Reactive Atmosphere Processing Research Articles

Verres fluores a base de metaux lourds. Traitement des impuretes oxydes et hydroxles sous atmosphere controlee

Resume Hydroxyl groups and oxide impurities in heavy metal ThF 4 BaF 2 fluoride glasses exhibit absorption bands in the mid-I.R. region near 2.9 μm (OH−) and in the 8–10 μm region (M–O bonds). OH groups are entirely removed in using He or N2 as non-reactive atmosphere. Reactive atmosphere processing using CS2 is successfully applied to decrease the O content improving directly the transmission in the 8–10 μm region.

Hydroxyl impurity effects in YAG (Y3Al5O12)

Evidence is reported of 1020 cm−3 hydroxyl impurity density affecting crystal fields and color center formations in YAG and Nd:YAG. Also reported is a high-temperature reactive atmosphere process to control H bonding and to bleach coloration. Absorption by O–H⋅⋅⋅O and O–D⋅⋅⋅O transitions in the 3–4 μm spectral region was observed in different crystals. Two independent O–H⋅⋅⋅O lines, both with weak satellites, were detected at 3340 and 3374 cm−1. H-bonding sites were assigned by (i) frequency dependence on O2−–O2− distance and (ii) Nd:YAG nonequivalent crystal field effects seen in Nd3+ fluorescence. H+ concentrations 1020 cm−3 were derived from these results. Bonding site relative densities also varied with the different crystals observed. Reactive atmosphere processing (RAP) could cause H+ or D+ to be coordinated into or out of crystals, concomitant with color center bleaching. Color center formations were dissociated by surface reaction with I2 + H2O or I2 + O2 reactive atmosphere at temperatures in the range of 1350–1880 °C. The RAP effect was to produce bulk crystal electronic transfers related to hydrogen in the crystal metal–oxide system. However, photoinduced coloration was again possible. Intense coloration also resulted from vacuum heating, causing an H2O effluent. The evidence indicated YAG and Nd:YAG contained a high concentration of largely mobile H+ affecting O− -center coloration and influencing crystal fields by bound-small polaron formations.

Halide glasses

An increasing interest is devoted to fluoride, and more generally, to halide glasses because of their potential use for making infra-red optical components and ultra-low-loss optical fibres. This paper presents the main fluoride systems based on ZrF 4, HfF 4, AlF 3, ZnF 2, on transition metals and on actinides and lanthanides, all of which act as glass progenitors. The main chloride glasses based on ZnCl 2, BiCl 3, ThCl 4 and CdCl 2 are also described. The aspects of glass synthesis reviewed include fluorination by NH 4HF 2 and reactive atmosphere processing, which are currently used to prepare and refine fluoride glasses. The manufacturing of glass samples and the drawing of fibres are discussed with the problems of nucleation and crystallization in mind. The characteristic temperatures for the major families of halide glasses are detailed in the text, but, generally speaking, the value of T G lies between 200 and 500° C for fluoride glasses and under 200° C for chloride and mixed halide glasses. The optical transmission range normally encompasses the U.V., visible and medium I.R. spectra. The position of the multiphonon absorption edge depends closely on the glass composition. The main optical characteristics are given and their potential use for making infra-red optical components, low-loss optical fibres, electrochemical barriers and laser hosts are presented and discussed. The occurrence of numerous new halide glasses provides experimental evidence for understanding the glass formation mechanism in non-oxide systems.

Reactive atmosphere processing of oxides

Concepts of reactive atmosphere process (RAP) are extended to the cleanup of OH − impurities in Ln 2O 3. Chlorine is unsuitable as this halogen converts Ln 2O 3 to LnOCl quantitatively. Iodine is shown to be suitable for OH − impurity cleanup of Ln 2O 3.

Infrared-transparent glasses derived from the fluorides of zirconium, thorium, and barium

Glasses consisting solely of high-purity ZrF 4, ThF 4, and BaF 2 have been synthesized using reactive atmosphere processing (RAP) techniques. RAP of the individual components and the molten material with anhydrous HF and CCl 4 is described. The glass molds easily at 312°C and 1920 psi with a high-fidelity replication of the die surface. The glass is water insoluble, unusually hard and strong, and continuously transparent from 0.3 to 7 μm.

Crystal growth of alkaline earth fluorides in a reactive atmosphere. Part IV

SF 6, CF 4, and BF 3 are compared as gas-phase reactants of reactive atmosphere processing (RAP) in the crystal growth of alkaline-earth fluorides. To varying degrees, all three agents are corrosive to carbon at the crystal-growth operating temperature of 1500°C. The corrosion rate decreases with an increase in the fluorine bond dissociation energy of the gas-phase reactant.

Bulk and surface absorptions in the 9.2–10.8-μm region in NaCl and KCl

The technique of tunable CO2 laser calorimetry has been used to study the wavelength dependence of optical absorption in the range 9.2–10.8 μm in NaCl and KCl. Samples grown using reactive atmosphere processing (RAP) as well as those cut from large-diameter ingots prepared commercially for fabrication of high-power CO2 laser windows have been studied. A differentiation between bulk and surface absorption has been made in several cases by employing long bar samples; these measurements have been performed at several wavelengths between 9.2 and 10.8 μm. Total absorption coefficient data at 9.27 and 10.61 μm on monocrystalline and polycrystalline commercially available NaCl are also presented. Measured bulk absorption values in RAP-grown NaCl are lower than commonly used values for the intrinsic absorption limit in the 10.1–10.8-μm wavelength region; these measured values are used to define a new operational multiphonon absorption limit for NaCl. Bulk absorption in RAP-grown KCl is nearly intrinsic at 10.61 μm, but increases to an order of magnitude above intrinsic near 9.2 μm. The residual bulk absorption observed in several KCl crystals may be related to impurities of the ClO3 type.

New coating materials for IR laser optical components

Concepts of reactive atmosphere processing are applied to the preparation, purification, and thermal characterization of three rare-earth fluorides (LaF 3, CeF 3, and PrF 3) and BiF 3, BiI 3, and KGaF 4. The rare-earth fluorides have optical absorption coefficients between 0.6 and 0.9 cm −1 at 9.27 μm; of the rare-earth fluorides, only LaF 3 is a good coating material at 3.8 μm. BiF 3 and KGaF 4 appear to have low absorption at 9.27 μm and 3.8 μm. BiI 3 shows a flat response between 2.5 and 20 μm.

Growth of doped-halide single crystals for infrared windows

Single crystals of KCl doped with RbCl have been grown by the Czochralski and Bridgman techniques. Czochralski crystals were grown in air, using platinum crucibles and Bridgman crystals were grown in a Trans-Temp glass furnace in air and also under a reactive atmosphere of CCl 4 in argon. This reactive atmosphere process (RAP) is known to greatly reduce impurities, such as hydroxyl ions, which degrade optical quality. A series of KCl-RbCl-SrCl 2 single crystals have been grown to stabilize microstructure. Some optical absorption and mechanical strength data is presented. An alternative approach to increase halide material strength is to hot-forge a single crystal under temperature and pressure sufficient to cause pressure-induced recrystallization (PIR). Hot-forging techniques to produce high-strength IR windows without degrading optical transmission are given.

Crystal growth of alkaline earth fluorides in a reactive atmosphere: Part III

A comparison of CF 4 and C 2F 4 as agents of reactive atmosphere processing (RAP) in the crystal growth of alkaline-earth fluorides is given. At temperatures much below the melting point of these metal fluorides, C 2F 4 decomposes into CF 4. CF 4 converts H 2O to HF, but has a much slower action on the dry metal oxides at ≳1000°C, and, therefore, it is preferred as a secondary RAP agent to the fast-acting HF.

Crystal growth of KBr in a reactive atmosphere

Reactive atmosphere processing (RAP) with bromomethane derivatives is employed in the crystal growth of KBr (mp = 730°C). Derivatives with higher C-Br bond dissociation energy are found to be applicable.

Crystal growth of KCl in a reactive atmosphere

Reactive atmosphere processing (RAP) with CCl 4 is employed in the crystal growth of KCl (mp = 776°C). Two carrier gases, He and CO 2, are compared. A carbon deposit accumulates in the growth of KCl in CCl 4 He , a drawback which is eliminated in crystal growth under CCl 4 CO 2 .

Crystal growth in a reactive atmosphere

Concepts of reactive atmosphere processing (RAP) are applied to crystal growth. Steady state pyrolysis of CBr 4 shows the material to be applicable to RAP growth at <600°C and, therefore, inapplicable to the growth of KBr (mp = 730°C). Steady state pyrolysis of CCl 4 shows the material to be applicable to RAP growth up to 900°C and, therefore, useful to metal chlorides in general.