
1 | ZnO | 0 | 0 | 3.67 · 101 | 0.05–0.07 |
2 | (ZnO)90(FeO)10-1 | 1.2 | 0.77 | 7.51 · 100 | 0.36 |
3 | (ZnO)90(Fe2O3)10-1 | 2.3 | 0.82 | 1.32 · 100 | 0.27 |
4 | (ZnO)90(Fe3O4)10-1 | 2.87 | 0.69 | 2.86 · 105 | 0.34 |
5 | (ZnO)90(FeO)10-2 | 1.0 | 0.81 | 7.89 · 102 | 0.36 |
6 | (ZnO)90(Fe2O3)10-2 | 2.81 | 0.66 | 3.18 · 103 | 0.37 |
7 | (ZnO)90(Fe3O4)10-2 | 2.98 | 0.79 | 6.12 · 102 | 0.37 |
8 | (ZnO)90(FeO + Fe2O3)10-2 | 5.04 | 0.87 | 4.37 · 101 | 0.24 |
Temperature dependences of electrical resistivity for (
As can be seen from Fig.
By and large, as can be seen from Fig.
One group (Curves
The second group of curves in Fig.
We will now discuss the experimental data on the structure and electrical properties of the ceramics. Table
with the conductivity activation energy Δ
Note that multiple researchers reported ionization or conductivity activation energies of ~0.15–0.4 eV for undoped zinc oxide which were attributed to the presence of the impurity band and/or tails of localized states [
As noted above the ρ(
where α = 0.25 for Mott’s mechanism in bulk semiconductors and α = 0.5 for Shklovskii-Efros’ hopping. The ρ01 and
The relevance of the ρ(
Temperature dependences of electrical resistivity for (
As can be seen from Curve
Diagram of the density of states for
where the conductivity activation energy Δ
As noted above the ρ(
It should be noted that a combination of hopping conductivity attributed to Mott’s law described by Eq. (2) at below 40 K with conventional band conductivity described by Eq. (1) at above 50 K was observed earlier in intrinsic ZnO single crystals [
The structure, type of doping Fe oxide and temperature were shown to affect the electrical properties of (Fe
The Authors express their gratitude to the State Research Program “Physical Materials Science, New Materials and Technologies” (Belarus) for financial support.