To better understand the basic solid state mechanism affecting select physical properties, structural measurements were made using a relatively new Nuclear Magnetic Resonance (NMR) technique. Historically, NMR has proved invaluable to the organic researcher for determining molecular structure(s). For about a decade, exciting work has been ongoing which has developed and continually refined the technique for solid state NMR. A family of ceramics known as spinels has a natural crystalline structure which is well defined. Synthetic spinels do not always adhere to the same strict coordination site scheme. Initial NMR measurements seem to indicate a possible correlation between the measured coordination site location(s) of cations and select physical properties.
Lanthana-strengthened yttria is of interest for infrared applications because of its high strength, long wavelength cutoff (8 gm), and low emissivity. Specimens were fabricated for various optical, thermal, and mechanical tests, and their properties measured. A new thermal shock test method was developed and the thermal shock behavior characterized. The toughened yttria was shown to be more resistant to thermal shock fracture than the standard material. Techniques were developed for fabrication of full size infrared domes, and specimens successfully fabricated and characterized. Various details of the material and component properties will be discussed.
The optical transmission of transparent polycrystalline lanthana-strengthened yttria has been measured in both the near ultraviolet and infrared regions at temperatures between 20°C and 1400°C. The absorption remains extremely low until about 900°C, then rises almost exponentially as the temperature is raised further. The magnitude of the increase is a function of the oxygen partial pressure (P02) in the firing atmosphere. The absorption increase with temperature is smallest when Po2 is between 10-11 and 10-8 atm, representing the range in which the concentration of stoichiometry-related point defects (oxygen interstitials and vacancies) is minimized. The temperature dependence is significantly greater in the UV than in the IR, but the optimal Po2 range is the same. The absorption behavior is also a function of processing variations, and provides a good criterion for comparison of specimen quality.
Proc. SPIE 0968, Optical, Mechanical And Thermal Properties Of Calcium Lanthanum Sulphide Compared With The Properties Of Current 8-12 µm Window Materials, 0000 (17 January 1989); doi: 10.1117/12.948121
The physical property data reported for calcium lanthanum sulphide indicates that it would be an attractive material for high performance 8-12 μm FLIR window applications. However, a number of other available materials may also be considered for aircraft windows operating at temperatures up to 200°C. These are low resistivity germanium (70°C to 100°C), gallium arsenide, multi spectral and standard grade zinc sulphide, zinc sulphide/zinc selenide laminate and zinc selenide. A comparison is presented of the transmittance properties of these materials at room temperature and at 175°C together with room temperature data on the reported optical, mechanical and thermal properties. A qualitative assessment of the likely effectiveness of calcium lanthanum sulphide as a FLIR window material in relation to these existing materials is presented.
Several long wavelength infrared (LWIR) transparent external window materials are compared with respect to their IR bandpass, rain erosion and thermal shock resistance. The materials of interest include: ZnSe, ZnS, multispectral ZnS, calcium lanthanum sulfide (CLS), Ge, GaAs and GaP. Relevant thermal and mechanical property data are presented. The current state of development and size capabilities of each material are also discussed.
The development of infrared-transmitting ZnS-based ceramics with a combination of high strength, hardness and toughness over a range of temperatures is highly desirable. The approach we are currently pursuing is a familiar one to metallurgists, namely the exploitation of thermal, and possibly mechanical, processing of ZnS-base "alloys". Knowledge of the phase equilibria of various ZnS-rich systems is essential to achieve our objectives. Unfortunately, the literature on this subject is sparse, and we have had to undertake such investigations ourselves. This paper describes the results of our initial studies of the solid-state phase equilibria in the ZnS-CdS and ZnS-Ga2S3 phase diagrams. We also discuss possible processing routes to achieve hard and tough ceramics utilizing the phase diagrams established.
Efforts to prepare new infrared-transmitting ceramic materials and to improve the mechanical properties of existing materials are in progress. Work on new materials is concentrating on phosphides, sulfides, and mixed phosphide/sulfides. Zinc sulfide with submicron grain size has been prepared from organometallic precursors with the hope of improving strength or fracture toughness. A survey of the reaction of hydrogen sulfide with organometallic compounds was conducted to evaluate this route to ceramic sulfides.
Recent works on bismuth germanate and lithium niobate are described with relation to the behavior of impurity or dopant ions and their effect on optical properties. For bismuth germanate crystals grown by Bridgman method the mean segregation coefficients for several elements were found to be very small as compared with those for Czochralski method. Radiation damage experiments revealed that Pb, Mn, Fe and Co have the most deteriorative effect. Whereas Ni, Ga and Mg affect slightly, and Al, Si, Ca and Cu seem to have no measurable effect on radiation damage of bismuth germanate crystals. For lithium niobate absorption edge of congruent crystals doped with Mg0 shows a minimum at some 5 mol% Mg. This threshold effect of Mg dopant finds its origin in its substitution behavior in host lattice and could be closely related to the occupancy (4.58%) of Nb in Li site in congruent crystal. For the first time the crystal structure of lithium niobate heavily doped with Mg0 was investigated directly, resulting in a best model with all the Mg accommodated in Li sublattice. On the basis of experimental data the substitution behavior of Mg could be summarized as the following: With Mg content below the threshold value, replacement of Nb in Li site by Mg is the dominant process, leading to a shift of absorption edge toward shorter wavelengths. As Mg content reaches the threshold value,most of the Nbin Li site are replaced, resulting in the observed minimum of the absorption edge curve. With further increase of Mg content, the substitution of Li by Mg becomes prevailing, causing a slight counter-shift of the absorption edge.
In this paper, a new figure of merit for steady-state diffraction efficiency for two-wave mixing in photorefractive materials is presented which includes total incident energy and absorption losses. The parameter is appropriate for use in systems where the reading and writing beams are the same and where efficient use of a fixed light budget is a design requirement. Design relationships between the exponential gain, I', the beam ratio, m , energy coupling, absorption, and Fresnel reflectance and transmittance are explored. Experimental results using BaTiO3 at X = 514.5 nm bear out analysis. The results provide a straightforward guide to the optical designer who wishes to use the coupling capabilities of photorefractive media in an optical system or subsystem.
The potential of Beta-Barium Borate (B-BaB204) for use as an EO material is evaluated. The clamped and unclamped values of ryyy and rc are rtyyy = 2.5 pm/V, rTC = 0.17 pm/V, IrSyyyl = 0.24 rSXyz(KDP), and RSC = 0.013 rSXyz(KDP), where rc = rzzz-(no/ne)3rx xz. The magnitude of the EO coefficient is due mainly to the electronic nonlinearity. The expected half-wave voltages and thermo-mechanical parameters of 13-BaB204 plates in various configurations are calculated and compared with those of other materials of interest. Its resistance to thermal fracture, high damage threshold, and its wide range of transparency, make 13-BaB204 an excellent candidate for use in a high average power Pockels cell, and as an intra-cavity laser Q-switch.
B-BaB204 has recently been demonstrated as a promising material for both SHG and OPO applications, particularly in the UV region. We have successfully grown 0-BaB204 in a novel flux composition consisting of NaCl and Na20. Crystal growth rate was very fast in a pure NaC1 flux with well developed facets. Na20 was added as a retardant to slow down both the growth rate and spurious nucleation. The crystal habit also changed from long-prismatic shape to more equant semispherical. As the Na20 concentration increased, the crystal clarity was also reduced because of more severe flux inclusion. We believed that this was due to the contamination of carbonate in the flux since Na2CO3 was used as a pre-cursor for Na20.
Changes in the point defect concentrations have been studied using Debye-Scherrer x-ray powder diffraction on samples equilibrated in various environments. The data have been integrated with information from the literature to derive an enthalpy of oxidation and an enthalpy of solution for LiNb3 08 into LiNb03 .
The literature pertaining to hydrogen defects dissolved in lithium niobate has been reviewed. Particular attention has been given to the infra-red absorption spectra. The polarization variations of the spectra give indications about the structure of hydrogen defects in lithium niobate. In undoped crystals hydrogen defects sit in the close-packed oxygen plane, adjacent to vacant octahedral sites that result from nonstoichiometry. In magnesium doped crystals the observed threshold effect influences the hydrogen site; at low Mg concentrations there are sufficient vacant octahedral sites, but at high concentrations the hydrogen must sit adjacent to cations and are therefore pushed out of the close-packed oxygen plane. This changes the IR spectra. The hydrogen solution model is discussed with respect to optical damage effects in lithium niobate.
Techniques for producing large single domain potassium niobate (KNb03? crystals suitable for optical devices are discussed. Single crystals measuring up to 25x25x25 mm have been grown from potassium-rich melts using a top seeded technique. An x ray diffractometer and goniometer capable of orienting bulk crystals and fabricated pieces with an accuracy of 15 arc seconds was designed and constructed. An improved method of poling permits the alignment of ferroelectric domains within minutes using infrared heat. Fabrication of crystals has been streamlined through the use of special fixtures. Efficient room temperature second harmonic generation (SHG) of the 532 nm laser line is obtained using the <101> cut.
KTiOPO4 (KTP) satisfies many of the criteria required of a second harmonic generating (SHG) material. KTP combines large nonlinear optical coefficients with broad spectral, temperature, and angular bandwidths. It is also chemically inert and easy to fabricate. Accordingly it has been called the material of choice for the doubling of 1.064 μm Nd:YAG radiation. A history of the development of KTP to its present status is presented with special emphasis on crystal growth and SHG performance. A summary of various physical, chemical, crystallographic, dielectric, ferroelectric, and optical properties of the material are given. Applications such as SHG of KTP under conditions of noncritical phase matching, optical parametric oscillation, blue light generation via wave mixing, and guided wave optics are discussed. The future of KTP as a nonlinear optical material is addressed stressing solutions to crystal size limitations and bulk damage problems.
KTP possesses superior properties in its use as a nonlinear optic material with Type II phase matching. It is not hygroscopic, has a large effective nonlinear coefficient, excellent optical damage resistance, small beam walk-off angle, and large thermal and angular bandwidths. These combined factors have dictated its choice as a second harmonic generator when compared to previous materials. Ba2NaNb5015 was difficult to grow, KH2PO4 is hygroscopic, LiO3 has small spectral bandwidth, and LiNbO3 suffers optical damage easily. Even the recent 13-BaB204 has a larger walk-off angle, smaller angular acceptance, and a smaller d coefficient. In addition, the larger acceptance angle for KTP results in higher output for miniature configurations at low power input. KTP has now found new applications for optical waveguiding, sum frequency mixing, and parametric oscillators. The applications requiring large size have been limited because of the difficulty of growing single crystals. The use of high temperature and pressure aqueous solvents (hydrothermal crystallization) or the use of high temperature molten salt solvents limits the size of useful crystals. Hydrothermal crystal growth at a lower temperature offers potential advantages for size, cost and quality. Recent laboratory experiments suggested that growth may be performed near 400° C. In this paper, we report the scale up of this method to results in commercial sized activities. Our preliminary data indicate that favorable growth can be maintained at temperatures of 100° C lower than previously. We have examined selected physical properties of our low temperature crystals and compared them to those from normal growth. All of our measurements indicate a KTP crystal fully comparable to previous samples. Our process represents a major advance in scale up of growth technology for this important material.
Potassium titanyl phosphate (KTP) is a very important material for use in nonlinear optics. In particular, its nonlinear coefficients and refractive indices are well suited for efficient generation of the second harmonic of the 1.06μm line of the Nd:YAG laser. There is growing interest in manufacturing compact diode-pumped solid-state lasers with wavelengths in the visible region. The nonlinear crystal is a key component in these devices. Although it is possible to obtain blue radiation by sum frequency generation of the diode wavelength and 1.06μm, the non-critical point for type I phase matching for the second harmonic falls at around 990 nm. KTP is, therefore, unsuitable for obtaining wavelengths below 495 nm by second harmonic generation. There are many analogs of KTP in which any of the potassium, titanium or phosphorous are isomorphously replaced by other elements. For example, K can be replaced by Rb, Tl or NH4' and P by As. The properties of several of these compounds are known. Very little is known about the compound in which Ti is replaced by Sn. We describe the growth of a series of crystals KTi,_xSnx0PO4 from high temperature solution, and the characterization of the optical properties of these materials by powder methods.