Showing papers in "Journal of the American Ceramic Society in 1960"

Trapped Electrons in Irradiated Quartz and Silica: II, Electron Spin Resonance

Abstract: The correlation of two paramagnetic defects, observed by the electron spin resonance (ESR) technique, with two optical absorption bands produced by r-ray or neutron irradiation is indicated. The peaks of the two absorption bards fall at ∼2100 8.u. and 2300 a.u. The effective g values for the ESR lines, for a crystal orientation with respect to the magnetic field of 100.1] parallel to H2, are 2.0006 plusmn; 0.0005 for the 2100 a.u. band, and there are two lines observed for the 2300 a.u. band at [g= 2.0007 and g= 2.00091 ± 0.0005. The correlation of the optical bands and ESR lines was established by a series of bleaching and annealing experiments. These experiments also establish the presence of another center which is not observed directly by either the optical or the ESR technique. ESR lines were observed which were attributed to a hyperfine interaction, A= 7.7 oersteds, of the g= 2.0006 (2100 a.u. band) defect with the Si29 isotope (I = 1/2, 4.7% abundance). Models for these defects, consistent with the experimental data, are suggested.

Trapped Electrons in Irradiated Quartz and Silica: I, Optical Absorption

Abstract: Evidence is given for the correlation between an optical absorption band at 2300 au and an electron spin reso nance system developed in quartz after Co/ sup 60/ gamma irradiation. The results of bleaching gamma irradiated quartz and silica at 78 deg K and at room temperature are presented and discussed. Also, further observations upon annealing these color centers are given. (auth)

Radiation Energy Transfer and Thermal Conductivity of Ceramic Oxides

Abstract: The heat-transfer mechanisms in several ceramic oxides were investigated from room temperature to 1250/sup o/C. The conductivity of normal sintered porous polycrystalline oxides results from phonon conduction, and results agree with theoretical expectations for the effect of impurities, structure, and temperature dependence. Single crystals, glasses, and low-porosity aggregates show deviations from the expected phonon conduction which are attributed to a second mechanism of heat transfer, namely, photon conduction. Measurements of photon conduction are related to infrared transmission, structure, and conditions of the measurements. Photon conduction can be a significant portion of the total heat conduction at high temperatures; however, for normal polycrystalline ceramic oxides of porosities greater than 2%, this contribution is small. (auth)

Phase Equilibria in the Ferrite Region of the System FeO‐MgO‐Fe2O3

Abstract: Phase equilibria in the ferrite region of the system FeO–MgO–Fe2O3 have been investigated up to 1300°C. over an oxygen-pressure range of 1.0 to 0.01 atmospheres. The five most important features of the equilibria in this system are as follows: (1) The spinel phase can exist with a deficiency of oxygen in the high-magnesium compositions, (2) Mg++ replaces Fe++ in Fe3O4 beyond the “stoichiometric MgFe2O4,” corresponding to the formula (MgO)xMgFe2O4, where x = 0.092 ± 0.004 and is independent of temperature, (3) the spinel field does not include single-phase composition corresponding to Mg-Fe2O4, (4) the lattice constant of spinels in this system depends on cation distribution, and the extent of distribution changes as a function of temperature in turn depends on the magnesium concentration in the spinel, and (5) the spinel field near its terminus has little or no width.

Vibrational Spectra of Vitreous B2O3.xH2O

Abstract: The infrared absorption spectra of boron oxide glasses of low and high water content have been obtained in the 400- to 4000-cm.−1 region using thin films or fine powders dispersed in a liquid. A structural interpretation of the glass spectra has been made with the aid of the spectra of the closely related materials boric acid, orthorhombic metaboric acid, and partly deuterated boron oxide glass of high water content. It has been shown that the glass spectra are consistent with a random-network structure in which each boron is triangularly coordinated by three oxygens and that the presence of water leads to weak hydrogen bonding between oxygen atoms. No evidence for a substantial amount of tetrahedral coordination of boron by oxygen has been found in glasses of either low or high water content.

The Kinetics of Gamma‐Ray Induced Coloring of Glass

Abstract: When glasses are colored by ionizing radiation the induced optical absorption increases as the radiation progresses and appears to be due to the superposition of a number of individual absorption bands. A detailed study of this process has been made using a specimen of Corning boro-silicate glass colored by exposure to 60Co gamma rays. This particular sample was chosen because only four bands are formed. If it is assumed that each band is Gaussian shaped the spectrum may be separated into four absorption bands. The peak energy E0 and full width U of these bands are, in electron volts: E0= 4.85, U= 1.19; E0= 3.95, U= 1.30; Eo= 2.58, U= 0.58; E0= 2.02, U= 0.52. For each band growth curves may be constructed showing how the density of absorption centers increases as a function of dose. These growth curves have been fitted with theoretical curves based on the following considerations: the radiation field creates ionization electrons in the glass; for each ionization electron one electron deficient region or hole is formed; absorption bands observed are due to centers formed by electron trapping although the possibility that some of the bands are due to hole trapping is not ruled out; and competition for ionization electrons exists between holes and the various kinds of electron traps. Satisfactory agreement between the observed and calculated curves is obtained. The theory indicates that the “radiation protection” imparted to glass by materials such as CeO2 may arise in several different ways and that it would be possible to decide between them from rather simple experiments.

Phase Diagram of the System BaO‐Fe2O3

Abstract: The phase diagram of the system BaO-Fe203 was determined by X-ray diffraction, melting-point measurement, and microscopic methods. Since the reduction of Fe3+ to Fe2+ was observed by chemical analysis in the samples heated at high temperature, especially in molten samples, the samples were heated at 1 atmosphere pressure of oxygen in the temperature region in which the liquid was in equilibrium; 1 atmosphere pressure of oxygen was su5cient to restrain the reduction of Fe3+. In the temperature region of solid-solid equilibrium, the dissociation was not observed even when the samples were heated in air. BaO - 6Fe2O3 formed a solid solution with BaO.Fe203. The BaO:Fe203 ratio of the solid solution was BaO.4.5Fe203 at 1350°C. and BaO. 5.0Fe20a at 800°C. The precipitation micro-structures of each primary solid solution were observed.

Processing of Positive Temperature Coefficient Thermistors

Abstract: The controlled addition of impurities such as lanthanum in barium titanate and barium strontium titanate systems results in thermistor materials which have large positive temperature coefficients of resistance over controlled temperature ranges. The resistivity in these systems is sensitive to the concentration of the trace additives, to undesirable impurities in the raw materials, and to the sintering and maturing conditions. Ceramic procedures well adapted to prepare these thermistors reproducibly have been developed.

Cell Dimensions, Solid Solution, Polymorphism, and Identification of Mullite and Sillimanite

Abstract: X-ray, optical, and chemical data are presented for sillimanites and for a wide range of natural and synthetic mullites. Single-crystal X-ray studies have revealed a new type of mullite (called S mullite) characterized by subsidiary sharp reflections in distinction to the extra diffuse reflections of common mullite (called D mullite). Atomic substitution of aluminum in D mullite leads to expansion of a and c but not of b, whereas substitution of iron and titanium in natural mullites gives a slight initial decrease in a and b followed by a subsequent increase; c increases uniformly for all concentrations. The data are consistent with solid solution ranging between the extremes 3Al2O3.2SiO2, 2Al2O3.-SiO2, and 3(Al0.9Fe0.1)2O3.2SiO2. Cell dimensions of sillimanite vary only slightly. Consideration of the various methods of distinguishing mullite and sillimanite shows that the measurement of cell dimensions is the most reliable and for mullite has the added advantage that the S and D polymorphs may be distinguished and an estimate of the composition obtained for D mullite.

X-Ray Study of the Solid Solution of TiO, Fe2O3, and G, O3 in Mullite (3A12O3a2Si02)

Abstract: A study of the solid solution of TiO2, Fe2O3, and Cr203 in mullite was made by measuring the changes in lattice parameters and unit-cell volume. Synthetic mullite (3O3-2SiO2) was reacted with up to 12 weight % of the oxides at temperatures ranging from 1000° to 17000C. The approximate minimum temperature required for the formation of solid solution was 12000C. for Fe203 and 1400°C. for Cr2O3 and TiO3. The maximum amount of solid solution found was 2 to 4% TiO2 at 1600°C., 10 to 12% Fe2Os at 1300°C., and 8 to 10% CrZO3 at 1600OC. Lattice parameters and unit-cell volumes for each solid solution series increased with increasing amounts of foreign oxide. There was good agreement between the calculated and observed increase in cell dimensions for the iron oxide series. Except in the case of titania, there was good agreement between X-ray data and petrographic observations.

Phase Equilibria in the System La2O3—Iron Oxide in Air

Abstract: Phase equilibria data are presented for compositions in the system La2O3-iron oxide in air. Liquidus and solidus curves were obtained by the quenching method in the iron-rich portion of the system. The remainder of the diagram was determined using a strip-furnace technique. Two compounds have been found, the ortho-rhombic perovskite LaFeO3 and a compound with the magnetoplumbite structure corresponding to a composition LaFe12O19. LaFeO3 was determined to melt congruently at about 1890°C. whereas LaFe12O19 has both a stability minimum and maximum at 1380° and 1421°C., respectively. The iron-rich portion of the system is essentially ternary whereas the remainder can be considered to be a simple binary. Liquid compositions have been determined and are plotted in terms of the ternary system La2O3-Fe2O3-FeO.

Mechanical Properties of Magnesia Single Crystals Corn pression

Abstract: 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.

Strength and Static Fatigue of Abraded Glass Under Controlled Ambient Conditions: III, Aging of Fresh Abrasions

Abstract: The effect of aging under various conditions on the strength and static fatigue of freshly abraded specimens has been studied. For both grit-blast and emery cloth abrasions the liquid nitrogen (fatigueless) strength increased with storage time after formation of the abrasions for specimens stored in liquid water or in an atmosphere containing water vapor. After 1 day of storage in water the strength increase was about 60% of the freshly abraded value for emery cloth abrasions and about 30% for grit-blast abrasions. The aging depended strongly on the medium in which the specimens were stored and on the relative humidity when they were stored in air. In very dry air or vacuum the aging effect appeared to be eliminated. Static fatigue curves (strength vs. load duration) in water were measured for specimens containing fresh abrasions and aged abrasions. Ordinary aging did not change the static fatigue behavior of fresh abrasions. The liquid nitrogen strength and static fatigue also were studied for specimens which had been baked at 470°C. in high vacuum after being abraded. This treatment increased the fatigueless strength by approximately the same amount as aging in water. The rate of static fatigue, however, which was unaffected by ordinary aging, was reduced by a factor of approximately 25 for vacuum-baked as compared with fresh abrasions. Possible mechanisms for aging are discussed in some detail. It is proposed that the observed aging effects on the fatigueless strength can be accounted for by an increase of the radii of the tips of the abrasion cracks in the surfaces of the specimens. The suppression of fatigue for the vacuum-baked specimens is believed to be related to the non-wettability of their surfaces which was observed in the course of the experiments.

Oxygen Mobility in Polycrystalline NiCr2O4 and α‐Fe2O3

Abstract: Oxygen diffusion coefficients were determined for polycrystalline samples of NiCr/sub 2/O/sub 4/ and alpha -Fe/sub 2/O/sub 3/ by exchange measurements with oxygen gas containing the stable isotope O/sup 18/, using mass spectrometer analysis. Oxygen diffusion in NiCr/sub 2/O/sub 4/ can be represented by the equation D = 0.017 exp (-65,400/RT). Oxygen diffusion in alpha -Fe/sub 2/O/sub 3/ can be represented by the equation D = 1 x l0/sup 11/ exp (146,000/RT). The large difference between D/sub 0/ and activation energy for these materials suggests that different diffusion mechanisms are involved. (auth)

Indentation Strength of Glass

Abstract: The tensile strength of plate and sheet glasses was investigated by means of indentation experiments with small spherical indenters. In agreement with earlier experiments by Powell and Preston, tensile strengths were observed about 10 to 100 times higher than those found in tensile or bending tests, the highest values being about one-half the greatest strength of thin fibers or freshly drawn rods. It did not seem that any further reduction of the diameter of the indenting sphere below 1 mm. would lead to a significant increase of the highest strength values. The highest values of the indentation strength of freshly cleaved surfaces were not above those of ordinary surfaces, although the average was considerably higher. This indicates that, unless the use of the Hertz stress distribution involves a significant inaccuracy, the upper limit of about 300,000 lb. per sq. in. of the tensile stress may represent the order of magnitude of the tensile strength of bulk glass in the absence of surface defects. This value is within a factor of two to five of the probable molecular tensile cohesion of a glass of maximum homogeneity. Fracture stress histograms and maximum endured stress histograms are given for a variety of flat glasses.

Low‐Temperature Equilibria Among ZrO2, Tho2, and UO2

Abstract: Studies made on low-hafnium-content ZrO2, show that the monoclinic-tetragonal inversion temperature is 1170°C., and it is raised to approximately 1190°C. in the “natural” ZrO2, which contains approximately 2% HfO2. No explanation could be found for the knownmarked hysteresis during cooling, when the reverse polymorphic transformation takes dace at 1040°C. In the system ZrO2-ThO2 the monoclinic-tetragonal ZrO2, inversion temperature is lowered to 1000°C., although the maximum solid solution extent of ZrO2, in Thon and vice versa is approximately only 2% at this temperature. Below about 400°C. under hydrothermal conditions it was possible to prepare a continuous, although metastable series of solid solutions with the fluorite structurewith compositions varying from ThO2, to nearly pure ZrO2. Contrary to earlier work only 8 mole ZrO2, dissolves in UO2 and less than 4 mole of UO, in ZrO2 at temperatures up to 13OO0C. A continuous series of solid solutions could be made between Th2 and UO2 at 13OO°C., and extensive defect fluorite solid solutions could be prepared between Tho2 and U3O8; there is some evidence for exsolution into uranium-rich and thorium-rich members at low temperatures.

High-temperature x-ray diffraction investigation of the uranium-carbon system

Abstract: The system UC-UC/sub 2/ was investigated at temperatures to 2200 deg C utilizing high-temperature x-ray-diffraction techniques. UC/sub 2/, which has the CaC/sub 2/ structure, was shown to transform to the CaF/sub 2/ structure at 1820 deg C. Mean coefficients of expansion of UC, U/sub 2/C/sub 3/, and UC/sub 2/ were determined. The decomposition of U/sub 2/C/sub 3/ to cubic UC/sub 2/ and to UC saturated metastably with carbon was observed. Solubilities of UC in UC/sub 2/ and UC/sub 2/ in UC were determined near 1800 deg C by a method based on lattice- parameter measurements. The formation of, and existence conditions for, U/sub 2/ C/sub 3/ were confirmed. It was observed that UC/sub 2/ tends to decompose toward UC at the extreme temperatures, probably by loss of carbon to CO. (auth)

Solid‐Solid and Solid‐Vapor Reactions and a New Phase in the System Mn‐O

Abstract: The univariant equilibrium pressure-temperature curve along which β-MnO2, Mn2O3, and O2 are stable together has been determined up to a Po2 of 200 atmospheres with a trace of water as a catalyst. The analogous curve for the assemblage Mn2O3, Mn3O4, and O2 has also been determined. In both cases the experimental data differ considerably from the thermodynamic calculations. Interconversion studies of the γ, ρ, ɛ and ramsdellite phases indicate only that γ-MnO, is probably not a poorly crystallized ramsdellite. A new phase probably containing mainly trivalent manganese has been obtained under a wide variety of conditions. The absence of any consistent X-ray diffraction line shifts in either the MnO or Mn3O4 phases when they were prepared with different stoichiometries raises some question regarding the extent of stable solid solution in these two phases in the system Mn-O.

The System UO2‐ZrO2

Abstract: A phase diagram was proposed for the system UO/sub 2/ZrO/sub 2/ on the basis of earlier work and the results of new studies in which the mixed oxides were melted in a solar furnace to avoid contamination. X-ray diffraction measurements were made on specimens melted in helium and on the same specimens after heating to 1350 deg C in helium. The system shows a narrow two-phase region about the eutectic at high temperatures which increases in extent at lower temperatures. The high-temperature fornn of zirconia cannot be stabilized by the addition of UO/sub 2/. (auth)

Studies on lead titanate ceramics containing niobium or tantalum

Abstract: Dielectric, lattice constant, and thermal-expansion measurements have been made on solid solution ceramics in the systems PbTiO3Pb0.5NbO3 and PbTiO3-Pbo.5TaO3. The ferro-electric Curie temperature is lowered to 465OC. at the solubility limit (2 to 5% Nb or Ta). Replacement of Ti4+ by Nb5+ or Ta5+ in these perovskite-type solid solutions is accompanied by the creation of cation vacancies. These vacancies seem to facilitate material transport and aid sintering. Thus ceramics were obtained with up to 91% of theoretical density. These compositions have peak dielectric constants of the order of 10,000 with tan δ less than 0.1 in the interval 25° to 525°C.

A Structural Study of Talc and Talc-Tremolite Relations

Abstract: About forty specimens of talc have been examined by X-ray powder methods. Variations in the degree of crystallinity of the mineral have been observed, but no evidence has been obtained for polymorphic structural varieties. Single-crystal data have confirmed the two-layer mono-clinic cell. Little evidence has been obtained of talcs containing mixed-layer sequences. Electron micrographs have revealed a variety of platy and fibrous forms, and in the latter a close structural relation between talc and tremolite has been established. Talcs prepared synthetically in the presence of aluminum showed small lattice parameter differences from talcs prepared in the absence of aluminum, and the results are consistent with the substitution of small percentages of A1 ions in the talc structure. New X-ray powder data and unit-cell parameters for talc and tremolite are listed.

Space‐Charge Development in Glass

Abstract: Data are given that graphically demonstrate space-charge build-up in a glass containing alkali and in a glass relatively free of alkali. The observed potential distributions show a decided asymmetry and have large potential drops near the electrodes with a linear potential change near the center of the sample. The observations qualitatively fit the expected theoretical distributions, according to Proctor and Sutton, for material having cations mobile and anions essentially immobile. Crude estimates arising from comparison with the theory yield carrier concentration near 2 × 1016 per cm3. mobility of the cation near 3 × 10-6 cm.2 per volt second, and diffusion constant near 2 × 10-6 cm.2 per second. These mobility and diffusion values are two orders of magnitude larger than values computed from conductivity data or computed from measurements of the diffusion coefficient for sodium cations, both in simple alkali silicates. According to the theory, these higher values are perhaps reasonable, since they are associated with only the more mobile portion of the cation population.

Hot Pressing of Fused Silica

Abstract: The hot pressing of powdered fused silica at temperatures below the melting and devitrification regions is described. The fabrication of high-density shapes of fused silica is possible by this process. A relation expressing densification as a function of viscosity, time, applied pressure, and initial compaction is essentially substantiated.

X‐Ray Study of Phase Transitions in Ferroelectric PbNb2O6 and Related Materials

Abstract: The phase transitions in PbNb2O6 and in compositions of the type Pb1+x (BxNb1-x)O6, where B = Ti4+, Zr4+, or Sn4+, have been investigated between 25° and 650°C. using X-ray and dilatometric techniques. The modified PbNb2o6 compositions possess orthorhombic PbNb2O6-type structure, with the additional Pb2+ ions occupying vacant lattice sites. The lattice parameters a and c expand and b contracts during heating until, at the ferroelectric Curie temperature, a and b suddenly coincide and c expands slightly. Besides this phase change at the Curie temperature, the nonstoichiometric compounds show an additional phase transition in the range 450° to 300°C. depending on composition. The intermediate phase of Pbl+x(Ti2Nb1-z)2O6 appears to possess orthorhombic symmetry.