Properties of Lithium-Doped Hydrothermally Grown Single Crystals of Zinc Oxide

The piezoelectric, dielectric, and optical proper ties of hydrothermally grown ZnO were exam ined. Although the material was strained it was not electrically twinned and the values of the electromechanical coupling factors, k 33 (= 0.47) measured by static loading and k 31 (= 0.188) measured by the resonance-antiresonance tech nique, compared favorably with those measured in vapor-grown crystals. The concentration of Li ⁺ substituted on a Zn site which was necessary to compensate the crystals was about 5 × 10¹⁸atoms/cm³. This concentration could be ob tained easily by growth from hydrothermal solu tions containing 2.0 molal LiOH. It was neces sary to heat-treat hydrothermally grown crystals at 800° for about 50 hours, to diffuse out inter stitial zinc, to obtain high-resistivity material. Typical dc resistivities of 10¹⁰ ohm-cm were ob tained by such treatment. There was a pro nounced anisotropy in the dielectric constant with K ₃₃ varying between 9.9 and 11.7 and K ₁₁ varying between 8.1 and 9.0 at 100 kc. The size and properties of Li ⁺-doped hydrothermal ZnO make it of interest for transducers.     

Electrical Properties of Single Crystals, Bicrystals, and Polycrystals of MgO

The high oxygen pressure conductivity of high-purity single-crystal magnesium oxide at high temperatures varied as the 1/4 power of the oxygen partial pressure with a 3 eV activation energy; the low pressure conductivity varied as the -1/6 power of the pressure with an activation energy of 4 eV. The predominant defects proposed are (1) holes and singly ionized Mg vacancies at high pressure and (2) electrons and doubly ionized oxygen vacancies at low pressures. The effect of impurities was noted. Changes in the grain boundary conductance relative to the crystal conductance of MgO were observed as a function of temperature and pressure, but not as a function of grain boundary orientation, transport direction, or total impurity content. Increased high pressure ionic conductivity in polycrystalline material offered possible evidence for increased grain boundary diffusion.     

Point Defects in Electron-Irradiated Oxide Single Crystals

Point defects have been produced in CaO, MgO, and α-Al₂O₃ single crystals by electron irradiation, and thresholds for atomic displacement have been measured using timeresolved luminescence spectroscopy. Oxygen displacement energies of approximately 50 eV have been determined; however, a temperature-dependent threshold observed for an emission band in MgO may arise from a magnesium displacement. A 300-nm emission in α-Al₂O₃ may be due to an F-center transition. Studies of radiative recombination kinetics are consistent with an electron-detrapping model.     

Defect Diffusion in Single Crystal Aluminum Oxide

A defect diffusion coefficient was determined in single crystal aluminum oxide from 1400° to 1850°C by measuring the movement of the color boundary in a Ti³⁺⁻ doped single crystal heated in air. In this temperature range the experimental data can be represented by    
The interpretation developed is that this equation represents the diffusivity for a defect which contributes to aluminum self-diffusion.     

Diffusion of Iron and Nickel in Magnesium Oxide Single Crystals

Diffusion kinetics and mechanisms were studied in the Fe_xO-MgO (vacuum), NiO-MgO (vacuum and air), and Fe₂0₃-MgO (air) systems. In the Fe_xO-MgO system, Fe entered MgO by a redox reaction; the diffusivity and activation energy depended on concentration. In the NiO-MgO system in air the diffusivity depended on concentration and the activation energy did not; in vacuum both the diffusivity and activation energy were concentration-independent. In the Fe₂O₃-MgO system in the MgO phase the activation energy and diffusivity did not depend on concentration. Because of impurities, the diffusion results were for the extrinsic region. Formation of trivalent ions and consequent chemically created vacancies in the Fe_xO-MgO and NiO-MgO (air) systems resulted in the concentration dependence of diffusivity. Concentration dependence of activation energy in the Fe_xO-MgO system is associated with structural changes due to a change with concentration of the Fe³⁺ octahedraI/Fe³⁺ tetrahedral ratio. In the Fe₂O₃-MgO system structural changes do not occur during diffusion because this ratio remains constant.     

Growth of Complex Oxide Single Crystals from Fluoride Melts

Single crystals of a barium magnesium tantalate, a lead tantalate, calcium niobate, and potassium germanate were synthesized by the reactions of oxides and fluorides at high temperatures. The technique is very much like the flux method, except that some of the fluoride cations combine chemically with the oxide nutrients. Starting compositions and conditions for growth are given. The most probable reactions were calculated using free energies of formation with known thermo-chemical data for an analogous system. Complex fluoride-forming reactions, hydrolysis reactions, and reactions which form fluorides and oxyfluorides are possible. The reaction products were also studied by X-ray analysis of the single crystal and polycrystalline products. Lattice parameters, space groups, and X-ray powder patterns are presented for the single crystal materials.     

Epitaxial Aluminum-Doped Zinc Oxide Thin Films on Sapphire: II, Defect Equilibria and Electrical Properties

The electrical transport properties of epitaxial ZnO films grown on different orientations of sapphire substrates have been measured as a function of partial pressure of oxygen. After equilibration, the carrier concentration is found to change from a p ^-1/4_O2 to a p ^-3/8_O2 dependence with increasing oxygen partial pressure. The partial pressure dependence is shown to be consistent with zinc vacancies being the rate-controlling diffusive species. In addition, the carrier concentration in ZnO films grown on A-, C-, and M-plane sapphire are the same but that of R-plane sapphire is systematically lower. Electron Hall mobility measurements as a function of carrier concentration for all the substrate orientations exhibit a transition from "single-crystal" behavior at high carrier concentrations to "polycrystalline" behavior at low carrier concentrations. This behavior is attributed to the effective height of potential barriers formed at the low-angle grain boundaries in the epitaxial ZnO films. The trap density at the grain boundaries is deduced to be 7 × 10¹²/cm². The electron mobility, at constant carrier concentration, varies with the substrate orientation on which the ZnO films were grown. The difference is attributed to the difference in dislocation density in the films produced as a result of lattice mismatch with the different sapphire orientations.     

High Temperature Tensile Creep of Magnesium Oxide Single Crystals

The creep behavior and the dislocation substructure developed during creep were investigated for 〈011〉 oriented MgO single crystals creep tested in tension. Creep deformation was studied over stress and temperature ranges of 29.0 to 86.2 MN/m² and 1200 to 1500°C, and the minimum creep rate, ε, was found to obey the relation:

where σ = applied tensile stress, k = the Boltzmann constant, T = absolute temperature, n = 3.8 to 4.5, and A = ll × 10⁻² (MN/m²)^-4 s^-1. Dislocation substructures developed during creep were studied by transmission electron microscopy and etch pitting techniques. At 1400°C, the dislocation density, ρ , at 0.10 tensile creep strain depended on applied stress as ρασ ^2.1. Numerous dislocation loops and long straight dislocations were present, but subboundaries were seldom observed. The results are discussed in terms of two possible operative creep mechanisms: (1) a recovery process based on annealing out of dislocation dipoles and loops, and (2) dislocation glide limited by atmospheres of charged defects surrounding dislocations.     

Growth and Properties of Single Crystals of Hexagonal Ferrites

Single crystals of hexagonal ferrites up to 1/2 in. long were grown from the system NaFeO₂-Fe₂O₃-BaO-MeO, where Me = divalent Zn, Co, Ni, or Mg. NaFeO₂ was the flux from which crystals were grown in platinum crucibles from a maximum temperature of 1250° to 1375°C. More than 150 single-crystal growth experiments were attempted and the reaction products were studied by X-ray diffraction and optical microscopy. Stability data for the principal single-crystal products are given for initial compositions lying in quaternary planes. Such planes were considered for BaO/MeO ratios corresponding to the compounds 1/1 for Ba₂Me₂Fe₁₂O₂₂ (Y) and Ba₂Me₂Fe₂₈O₄₆ (X) , 3/2 for Ba₃Me₂Fe₂₄O₄₁ (Z) , 2/1 for Ba₄Me₂Fe₃₆O₆₀ (U) , and 1/2 for BaMe₂-Fe₁₆O₂₇ (W) (Me = Zn, Co, Ni, or Mg). A number of adjoining planes were also considered. All the five known structure types were grown as single crystals. A series of four new hexagonal ferrite structures was discovered. These were derived from Z by systematically adding blocks of Y. Magnetization as a function of temperature and field was investigated for crystals of two structure types. The room-temperature ferromagnetic resonance and effective anisotropy were studied for substituted crystals of the same two structure types.     

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.     

Microhardness and Fracture Toughness Anisotropy in Cubic Zirconium Oxide Single Crystals

Vickers and Knoop indentation tests have been used to study the fracture and deformation characteristics of 9.4-mol%-Y₂O₃-stabilized ZrO₂ single crystals. K_c is anisotropic, with values of 1.9 and 1.1 MPa·m^1/2 for radial cracks propagating along (100) and (110), respectively. The toughness for these two orientations was also determined using the single-edge notched-beam geometry, and yielded values of 1.9 and 1.5 MPa·m^1/2.     

中子辐照半绝缘SiC单晶的光学性质

利用荧光光谱、紫外-可见-近红外透射光谱和Raman光谱对经1.67×1020n/cm2中子辐照的半绝缘SiC的光学性质进行了研究。结果表明:中子未辐照的和辐照后退火温度低于1000℃的样品未出现任何发射峰;1200℃退火的样品在510、540和575nm处出现了3个绿色发射峰,其中,510和540 nm处的发射峰在经过1 600℃退火后依然存在。中子辐照导致SiC的截止波长由393 nm增大到1 726 nm;随退火温度提高,截止波长逐渐减小,并在1600℃退火后完全回复。Raman光谱显示:辐照诱发了许多新的振动模,如187、278、435和538cm-1处的Si—Si键振动模,600、655和709 cm-1处的Si—C键振动模和1419cm-1处C—C键振动模。辐照缺陷引起声子限制效应,表现为FTO2/6和FLO0/6Raman特征峰的不对称性展宽和红移。在低于1000℃退火阶段,主要表现为不对称性展宽和红移逐渐减弱;在高于1000℃退火阶段,主要表现为辐照产生的新Raman峰逐渐消失。     

Mechanical Properties of Magnesia Single Crystals Corn pression

Stress-strain curves of single crystals of magnesia compressed in the [100] direction are reported at temperatures from –196° to 1200°C.; curves are also shown for different rates of loading at room temperature. The crystals show considerable ductility at all temperatures and at room temperature can be deformed plastically about 6% before fracture at stresses which are about one-quarter of reported polycrystalline fracture strengths. The macroscopic yield drops apparently exponentially from an extrapolated value of 50,000 lb. per sq. in. at absolute zero to about 4500 lb. per sq. in. at temperatures of 900°C. and higher. Heat-treatment has an appreciable effect on the yield stress. The resistance of the material to deformation increases with the number of slip systems and bands activated because of the barriers to dislocation movements which occur at slip band intersections. At about 2 to 3% strain, stress concentrations begin to be relieved by small internal cracks which are not easily propagated. This effect is extensive before final macroscopic failure of the crystal occurs. Preliminary creep tests above the macroscopic yield stress and in the temperature range 800° to 1000°C. show large instantaneous plastic deformations followed by slow constant-rate creep.     

Growth of Nickel Oxide Single Crystals and Bicrystals via hemical Vapor Transport

A chemical vapor transport (CVT) method was successfully used to grow NiO single crystals and bicrystals heteroepitaxially on single-crystal MgO substrates. The most favorable growth conditions were obtained at 1400 K using 250 torr (∼3.33 × 10⁴ Pa) of HCl( g ) as the transport agent. Average growth rates greater than 100 μm/h were easily achieved under these conditions. The CVT-grown NiO single crystals and bicrystals usually displayed highly reflective facets along the growth direction that suggest high mechanical quality. The grain boundaries in the bicrystals were observed to be perpendicular to the (001) growth surface. The epitaxial NiO crystals were easily separated from the MgO substrate by dissolving away the latter in 85% H₃PO₄ at 190°C. The crystallinity and purity of the deposits were checked using these free-standing NiO crystals. The concentration of cation impurities and the Cl content in the CVT-grown crystals were investigated by inductively coupled plasma (ICP) mass-spectrometric analysis and neutron activation analysis, respectively. High-resolution transmission electron microscopy of a Σ13 (510) boundary revealed a structure at the atomic scale that provided no evidence for segregated phases.