Defect Structure and Electrical Conductivity of ThO2-Y2O3 Solid Solutions

Compositions in the system ThO₂-YO_1.5 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 % YO_1.5 at 1400°C and up to 45 to 50 mole % YO_1.5 at 2200°C. Density data showed that these solid solutions correspond to Th_{1— x} Y _x O_{2—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 YO_1.5content is consistent with charge transport by oxygen ions through a vacancy mechanism. Approximately 7 mole % ThO₂ is soluble in YO_1.5 at 2200°C. Density results indicate an anion interstitial structure for the Y₂O₃ phase. Transference number measurements indicate that the electrical conductivities are only partly due to ions.     

Electrical Properties and Defect Structure of HfO2

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.     

Electrical Properties and Defect Structure of a (0.13YO1.5·0.87ThO2) Electrolyte

High-density sintered disks of the composition 0.13YO_1.5·0.87ThO₂ are shown to be mixed conductors at high oxygen pressures (>10⁻⁶ atm) by electrical conductivity and electrochemical cell measurements. The ac and dc conductivity measurements were made between 900° and 1600°C over a wide range of oxygen partial pressures. A blocking-electrode polarization technique for determining transference numbers was not applicable at high oxygen pressures but appeared to work at the lower pressures, indicating a transition to n -type behavior. The electrochemical cell measurements show essentially completely ionic behavior at low oxygen pressure but indicate at least 0.1% electronic contribution at 10⁻¹³ atm at 1000°C. The lower oxygen pressure limit for completely ionic behavior has not been determined but extends below the equilibrium pressures of an Mn-MnO₂, Cr-Cr₂O₃ electrochemical cell at 1000°C.     

Electrical Properties and Defect Structure of Y2O3

Guarded measurements of the electrical conductivity of high-purity, polycrystalline Y₂O₃ 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.     

Defect Structure and Electrical Properties of KTaO3

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).     

High-Temperature Electrical Properties in the ThO2-Rich Region of ThO2-RE2O3 Systems

In the present study the n -type electronic conduction in terms of the parameter p e, and the phase relations in several ThO₂-RE2O₃ systems were examined. Large fluorite solid solution regions exist at elevated temperatures. It was demonstrated that RE₂O₃-doped thoria compositions feature lower parameters p e, and higher chemical stability than the conventional stabilized ZrO2 electrolytes. The results are given in terms of the characteristic parameter p e, in the temperature range from 1000° to 1600°C. The experimental investigations were made using a new thermodynamic measuring system.     

Activated Sintering of ThO2 and ThO2-Y2O3 with NiO

The effect of additives on the sintering of ThO₂ and ThO₂-Y₂O₃ compacts and loose powders was studied by isothermal shrinkage measurements and by scanning electron micrography. Small amounts of the oxides of Ni, Zn, Co, and Cu reduced the sintering temperature. The behavior of NiO at a concentration of 0.8 wt% (2.5 mol%) was studied in detail and found to yield high-density bodies at temperatures below 1500°C. The presence of Y₂O₃ as a separate phase increases the rate of sintering of ThO₂, but smaller amounts of NiO are much more potent. The major portion of the densification occurs very rapidly and is followed by a much slower sintering process typical of volume diffusion. The fast early shrinkage may be caused by the capillary forces of a liquid, but since no evidence of melting was found, a solid-state mechanism may be responsible.     

Effect of Oxide Defect Structure on the Electrical Properties of ZrO2

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≤P_o2≤1 atm.     

Defect Structure and Electrical Properties of Single-Crystal Ba0.03Sr0.97TiO3

The electrical conductivity and thermoelectric power of single-crystalline Ba_0.03Sr_0.97TiO₃ were measured over a wide temperature (800° to 1100°C) and oxygen partial pressure (10⁵ to 10^-15 Pa) range. Our experimental data, like those of previous workers on nominally undoped BaTiO₃ or SrTiO₃, support a defect model based on doubly ionized oxygen vacancies, electrons, holes, and accidental acceptor impurities. The simultaneous measurement of electrical conductivity and thermoelectric power, together with precise experimental data obtained with an advanced thermoectric power measurement technique, enabled us to determine for the first time reliable values for the preexponential factors and the activation energies which characterize the defect equilibrium constants. These calculated values, together with the defect model, were found to give an excellent fit to the experimental data, and were used to generate the boundaries, in P _o2-1/ T space, of the various defect regimes.     

Final Stage Sintering of ThO2

The addition of CaO to ThO₂ inhibits discontinuous grain growth and allows sintering to proceed to theoretical density by maintaining a high diffusion flux of vacancies from the pores to the grain boundaries. The optimum amount of CaO added is 2.0 mol%, which exceeds the solid solubility limit. A model of second-phase inclusions impeding grain boundary motion was used to explain the results.     

The System ThO2-P2O5

The existence of compounds with 1:1, 3:2, and 3:1 ThO₂:P₂O₅ ratios in the system ThO₂-P₂O₅ was confirmed. A 1:2 compound found by previous workers was not investigated, and their 2:1 compound was not detected; however, extensive solid solution on either side of the 3:2 compound was established. The linear thermal expansion behavior of the compounds and solid solutions was determined.     

Optical Absorption Spectroscopy of ThO2

The optical absorption spectra from 200 to 2700 nm were determined for arc-fused ThO₂ crystals of much higher purity than any available hitherto. The fundamental absorption edge for these crystals lies at a much higher energy (∼5.9 eV) than the apparent edge reported previously for less pure specimens. This apparent edge is shown to result from one or more prominent high-energy absorption bands superimposed on the true edge for such specimens. By comparing the spectra for different specimens subjected to thermal and γ-irradiation treatments, 31 distinct absorption bands were revealed in the ThO₂ spectrum. A full analysis of certain crystals by spark-source mass spectrometry gave no strong correlation between specific band areas and individual impurity-element concentrations. Most of the bands, however, are believed to be associated with the impurity content.     

Defect Structure of Ta2O5

Electrical conductivity, thermoelectric power, and weight change were measured for polycrystalline Ta₂O₅ 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 × 10³exp (−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.     

Revised ThO2-Nb2O5 Phase Diagram

The phase equilibrium diagram of the system ThO₂-Nb₂O was redetermined near the composition Th₂Nb₂O₉. This phase was found to melt incongruenlly at 1362°C, with a eutectic temperature at ∼1350°C. The peritectic and eutectic compositions must occur between 60 and ∼64 mol % ThO₂. From single crystal and powder X-ray diffraction data, Th₂ Nb₂O₉ was found to have a primitive monoclinic unit cell with a = 6.711(1), b = 25.254(5), c=7.757(1)×10⁻¹nm, β=90.46 (1)°.     

Defect Structure and Dielectric Properties of Nd2O3-Modified BaTiO3

The defect structure and dielectric properties of BaTiO₃ with 1 to 10 mol% Nd₂O₃ additions were studied. The results indicated that neodymium occupies the barium site and charge compensation takes place by creation of titanium vacancies. The dependence of inverse electric susceptibility, spontaneous polarization, and specific heat on temperature for samples containing more than 2 mol% of Nd₂O₃ were characteristic of a diffuse transformation resulting from a disordering of defects. The addition of Nd2O3 leads to a very drastic sfiift in the Curie temperature ( Tc ) of BaTiO₃; 3 mel% Nd₂O₃ addition moves Tc below room temperature.     

Thermal Simulation of In-Reactor Microstructures in ThO2-UO2

A device was built to provide the capability for out-reactor simulation of the microstructures seen in irradiated fuel. Out-reactor simulation of nuclear heating makes possible the precise measurement of temperature gradients in the fuel and provides data for correlating microstructural alterations with temperature. The simulation apparatus was designed to produce controlled thermal gradients sufficient to produce both equiaxed and columnar grain growth in the specimens. Provision was made for calorimetric measurements from which thermal conductance could be determined. Micro-structural alterations in UO₂ and ThO₂-UO₂ materials were achieved, and thermal conductivity data were recorded. Microstructural alterations in the ThO₂-10 wt% UO₂ specimens were similar to those that other investigators have observed in UO₂ at lower temperatures.