Edward Carnall

The densification of MgO in the presence of a liquid phase

Abstract The use of LiF allows MgO to be pressed at 47,000 psi less pressure than what is normally used for pure MgO and it is effective in the removal of surface impurities from the MgO. Densification during the soak period of the hot pressing process of MgO doped with 1% LiF occurs by a three step mechanism. Kinetic data reveal the first step to occur by the crystals moving about to achieve dense packing. Next, solution and reprecipitation of the MgO in LiF causes additional densification and grain growth. Finally densification slows down as a result of reduced solubility of MgO in LiF.

Hot pressed CoCr2S4: A potentially useful magneto-optic material

Transparent CoCr2S4 has been vacuum hot pressed at 850°C and 50,000 psi. This material is a ferrimagnetic semiconducting spinel with a Curie temperature of 221°K. In the spectral range from 1500 to 800 cm−1 (6.6–12.5μ) it has a refractive index of 3.56 and a residual absorption coefficient of about 7 cm−1. The Faraday rotation at 2000 cm−1 (5μ) and 944 cm−1 (10.6μ) at 80°K in a magnetic field of 7 kOe is 2100°/cm and 320°/cm, respectively. CoCr2S4 has been used to modulate a CO2 laser.

Transparent Gd2O3 ceramics and phosphors

Abstract Transparent discs of Gd 2 O 3 , two inches in diameter, have been vacuum hot pressed at 850°C and 40, 000 psi. LiCl was used as a pressing aid. Europium doping has resulted in a transparent phosphor screen which is excited by X-ray and ultraviolet radiation.

The preparation of hot-pressed chalcogenide spinels

Abstract The preparation of chalcogenide spinel powders is reviewed. Microcrystalline powders are formed by the thermal reaction of mixtures of anhydrous cadmium and chromium chlorides or coprecipitated cadmium and chromium hydroxides with H 2 S, Ar + CS 2 , or H 2 + Se. These spinel powders are hot pressed to form highly dense, polycrystalline disks. The effects of pressing variables on optical absorption coefficients are given. The spinel powders and hot-pressed disks are characterized and optical spectra, refractive index, and Faraday rotation data are given.

Superconducting Y1Ba2Cu3O7-y thick films from nitrate pastes

A novel technique for forming superconducting Y1Ba2Cu3O7-y, thick films by screen printing pastes made from spray-dried nitrate powders on alumina and magnesia is described. Manufacture and characterization of nitrate powders are discussed. A model based on dissolution of yttrium nitrate is proposed to explain the rheological and film-forming properties of the nitrate pastes. Low-viscosity pastes are found to give better performance in terms of printability and quality of prints. Barium carbonate formation can be avoided by use of the nitrate pastes and by firing in an oxygen atmosphere. Superconducting thick films were achieved on single-crystal (1 0 0) MgO (T0=77 K) and on polycrystalline alumina with a prereacted barrier layer (T0=55 K).

The preparation of hot-pressed chalcogenide spinels. II. Stoichiometry and optical transparency in CdCr2S4 and CoCr2S4

The compounds CdCr2S4 and CoCr2S4 have been hot pressed into disks that are highly transparent in the infrared. Stoichiometry has been altered by varying the Cr3+M2+ ratio, where M2+ is Cd2+ or Co2+. The effects of nonstoichiometry on optical transmission were determined. Excess M2+ attenuates the transmission much more than excess Cr3+.

The Preparation and Magneto‐Optical Properties of Hot‐Pressings of the Magnetic Semiconductors CdxCo1−xCr2S4

The preparation and hot‐pressing of microcrystalline spinel powders of the composition CdxCo1−xCr2S4, where x varies from 1 to 0, are described. The Curie temperatures increase and the magnetic moments decrease as x goes from 1 to 0. The optical absorption and Faraday rotation of these pressings have been measured in the wavelength range from 1 to 25 μm. Compositions with x slightly less than 1 (low Co doping) reveal optical absorption bands at 1.61, 1.72, and 1.96 μm, which are assigned to the 4A2 to. 4T1(F) crystal field transitions of the Co++ in a tetrahedral field. Between 5 and 12μm, as x goes from 1 to 0, the magnitude of the figure of merit decreases to zero and then increases to the same order at x=0 as at x=1. The increase of the Curie temperature (Tc) from 84.5°K at x=1 to 221°K at x=0 reduces the cooling requirements and CoCr2S4, will probably be a more useful magneto‐optical material than CdCr2S4.

Optical Storage in Hot‐Pressed Ferrimagnetic Spinels

Unusual optical storage effects in hot‐pressed CoCr2S4 have been recently reported.1 Even though thesample is cubic and polycrystalline, remanence of the polar Kerr effect is observed at normal incidence. Similar effects have been observed recently in hot‐pressed FeCr2S4 and CoFe2O4, which have transition temperatures of about 190°K and 790°K, respectively. The origin of the polar remanence is related to the residual hydrostatic strain in a hot‐pressed polycrystal. The magnetostriction of the material produces a strain anisotropy term in the magnetic energy. Assuming a single‐domain particle model for the individual crystallites in the hot‐pressed material, the magnetic energy of the ensemble has been calculated. The magnetization will be stable in the normal direction provided certain criteria are met. The magnetostriction coefficient λ must be negative and the product λS must be greater than 3/2 DM2, where S is the residual stress and D is the average inplane demagnetizing factor. The normal remanence, ...

A new class of room‐temperature magneto‐optic insulators: The cobalt ferrites

Large magneto‐optical activity has recently been associated with the crystal field transitions of tetrahedral Co++ in a magnetic medium. Most of the materials, which have been reported, are sulfo‐spinels with transition temperature below room temperature. The ferrite CoFe2O4 has a transition temperature of 790 °K but it is an inverse spinel and has fairly small magneto‐optic activity.1 The addition of certain trivalent ions such as Rh3+, Cr3+ and Al3+ convert the spinel to a normal configuration with the distribution Co[FeT]04. Here T is one of the above trivalent ions. This substitution has produced large magneto‐optic effects in the range of the Co++ (Td) crystal field transitions. Peak Faraday effects of about 105 deg/cm are seen in the region of the 4A2(F) →4T1 (P) transition at 15,800 cm−1 (0.63 μm). These effects are seen at room temperature as the transition temperatures of the above compounds are in the range 350 °K to 500 °K. Thus, the substituted cobalt ferrites are a new family of room temperature compounds with large magneto‐optical activity. Possible applications of these materials are modulators, isolators, and optical memories.Large magneto‐optical activity has recently been associated with the crystal field transitions of tetrahedral Co++ in a magnetic medium. Most of the materials, which have been reported, are sulfo‐spinels with transition temperature below room temperature. The ferrite CoFe2O4 has a transition temperature of 790 °K but it is an inverse spinel and has fairly small magneto‐optic activity.1 The addition of certain trivalent ions such as Rh3+, Cr3+ and Al3+ convert the spinel to a normal configuration with the distribution Co[FeT]04. Here T is one of the above trivalent ions. This substitution has produced large magneto‐optic effects in the range of the Co++ (Td) crystal field transitions. Peak Faraday effects of about 105 deg/cm are seen in the region of the 4A2(F) →4T1 (P) transition at 15,800 cm−1 (0.63 μm). These effects are seen at room temperature as the transition temperatures of the above compounds are in the range 350 °K to 500 °K. Thus, the substituted cobalt ferrites are a new family of room temperat...