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1.
The electrical conductivity and thermoelectric power of KTaO3 were measured from 900° to 1300°C over a range of oxygen partial pressures. The isothermal electrical conductivity showed a minimum at an oxygen partial pressure corresponding to the transition between p-type and n-type behavior. A point defect model was developed which involved fully ionized potassium and tantalum vacancies, singly and doubly ionized oxygen vacancies, holes, and electrons. The values of all pertinent equilibrium constants were calculated from the experimental data and the nonsimplified neutrality condition was solved to give each of the defect concentrations as a function of temperature and oxygen partial pressure. The calculated conductivity agreed extremely well with the experimental data over the full temperature and oxygen partial pressure range, and the band gap derived from these calculations (3.43 eV) was in excellent agreement with the reported value (3.5 eV). 相似文献
2.
Equilibrium electrical conductivity data for large-grained, poly crystalline, undoped BaTiO3 , as a function of temperature, 750° to 1000°C, and oxygen partial pressure, 10−20 < P O2 <10−1 MPa, were quantitatively fit to a defect model involving only doubly ionized oxygen vacancies, electrons, holes, and accidental acceptor impurities. The latter are invariably present in sufficient excess to control the defect concentrations through the compensating oxygen vacancies, except under the most severely reducing conditions. Singly ionized oxygen vacancies play no discernible role in the defect chemistry of BaTiO3 within this experimental range. The derived accidental acceptor content has a slight temperature dependence which may reflect some small amount of defect association. Deviation of the conductivity minima from the ideal shape yields a small P O2 -independent conductivity contribution, which is tentatively identified as oxygen vacancy conduction. 相似文献
3.
The defect structure of high-purity, polycrystalline HfO2 was investigated by measuring the oxygen partial pressure dependence of the electrical conductivity and the sample weight. From 1000° to 1500°C and above oxygen partial pressures of 10 −6 , the conductivity is electronic and proportional to p o2 1/5 . The predominant defect is completely ionized hafnium vacancies. At lower oxygen partial pressures a broad shallow minimum in the lower temperature conductivity isotherms indicates the presence of an oxygen pressure independent source of electronic charge carriers. By combining the weight change and conductivity data, mobility values were found to vary from 1.6 × 10−3 to 3 × 10−4 cm2 /V-sec. The activation energies for the hole mobilities were calculated to be 0.2 ev above 1300° C and 0.7 ev below this temperature. 相似文献
4.
Electrical conductivity, thermoelectric power, and weight change were measured for polycrystalline Ta2 O5 from 900° to 1400°C. The predominant ionic and electronic defects in this temperature range are oxygen vacancies and electrons. The oxygen-vacancy and electron mobilities are 8.1 × 103 exp (−1.8 eV/ k T) and ∼0.05 cm2 /V-s, respectively. At O2 partial pressures near 1 atm, the ionic-defect concentration is essentially fixed by the presence of lower-valence cation impurities, and the total electrical conductivity is predominantly ionic, whereas at low P o2 's the conductivity is electronic and proportional to P P o2 −1/6 . 相似文献
5.
The electrical conductivity and thermoelectric power of highpurity polycrystalline ThO2 in thermodynamic equilibrium with the gas phase were measured as a function of temperature from 1000° to 1600°C and as a function of oxygen partial pressure from 1 to 10−22 atm. An n -type electronic contribution to the conductivity is observed above 1400°C at low oxygen pressures. An analytic solution is presented for the oxygen pressure dependence of the total conductivity in the mixed ionicelectron hole conduction region observed at higher oxygen pressures. The activation energies for p -type and ionic conduction are 1.0 and 0.9 eV, respectively. The combined conductivity and thermal emf data give a lower limit of ∼6 cm2 /V-s for the electron hole mobility. 相似文献
6.
MASARU MIYAYAMA SHUYA OGI HIROAKI YANAGIDA 《Journal of the American Ceramic Society》1983,66(5):351-352
Electrical conductivity and thermoelectric power were measured on sintered α-Sb2 O4 at 250° to 780°C. Oxygen partial pressure dependence of the conductivity and sign of the Seebeck coefficient showed α-Sb2 O4 to be a p -type semiconductor above 600°C in the oxygen pressure range of lo5 to 102 Pa. A hopping conduction was proposed from very small hole mobility with an activation energy of 18 kJ/mol. 相似文献
7.
The electrical conductivity of PrFeO3 and Pr2 NiO4 was investigated at 300° thd 1000°C and at oxygen partial pressures of 1 to 10−20 atm and the phase relations and nonstoichiometry of these materials were studied. The results suggest that PrFeO3 is a semiconductor exhibiting intrinsic behavior at T>300°C and Po2 >10−5 atm. The conductivity of Pr2 NiO4 depends on Po2 and is thus related to deviations from stoichiometry. These results for Pr2 NiO3 raise questions about the validity of the suggested semiconductor-to-metal transition model for explaining the electrical properties of La2 NiO3 and Nd2 Ni04 . 相似文献
8.
Yu-Dong Lu Xin Wang Zhi-Qiang Zhuang Bao-Ling Liu 《Journal of the American Ceramic Society》2007,90(11):3628-3631
The equilibrium defect chemistry of polycrystalline, undoped, and acceptor-doped BaPbO3 was studied by measurement of the equilibrium electrical conductivity as a function of temperature, 800°–900°C, and oxygen activity, 10−18 –1 atm. Both equilibrium electrical conductivity data of undoped and acceptor-doped samples were quantitatively fit to a defect model involving only doubly ionized oxygen vacancies, lead vacancies, holes, and acceptor impurities. The results in low and midrange of oxygen activity are dominated by acceptor impurities, whether deliberately added or not. Only in the highly oxidized condition is the conductivity independent of impurity content, confirming that this region represents the intrinsic behavior of BaPbO3 . 相似文献
9.
A combination of ac and dc electrical conductivity measurements was used to characterize the charge transport in single crystals of CaW04 which were equilibrated with H2 -H2 O-Ar gas mixtures. Measurements were made from 900° to 1300°C and at PH2 O /PH2 ratios from 0.02 to 3.0. The ac conductivity at 1000°C varied from 51.4×10−6 to 5.89×10−6 mho/cm for PH2 O /PH2 =0.02 and 2.0, respectively; the dc conductivity changed from 51.0±10−6 to 5.42×10−6 mho/cm under the same experimental conditions. The partial logarithmic derivative of dc conductivity with respect to PH2 O /PH2 was −1/2 between 1000° and 1300°C. The results may be described by a paired-defect model of oxygen vacancies and oxygen interstitials (majority defects and minority electrons). 相似文献
10.
The defect structure of monoclinic ZrO2 was studied by measuring the transfer numbers and electrical conductivity as functions of O2 pressure and temperature. The data suggest a defect structure of doubly ionized oxygen vacancies at low pressures, i.e. <10−19 atm, and singly ionized oxygen interstitials at pressures >10−9 atm. Zirconia is primarily an ionic conductor below #700°C and an electronic conductor at 700° to 1000°C for 10−22 ≤Po2 ≤1 atm. 相似文献
11.
Studies of the oxidation of Gd and Dy at P O2 's from 10−0.3 to 10−14.5 atm and temperatures from 727° to 1327°C indicate both semiconducting and ionic-conducting domains in the sesquioxides formed. At higher temperatures, where dense coarsegrained oxide layers developed, the rate of oxidation in the high- P 02 semiconducting domain yielded oxygen diffusion coefficients in Dy2 O3 in excellent agreement with literature values derived from oxidation of partially reduced oxide single crystals. Under the same conditions, the oxidation of Gd yielded oxygen diffusion coefficients in cubic Gd2 O3 which are considerably below literature values for monoclinic single-crystal Gd2 O3 . At lower temperatures, porous scales were formed, and apparent diffusion coefficients derived from oxidation rates show a smaller temperature dependence than the high-temperature data. At low P O2 , the oxides behave as ionic conductors, and metal oxidation rates result in estimates of the electronic contribution to the electrical conductivity of the order of 10−6 to 10−7 Ω−1 cm−1 . 相似文献
12.
Bernhard Ostrick Maximilian Fleischer Hans Meixner 《Journal of the American Ceramic Society》1997,80(8):2153-2156
Simultaneous Hall and conductivity measurements were performed in situ between 650° and 1050°C on n-type semiconducting BaSnO3 ceramics. The variation of the Hall mobility and the Hall carrier density as a function of oxygen partial pressure between 102 and 105 Pa and of temperature was investigated. At temperatures below 900°C the conductivity exhibits a dependence on temperature and oxygen partial pressure which is mainly determined by variations of the Hall mobility. Above 900°C most of the significant dependence is due to a variation in carrier density. Furthermore, a simple defect model assuming doubly ionized oxygen vacancies and acceptor impurities is discussed for BaSnO3 . 相似文献
13.
Yuan-Hua Lin Yong Liu Bo-Ping Zhang Ce-Wen Nan Jing-Feng Li Zhijian Shen 《Journal of the American Ceramic Society》2007,90(12):4005-4008
Highly dense La2 CuO4 ceramics have been prepared by the spark plasma sintering technique. Temperature dependence of electrical conductivity indicates that La2 CuO4 ceramics sintered at over 875°C exhibit a metal-like behavior, which should be ascribed to the special La2 CuO4 crystal structure and its correlation splitting of the half-filled d x 2− y 2 band. Our experimental data indicate that all of the La2 CuO4 samples exhibit positive thermoelectric power in the whole measuring temperature range, indicating that the majority of charge carriers are holes. It is desirable to obtain good thermoelectric performance for this system by optimizing the electrical properties and reducing the thermal conductivity. 相似文献
14.
Yuheng Liu Yuanhua Lin Zhan Shi Ce-Wen Nan Zhijian Shen 《Journal of the American Ceramic Society》2005,88(5):1337-1340
The precursor powders of Ca3 Co4 O9 were synthesized by a sol–gel method. The results of X-ray diffraction and thermogravimetric and differential thermal analyses patterns indicate that pure Ca3 Co4 O9 powders could be obtained by calcining the precursor at 800°C for 2 h. High dense Ca3 Co4 O9 ceramic samples (∼99% of theoretical density) were prepared by the spark plasma sintering (SPS) method. Compared with the conventional sintering (CS), the SPS samples exhibit much higher electrical conductivity and power factor which are respectively about 118 S/cm and 3.51 × 10−4 W·(m·K2 )−1 . The SPS method is greatly effective for improving the thermoelectric properties of Ca3 Co4 O9 oxide ceramics. 相似文献
15.
Compositions in the system ThO2 -YO1.5 were coprecipitated as oxalates and converted to oxides. Disks were pressed and sintered in oxygen at 1400° to 2200°C. Densities of the sintered disks were 96 to 98% of theoretical. Solid solutions with the fluorite-type structure were formed up to 20 to 25 mole % YO1.5 at 1400°C and up to 45 to 50 mole % YO1.5 at 2200°C. Density data showed that these solid solutions correspond to Th1— x Y x O2—0.5 x , having a complete cation sub-lattice filled by Th4+ and Y3+ ions, and vacancies in the anion sublattice. The observed increase in electrical conductivity with increase in YO1.5 content is consistent with charge transport by oxygen ions through a vacancy mechanism. Approximately 7 mole % ThO2 is soluble in YO1.5 at 2200°C. Density results indicate an anion interstitial structure for the Y2 O3 phase. Transference number measurements indicate that the electrical conductivities are only partly due to ions. 相似文献
16.
The electrical conductivity of polycrystalline Y2 O3 has been studied as a function of the partial pressure of oxygen (10–14 to 105 Pa) at 900° to 1500°C in atmospheres saturated with water vapor at 12°C or dried with P2 O5 . Yttria is a p -conductor at high oxygen activities. The p -conductivity increases with increasing P O2 and decreases with increasing PH2O . At low oxygen activities the oxide is a mixed ionic/electronic conductor. The ionic conductivity is approximately independent of P O2 and increases with increasing P H2O . In the Y2 O3 samples, excesses of lower-valent cation impurities (in the 10 to 100 mol-ppm range) are the dominating negatively charged defects, and in the presence of water vapor they are compensated by interstitial protons. At high P H2O levels additional protons are probably compensated by interstitial oxygen ions. At high temperatures (±1100°C) and for high P O2 and low P H2O , the protons are no longer dominant, and the lower-valent cations are mainly compensated by electron holes. The electrical conductivity exhibits hysteresis-like effects which are interpreted in terms of segregation/desegregation of impurities at grain boundaries. The mobility of electron holes in yttria at 1500°C is estimated to be of the order of magnitude of 0.05 cm2 . s–1 . V–1 相似文献
17.
MASARU MIYAYAMA SHINICHI KATSUTA YOSHIHIRO SUENAGA HIROAKI YANAGIDA 《Journal of the American Ceramic Society》1983,66(8):585-588
Electrical conduction in tetragonal β-Bi2 O3 doped with Sb2 O3 was investigated by measuring electrical conductivity, ionic transference number, and Seebeck coefficient. The β-Bi2 O3 doped with 1 to 10 mol% Sb2 O3 was stable up to 600°C and showed an oxygen ionic and electronic mixed conduction, where the electron conduction was predominant at low oxygen pressures. The oxygen-ion conductivity showed a maximum at 4 mol% Sb2 O3 , whereas the activation energy for the ionic conduction remained unchanged for 4 to 10 mol% Sb2 O3 -doped specimens. These results were interpreted in terms of the oxygen vacancy concentration and the distortion of the tetragonal structure. The electron conductivity and its oxygen pressure dependence decreased with increasing Sb2 O3 content. The fact that Sb5+ is partially reduced by excess electrons in heavily doped β specimens at low oxygen pressures is explained. 相似文献
18.
Guarded measurements of the electrical conductivity of high-purity, polycrystalline Y2 O3 in thermodynamic equilibrium with the gas phase were made under controlled temperature and oxygen partial pressure conditions. Data are presented as isobars from 1200° to 1600°C, and as isotherms from oxygen partial pressures of 10−1 to 10−17 atm. The ionic contribution to the total conductivity, determined by the blocking electrode polarization technique, was less than 1% over the entire range of temperatures and oxygen partial pressures studied. Yttria is shown to be an amphoteric semiconductor with the region of predominant hole conduction shifting to higher pressures at higher temperatures. In the region of p -type conduction, the conductivity is represented by the expression σ= 1.3 × 103 p O2 3/16 exp (-1.94/kT). The observed pressure dependence is attributed to the predominance of fully ionized yttrium vacancies. Yttria is shown to be a mixed conductor below 900°C. 相似文献
19.
JUNICHIRO MIZUSAKI TADASHI SASAMOTO† W. R. CANNON‡ H. KENT BOWEN 《Journal of the American Ceramic Society》1982,65(8):363-368
Electronic conductivity and Seebeck coefficients of LaFeO3 were measured as a function of temperature (1000° to 1400°C) and P ( O 2 ) (105 to 10−13 Pa). Electronic conduction was found to be n-type in the lower P ( O2 ) range, and p -type in a higher P(O2 ) range. The calculated carrier mobilities suggest a hopping-type conduction mechanism. The carrier concentrations were calculated as a function of P ( O2 ) and the defect structure was described. It was found that the electrical properties of LaFeO3 are determined not only by the concentration of oxygen vacancies, but also by the La/Fe ratio. 相似文献
20.
The electrical conductivity and ion/electron transference numbers in Al3 O3 were determined in a sample configuration designed to eliminate influences of surface and gas-phase conduction on the bulk behavior. With decreasing O2 partial pressure over single-crystal Al2 O3 at 1000° to 1650°C, the conductivity decreased, then remained constant, and finally increased when strongly reducing atmospheres were attained. The intermediate flat region became dominant at the lower temperatures. The emf measurements showed predominantly ionic conduction in the flat region; the electronic conduction state is exhibited in the branches of both ends. In pure O2 (1 atm) the conductivity above 1400°C was σ≃3×103 exp (–80 kcal/ RT ) Ω−1 cm−1 , which corresponds to electronic conductivity. Below 1400°C, the activation energy was <57 kcal, corresponding to an extrinsic ionic condition. Polycrystalline samples of both undoped hot-pressed Al2 O3 and MgO-doped Al2 O3 showed significantly higher conductivity because of additional electronic conduction in the grain boundaries. The gas-phase conduction above 1200°C increased drastically with decreasing O2 partial pressure (below 10−10 atm). 相似文献