Calcium Lanthanum Sulfide Research Articles

First-principles analysis of the local atomic environment in calcium lanthanum sulfide ceramics

Density functional theory is utilized to investigate the energetics and mechanisms of sulfur loss across different stoichiometries of La2S3:CaS ceramics. These atomistic results reveal that the local atomic environment around each sulfur atom, consisting of six cation sites, can be used to predict the vacancy formation energy of the atom. Three nominal compositions are considered, 50:50, 65:35, and 90:10 La2S3:CaS, for two different atomic configurations, La2S3-like and La3S4-like. Relations were developed utilizing multivariable linear models. It is shown that the number of cation vacancies and lanthanum atoms in the local environment are among the most important factors in determining the energetics of a sulfur vacancy. Bader charge analysis reveals this is related to the degree of localization of charge change around the vacancy site and how close to the preferred oxidation states the lanthanum is. Results also suggest that while defect-free La2S3-like structures are energetically favorable, once a critical vacancy content is reached, the La3S4-like structure becomes the lower energy structure, which agrees qualitatively with recent experimental observations. This has important implications on how defect content and stoichiometry in this system are related. These findings also provide useful guidance on how these materials might best be processed.

Electronic, mechanical, and vibrational properties of calcium lanthanum sulfide solid solution: A DFT study

The combination of optical and mechanical properties of calcium lanthanum sulfide (CLS) makes it an attractive material for optical applications. CLS is a γ-phase La2S3/CaS solid solution above 50:50 La2S3/CaS. Experiments have characterized the effect of composition on the structural, thermal, and optical properties. However, the effect of impurities and other defects, such as porosity, on resulting properties is difficult to deconvolute. We used the density functional theory (DFT) simulations to characterize the effect of La2S3/CaS relative fractions on structural, electronic, mechanical, and vibrational properties of single crystalline CLS solid solutions. We find a decrease in the stiffness with CaS fraction and confirm the experimentally observed increase in the lattice parameter with La2S3 fraction. We also characterized the point defects, including sulfur vacancies, oxygen substituting sulfur, oxygen, and sulfur impurities on lanthanum sublattice vacant sites, and sulfur substituting calcium (vS, OS, OLa, and SLa, respectively). Our defect studies in 90:10 CLS find the neutral charge state configuration to be energetically favorable for all tested configurations except for the sulfur vacancies (vS).

Field-assisted sintering and phase transition of ZnS-CaLa2S4 composite ceramics

In the present study, zinc sulfide (ZnS) and calcium lanthanum sulfide (CaLa2S4, CLS) composite ceramics were consolidated via field-assisted sintering of 0.5ZnS-0.5CLS (volume ratio) composite powders at 800–1050°C. Through sintering curve analyses and microstructural observations, it was determined that between 800 and 1000°C, grain boundary diffusion was the main mechanism controlling grain growth for both the ZnS and CLS phases within the composite ceramics. The consolidated composite ceramics were determined to be composed of sphalerite ZnS, wurtzite ZnS and thorium phosphate CLS. The sphalerite-wurtzite phase transition of ZnS was further demonstrated to be accompanied by the formation of stacking faults and twins in the ceramics. It was also found that the addition of the CLS phase improved the indentation hardness of the ceramics relative to pure ZnS by homogeneous dispersion of ZnS and CLS small grains.

Sintering behavior of calcium lanthanum sulfide ceramics in field‐assisted consolidation

AbstractIn this study, calcium lanthanum sulfide (CaLa2S4, CLS) ceramics with the cubic thorium phosphate structure were sintered at different temperatures by field‐assisted sintering technique (FAST). Densification behavior and grain growth kinetics were studied through densification curves and microstructural characterizations. It was determined that the densification in the 850°C‐950°C temperature range was controlled by a mixture of lattice or grain‐boundary diffusion, and grain‐boundary sliding. It was revealed that grain‐boundary diffusion was the main mechanism controlling the grain growth between 950°C and 1100°C. The infrared (IR) transmittance of the FAST‐sintered CLS ceramics was measured and observed to reach a maximum of 48.1% at 9.2 μm in ceramic sintered at 1000°C. In addition, it was observed that the hardness of the CLS ceramics first increased with increasing temperature due to densification, and then decreased due to a decrease in dislocations associated with grain growth.

Elaboration of CaLa2S4 transparent ceramics from novel precursor powders route

A cost-effective processing route for the production of calcium lanthanum sulfide CaLa2S4 (CLS) via a novel fast fabrication of precursor powders is reported. The sinterability of the newly developed powders was investigated by use of Hot-Pressing and pressureless sintering. Complementary techniques (XRD, SEM-EDS, chemical analysis, SSA, FT-IR spectroscopy) were employed to correlate the sintering processes and parameters to the microstructural/compositional developments and optical performances of the densified ceramics. Dense (>99.8% theor.) and homogeneous CLS ceramics were produced by pressureless sintering at 1250°C for 12h in H2S followed by hot-pressing at 1000°C for 6h in a powder bed to prevent sulfur loss. Transparency free of impurity absorption has been achieved in the LWIR region (optical transmission of 51% in the 12–14µm range).

ChemInform Abstract: Synthesis and Characterization of Calcium Lanthanum Sulfide via a Wet Chemistry Route Followed by Thermal Decomposition.

AbstractCaLa2S4 is synthesized from mixtures of Et2NCS2Na, phen, LaCl3, and CaCl2 in EtOH containing thioglycolic acid as a capping agent (1 h) followed by heating the obtained precursor at 1000 °C for 5 h in flowing Ar.

Synthesis and characterization of calcium lanthanum sulfide via a wet chemistry route followed by thermal decomposition

In the present research, the ternary sulfide CaLa2S4 has been successfully synthesized via a wet chemistry method followed by thermal decomposition and characterized to investigate the microstructural features and optical properties.

ChemInform Abstract: Preparation of Calcium Lanthanum Sulfide Powders Via Alkoxide Sulfurization.

This paper reports that calcium lanthanum sulfide powders are prepared by reacting methoxides of calcium and lanthanum with H[sub 2]S. Alkoxide mixtures of various La/Ca ratios dispersed in methanol were reacted with H[sub 2]S at 25[degrees] to 85[degrees] C and the amorphous gels obtained after the removal of methanol were heat-treated in H[sub 2]S for full sulfurization at various temperatures. Up to 500[degrees] C, cubic LaS[sub 2] was the only crystalline phase present. Calcium lanthanum sulfide phase started to form at 550[degrees] C. In the temperature range of 650[degrees] to 750[degrees] C, single-phase calcium lanthanum sulfide powders were obtained for La/Ca = 2.41 and 2.68.

Normal and interfacial stresses in thin-film coated optics: the case of diamond-coated zinc sulfide windows

Optical components such as mirrors or windows consisting of a substrate and a coating made up of thin films created at elevated temperatures exhibit substantial residual stresses induced by growth strains and/or thermoelastic strains that develop during the cool-down phase. A comprehensive description of these stresses must include not only the normal stresses in the film layers and the substrate but also the interfacial shearing stresses, which may cause delamination to occur. We take advantage of recent progress in describing elastic interactions in multilayered laminates for obtaining conceptually correct formulas for the residual stresses and the substrate's curvature of thin-film coated optics. Available analytical solutions for the normal stresses of elastically isotropic structures make no assumptions regarding layer thicknesses, but disregard the potential impact of edge effects. For circular structures such as coated optics, we show that recent work by Suhir now allows us to describe the distribution of both normal and interfacial stresses as long as the thin-film conditions are satisfied. The task of evaluating the deflection turns out to be fairly straightforward, leading to the conclusion that edge effects do not alter the bow of large compliant structures. The case of diamond-coated ZnS windows illustrates how thermal expansion mismatches can give rise to compressive film stresses of gigapascal intensity, which cause substrate deformations that are unacceptable in terms of the optical performance. Since the deflection of a multilayer- coated substrate reflects the sum of the contributions (positive or nega- tive) induced by each film, the deflection can be minimized by properly designing the film stack. For a diamond-coated ZnS window, this means that a suitable buffer must be in tension; in principle, a buffer made of calcium lanthanum sulfide, about 350mm thick, can mitigate the bending force exerted by a 50 mm thick diamond film and suppress the shear at the substrate/coating interface, thus enhancing the adhesion. © 2001

Solid particle erosion of IR-transmitting materials and diamond composites

A solid particle erosion facility has been used to study the erosion of two groups of materials. The first contains IR-transmitting solids which have potential application as IR “windows” and include zinc sulphide, germanium, calcium lanthanum sulphide, diamond (both natural and free-standing chemically vapour deposited (CVD) diamond), and CVD coated substrates. The second contains highly erosion resistant composites such as polycrystalline diamond (PCDs). The sections on the erosion of diamond in its various forms review results produced in the laboratory over recent years. The paper concentrates on providing quantitative data on erosion rates and the processes of material loss.

Formation of Calcium Lanthanum Sulfide by Carbonate Coprecipitation and CS2 Sulfurization

Pure calcium lanthanum sulfide (CLS) powders with various La/Ca ratios were prepared by a carbonate coprecipitation method and CS2 sulfurization. Four coprecipitation processes were investigated by conventional titration, and a suitable means for coprecipitating CLS was found. By that process, a cation solution containing La3+ and Ca2+ ions was added to the precipitation medium (an anion solution of ammonium carbonate) with stirring. Precursors with various La/Ca ratios were synthesized and then sulfurized for 3 h using CS2 as a sulfurizing agent. In the range of La/Ca ratios from 2 to 5, the intermediate phase of sulfide powder was LaS2, and the CLS formation temperature could be increased by increasing the La/Ca ratio. For powder with high La/Ca ratios (La/Ca = 10, 15), and LaS2intermediate phase formed at low temperature (700°C), a (β‐La2S3 minor phase at 800°C, and a pure CLS phase after 950°C treatment. The resulting pure CLS powder was spherical, with slight necking in the submicrometer range.

Fabrication of calcium lanthanum sulfide ceramic by carbonate coprecipitating method

Translucent CLS (calcium lanthanum sulfide) pellets were formed by the carbonate coprecipitation method, CS[sub 2] sulfurization, and hot press sintering. For a La/Ca = 2.5 pellet with 1.3 mm in thickness, the transmittance at 13 [mu]m is about 25% after sintering at 1,150 C for 30 minutes and resulfurizing at 950 C for 1 hour. For a La/Ca = 15 pellet with 0.9 mm in thickness the IR transmittance is about 51% at 13 [mu]m after sintering at 1,050 C for 2 hours and resulfurizing at 950 C for 1 hour. Beta-La[sub 2]S[sub 3] is present as a second phase after sintering at 1,150 C for 30 minutes. After resulfurization, the second phase disappears for the La/Ca = 2.5 pellet, but still exists in the La/Ca = 15 pellet.

Hot-press sintering and the properties of lanthanum-rich calcium lanthanum sulfide ceramic

The lanthanum-rich calcium lanthanum sulfide (CLS) was formed by a carbonate coprecipitating method followed by sulfurizing in a CS2 atmosphere at 950 °C for 3 h sulfurization. This powder then was densified by hot-press sintering at the temperature range from 1050 to 1150 °C for 0.5-2 h. The sintered pellets were resulfurized in a CS2 atmosphere at 950 °C for 1 h to restore the sulfur loss. The pellet sintered at 1050 °C for 0.5 h with 1.3 mm thickness had about 46% transmission at 13 μm.

The effects of sulphurizing agent on the formation of calcium lanthanum sulphide

For calcium lanthanum sulphide (CLS) formation, the effect of the sulphurizing agent is studied. CS 2 and sulphur are used as sulphurizing agents to sulphurize the lanthanum and calcium carbonate precursors which are formed by a coprecipitated method. With CS 2 as a sulphurizing agent, the intermediate product formed is LaS 2 whereas when sulphur is employed, the La 2O 2S was the intermediate product. If LaS 2 is formed first, it is not difficult to complete the sulphurizing reaction for CLS at a low temperature (700°C) for a short soaking time (3 h). However, if the La 2O 2S is formed first, the phase formation reaction of CLS should be completed at a high temperature (800–1050°C) for a long soaking time in order to dissociate the LaO bond.

Phase formation and thermal stability of calcium lanthanum sulfide powder

The phase formation and decomposition of calcium lanthanum sulfide (CLS) for various La/Ca ratios were determined by differential thermal analysis-thermogravimetric analysis, X-ray diffraction, Fourier transform IR spectroscopy and scanning electron microscopy. In the formation of CLS, the initial phases are LaS 2 and β-La 2S 3 for high La/Ca ratio, but the initial phase is LaS 2 only for low La/Ca ratio on sulfurizing in CS 2 atmosphere. For the high-La/Ca powder, the thermal stability is better than that of the low-La/Ca powder, and La 2O 2S is formed first during phase decomposition. For the low-La/Ca CLS, La 2O 2S 2 is the first decomposition phase. The major phase of these powders finally decomposes to La 2O 2(SO 4) at 860°C. The particle size of the CLS powders decreases as a result of heat treating in air.

The near millimetre wavelength optical constants of calcium lanthanum sulphide

Spectroscopic measurements at near millimetre wavelengths made on calcium lanthanum sulphide, a potential material for infrared (IR) window applications in high speed aircraft, have revealed a region of transparency below 40 cm −1. The optical constants of the material have been measured from 5 to 40 cm −1 at 293 K, in order to consider its suitability for near millimetre wavelength radome applications.

Characterization of lanthanum sulphides

With application in infrared lenses, lanthanum sulphide and calcium lanthanum sulphide are ceramic materials which require a low defect density in order to minimize light scattering and maximize mechanical strength. Solution routes to monodispersed ceramics are investigated, offering materials with improved packing in the green body and considerable energy saving through the lower preparation temperatures.

Preparation of Calcium Lanthanum Sulfide Powders via Alkoxide Sulfurization

Calcium lanthanum sulfide powders were prepared by reacting methoxides of calcium and lanthanum with H2S. Alkoxide mixtures of various La/Ca ratios dispersed in methanol were reacted with H2S at 25° to 85°C and the amorphous gels obtained after the removal of methanol were heat‐treated in H2S for full sulfurization at various temperatures. Up to 500°C, cubic LaS2 was the only crystalline phase present. Calcium lanthanum sulfide phase started to form at 550°C. In the temperature range of 650° to 750°C, single‐phase calcium lanthanum sulfide powders were obtained for La/Ca = 2.41 and 2.68.

Fabrication and ESR Characterization of Transparent CaLa2S4

The preparation of calcium lanthanum sulfide (nominally CaLa{sub 2}S{sub 4}) powder and its processing into a fully dense optical ceramic by sintering and hot isostatic pressing are described. Maximum infrared transmission is observed for the atomic ratio La/Ca = 2.7. Observation of the characteristic ESR signal of the impurity Mn{sup 2+} shows that CaS is present in the CaLa{sub 2}S{sub 4} powder. The CaS is not seen after sintering at 1,150{degree}C under 100 kPa (1 atm) of H{sub 2}S. A second cycle of hot isostatic pressing above 990{degree}C causes loss of sulfur from the ceramic, decreases the ir transmission, and introduces paramagnetic defects. Sulfur deficiency is inferred from the reduction of H{sup +} to H{sub 2} when the ceramic is dissolved in acid.