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Goldstein A.,Israel Ceramic and Silicate Institute | Shames A.I.,Ben - Gurion University of the Negev | Stevenson A.J.,University Pierre and Marie Curie | Stevenson A.J.,Saint - Gobain | And 2 more authors.
Journal of the American Ceramic Society | Year: 2013

Parasitic radiation absorption by impurities located in transparent polycrystalline ceramic hosts may significantly impair their functioning. At least two classes of impurity which can act as parasitic absorbers were encountered: transition or rare-earth metal cations and neutral species (molecules or atoms) trapped in lattice void spaces. The most ubiquitous parasitic absorbers, able to affect transmission in the visible domain, have been identified for the case of MgAl2O4-spinel and Y 3Al5O12-garnet hosts; their spectral effect was described and explained. In the case of spinel, S8 type molecules and carbon atom clusters are the most damaging impurities, with Ce3+ and Y2+ also of significance for YAG host. Oxygen vacancy formation assists some of the redox processes which generate uncontrolled absorption. For instance, the Ti4+ reduction to Ti3+ is facilitated by such a process. © 2013 The American Ceramic Society.

Goldstein A.,Israel Ceramic and Silicate Institute | Raethel J.,Fraunhofer Institute for Ceramic Technologies and Systems | Katz M.,Israel Ceramic and Silicate Institute | Berlin M.,Israel Ceramic and Silicate Institute | Galun E.,Elbit Systems
Journal of the European Ceramic Society | Year: 2016

Hot-pressing schedules - able to ensure a proper balance between positive and negative effects of LiF - allow fabrication of highly transparent MgAl2O4 parts under low-pressure (≤50 MPa); peak temperatures in excess of 1550 °C are needed, with best results obtained at ~1650 °C. At the concentration-levels and conditions present during hot-pressing, LiF does not react with spinel; it is inert also toward graphite. The lubricant and solvent abilities of liquid LiF make possible achievement of densification levels >90%TD under 1200 °C. All LiF leaves the specimens as such at temperatures ≤1550 °C. LiF's ability to close porosity at temperatures lower than those (>1350 °C) - which allow significant carbon penetration into spinel - is essential in achievement of transparency. Light absorption - occurring when carbon is present - is thus prevented. The main light-scattering defect produced by HPing, in the presence of LiF, is micro and macro-cracking; opaque spots, due to un-complete densification, are rare. © 2016 Elsevier Ltd.

Goldstein A.,Israel Ceramic and Silicate Institute | Loiko P.,Belarusian National Technical University | Burshtein Z.,Ben - Gurion University of the Negev | Skoptsov N.,Belarusian National Technical University | And 4 more authors.
Journal of the American Ceramic Society | Year: 2016

We describe the development of a reasonable cost Co2+:MgAl2O4 transparent ceramic plates fabrication technology that allows the producing of parts functioning as passive laser Q-switches in the 1.3-1.7 μm domain. The main relevant material characteristics were measured. The absorption band, positioned between 1.2 and 1.7 μm, is typical of the 4A2 (4F) → 4T1 (4F) transition of Co2+ substituting Mg2+ ions in their Td symmetry sites. The measured ground-state absorption cross section σgs = 2.9 × 10-19 cm2, saturation contrast γ = 0.12, and depleted ground-state recovery time τ2 = 110-430 ns render such parts suitable for the intended application. The radiative lifetime was estimated as τ2rad=14.25μs. The spin-orbit splitting constant was estimated as ξSL≅-150 cm-1 for the 4F parent ground state, and ξSL ≅ -575 cm-1 for the 4P parent excited state. Obtained specimens had a transmission of ~80% (t = 2 mm, λ = 600 nm) and included some opaque, white spots. Further improvement of host optical transmission and resistance to laser damage are necessary. © 2016 The American Ceramic Society.

Goldstein A.,Israel Ceramic and Silicate Institute | Cohen Z.,Israel Ceramic and Silicate Institute | Ish-Shalom M.,Israel Ceramic and Silicate Institute
Physics and Chemistry of Glasses: European Journal of Glass Science and Technology Part B | Year: 2013

The electron-donor ability of the Oδ- anions has a significant influence on the chemical bonding pattern in phosphate glasses. The variation in δ- electron-donor ability with composition can be qualitatively estimated by monitoring the evolution of the equilibria between the oxidation states of a transition metal dopant. We report the use of optical spectroscopy to examine the equilibria between the oxidation states of dopants like Cu, Mo, or Ti, for the case of Rm+O-P2O5 (m=1 or 2) type glassy hosts (melted under reductive conditions). It was determined that the presence of an R2+O metal oxide increases the stability of the lower dopant oxidation states (Cu0, Cu+, Mo3+, Ti3+), while the presence of a R1+O oxide favours the higher oxidation states (Cu2+, Mo5+, Ti 4+). It follows that, while Oδ- donor ability is increased by R1+O oxides, it is reduced by R2+O. A possible explanation for this finding is that small R2+ type cations form strong coordinative bonds with Oδ-, from different phosphate layers or chains, leading to overall enhancement of network cohesion despite the depolymerisation they also cause. The R1+ cations establish predominantly ionic inter-layer bonds, weaker than the broken -P-O-P- bonds; this leads to a less compact network. Such a model is consistent with the observed effect on phosphate glass properties of, respectively, R1+O and R2+O oxides.

Goldstein A.,Israel Ceramic and Silicate Institute | Goldenberg A.,Israel Ceramic and Silicate Institute | Hefetz M.,Israel Ceramic and Silicate Institute
Ceramic Transactions | Year: 2010

The sinterability of two fine Mg-spinel powders, which have similar characteristics, was investigated. One was prepared by wet-chemistry followed by calcination, while the other - by gas-phase reaction. Marked differences have been observed regarding their densification. The gas-phase powder could be sintered to a bulk density level of 97.5 %TD at a temperature as low as 1280°C. HIPing at 1350°C/3h of the sintered disks generates transparent (total transmission of ∼80% at λ=700 nm) specimens, having a grain size of ∼1 um. For the obtainment of transparent disks, the wet-chemistry powder requires sintering and HIPing at temperatures in the 1600 to 1700°C range. The reason(s) for the observed difference in sinterability are not clear. The more compact packing (possibly related to the spheroidal morphology of the basic particles), the lack of large (micron size) voids and the lower calcination temperature (a more disordered lattice, allowing faster diffusion) proper to the gas-phase powder may be among the factors conferring a higher sinterability.

Goldstein A.,Israel Ceramic and Silicate Institute
Journal of the European Ceramic Society | Year: 2012

Efforts were and are made to develop performant fabrication technologies, for transparent polycrystalline spinel - a material used for armor, infrared windows and other products. Significant progress was made - during some fifty years of research - regarding the understanding of the structure of spinel, at various scales, and the best ways to correlate processing with the relevant structural features so as to improve properties. This review compiles and comments the results of this progress, using as sources the literature and the author's own work. As of now the best specimens obtained combine submicron grains with an optical transmission close to the theoretical and a Vickers hardness of 15. GPa (size ≤25. cm). Larger plates, more than 0.5. m in size, but with coarse microstructure and lower hardness, have also been produced, together with quite large dome shaped parts, exhibiting highly uniform optical properties. © 2012 Elsevier Ltd.

Goldstein A.,Israel Ceramic and Silicate Institute | Yeshurun Y.,Israel Ceramic and Silicate Institute | Vulfson M.,Israel Ceramic and Silicate Institute | Kravits H.,Israel Ceramic and Silicate Institute
Journal of the American Ceramic Society | Year: 2012

There is interest in the enlarging of the bulk, transparent ceramics family. Polycrystalline ZnAl 2O 4 may constitute a useful addition to the existent set of such materials. The objective of this work was the development of a technology allowing the fabrication of bulk, polycrystalline, transparent ZnAl 2O 4 parts. It was determined that a nano ZnAl 2O 4 powder, synthesized by the hydrothermal technique, can be formed into green bodies having an average pore size of 30 nm and a bulk density of ∼55% TD. A two stage sintering process, in which air heating (1500°C) was followed by hot-isostatic pressing (1550°C), densified small (20 mm in diameter) disks to a bulk density ≈TD. Such specimens exhibit an in-line transmission of 78% (λ = 800 nm; t = 2 mm), a hardness of 10.5 GPa and an average grains size of 1.1 μm. © 2012 The American Ceramic Society.

Goldstein A.,Israel Ceramic and Silicate Institute | Goldenberg A.,Israel Ceramic and Silicate Institute | Vulfson M.,Israel Ceramic and Silicate Institute
Journal of Ceramic Science and Technology | Year: 2011

The objective of the work was the establishment of a procedure allowing the obtainment of high-optical-transparency MgAl2O4 polycrystalline parts that exhibit a fine microstructure (average grains size lower than 5 μm). The technology had to be relevant for industrial-scale production. A nano powder, commercially available in large quantities, at a reasonable price, was used as raw material. It was determined that by applying a suitable combination of powder processing and green-bodies-forming procedure, a configuration favorable for advanced densification can be derived from the selected powder. It was also found that the pore-closing ability of HIPing was best exploited when the specimens subjected to this treatment (predensified by pressureless sintering, in air) exhibited an optimal tradeoff between densification level and microstructural configuration. Specimens combining an average grain size of 2.5μm with an in-line transmission of 77% (λ = 750 nm; thickness ~2 mm) were obtained; such parts possess a Vickers hardness of 13.8 GPa and a transverse rupture strength of ~200 MPa. Because the technology developed generates transparent spinel exhibiting properties acceptable for some of the existent applications, at reasonable cost, it may be of interest to industry. © 2011 Göller Verlag.

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