Fifty years have elapsed from the moment the first light transmitting ceramic-based commercial item-a sodium vapor-based street lamp component-reached the market. This paper intends to be a brief chronicle (albeit not a chronology) of this first half century of translucent and transparent ceramic history. The main ceramic materials now available in a transparent state are presented and their main applications are described. Applications range from aerospace and relativistic optics to medical care, supermarket shopping, and modern warfare. Light transmitting ceramics usable as laser gain-media, armor windows, IR domes, phosphors, scintillators, and electro-optical components have been developed. The principal achievements this research produced are discussed. Processing strategies for full densification have been devised and quantitative relationships were established between different microstructural features such as amount and size distribution of porosity or level of birefringence, and the level of electromagnetic radiation attenuation they cause. Future prospects list ends the paper.

The validity of the Hall–Petch relationship for transparent spinel ceramics with grain sizes of 0.2–0.1 μm is confirmed here in context with data for coarser microstructures >0.3 μm and up to 200 μm whereas there is no further hardness increase on the transition to nanoscale spinel microstructures. Although such cubic ceramics do not suffer birefringent scattering, all active optical losses are subject to a strong influence of the thickness. Most technical applications of transparent spinel request components thicker than 1–2 mm and need, therefore, very high in-line transmission data when measured on thinner samples.

International Journal of Applied Ceramic TechnologyVolume 12, Issue S1 p. E174-E175 Reply Comment: The Effect of Grain Size on the Mechanical and Optical Properties of Spark Plasma Sintering-Processed Magnesium Aluminate Spinel MgAl2O4 (Rothaman et al.) Andreas Krell, Corresponding Author Andreas Krell Fraunhofer Institute for Ceramic Technologies and Systems (IKTS), Dresden, 01277 Germany Member, The American Ceramic Societyandreas.krell@ikts.fraunhofer.deSearch for more papers by this author Andreas Krell, Corresponding Author Andreas Krell Fraunhofer Institute for Ceramic Technologies and Systems (IKTS), Dresden, 01277 Germany Member, The American Ceramic Societyandreas.krell@ikts.fraunhofer.deSearch for more papers by this author First published: 10 July 2014 https://doi.org/10.1111/ijac.12280Citations: 1 Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article.Citing Literature Volume12, IssueS1Special Issue: Ferroelectric CeramicsJanuary/February 2015Pages E174-E175 RelatedInformation

The study investigates the transmittance and hardness of Al‐rich spinel ceramics (MgO·nAl2O3, 1 ≤ n ≤ 2.5) prepared by reaction air sintering (up to closed porosity) of different ratios of fine and coarse‐grained commercial Al2O3 and MgO raw powders completed by subsequent hot isostatic pressing (HiP). Different compositions give rise to a wide range of presintering temperatures. With starting compositions 1 ≤ n ≤ 1.5, presintering results in a formation of single‐phase spinel, in which the excess of Al is solved. With higher Al contents (n > 1.5), however, a biphasic ceramic of stoichiometric MgAl2O4 and residual alumina is formed first. This excess alumina is incorporated into the spinel lattice during the final HiP at a temperature of 1750°C. Single‐phase, highly transparent spinel is obtained by increasing the Al‐content up to n = 2.5, which gives about 85% in‐line transmittance in the visible range of light and about 63% at a UV wavelength of 200 nm. Whereas the optical properties can be improved, the hardness (HV1) slightly decreases with increasing Al content. Depending on the raw powders, the hardness of samples prepared by finer powders tend to higher values enabled by the development of a bimodal microstructure with a finer grain fraction (≤2 μm) between coarser grains (≤156 μm). In contrast, samples made of coarser powders need higher sintering temperatures and exhibit, then, a monomodal microstructure of very large grains (≤622 μm) only.

The validity of the Hall–Petch relationship for transparent spinel ceramics with grain sizes of 0.2–0.1μm is confirmed here in context with data for coarser microstructures >0.3μm and up to 200μm whereas there is no further hardness increase on the transition to nanoscale spinel microstructures. Although such cubic ceramics do not suffer birefringent scattering, all active optical losses are subject to a strong influence of the thickness. Most technical applications of transparent spinel request components thicker than 1–2mm and need, therefore, very high in-line transmission data when measured on thinner samples.

Absorption and fluorescence of chromophoric dissolved organic matter (CDOM) in sea ice and surface waters in the southern Sea of Okhotsk was examined. Sea-water CDOM had featureless absorption increasing exponentially with shorter wavelengths. Sea ice showed distinct absorption peaks in the ultraviolet, especially in younger ice. Older first-year sea ice had relatively flat absorption spectra in the ultraviolet range. Parallel factor analysis (PARAFAC) identified five fluorescent CDOM components, two humic-like and three protein-like. Sea water was largely governed by humic-like fluorescence. In sea ice, protein-like fluorescence was found in considerable excess relative to sea water. The accumulation of protein-like CDOM fluorescence in sea ice is likely a result of biological activity within the ice. Nevertheless, sea ice does not contribute excess CDOM during melt, but the material released will be of different composition than that present in the underlying waters. Thus, at least transiently, the CDOM introduced during sea-ice melt might provide a more labile source of fresher protein-like DOM to surface waters in the southern Sea of Okhotsk.

The recently suggested validity of the Hall–Petch relationship for transparent spinel ceramics with grain sizes down to 28 nm is discussed here regarding the equivalence of grain size and indentation size effects. The quantitative characterization of the samples investigated as transparent needs a correction of the authors' calculation of the theoretical transmittance. For fundamental physical reasons, this theoretical transmission of ceramics with a cubic crystal lattice does not exhibit an intrinsic grain size influence.

Laboratory-made Al2O3 and spinel ceramics were investigated so as to enable ballistic tests with a variation of individual influences while keeping other factors constant. The study was aimed at answering the question about influences of different microstructures and of basic mechanical properties and included also a systematic investigation of the effects of backing materials. Contradictive findings of the past are explained by the observation that the ballistic impact stability of ceramics and single crystals with different backings (steel, aluminum, glass) is governed by a strict hierarchy of few major influences: (1) Top priority is the mode of ceramic fragmentation governed by microstructural features and by the dynamic stiffness of the ceramic/backing target; these influences also affect the relative importance of dwell and penetration. (2a) On a lower rank, Young’s modulus of the ceramic is responsible for projectile damage during dwell but the importance of this influence depends on the priority of ceramic fragmentation. (2b) On penetration, the abrasive benefit of a high ceramic hardness depends on the size of the ceramic debris, i.e. on priority (1). In contrast, all average strength data are weakly correlated with the ballistic efficiency.

This chapter contains sections titled: Introduction Material Preparation Uniaxial Hydraulic Pressing Thermal Processing Sample Characterization Summary and Outlook

For the past years most reports on transparent ceramics were focused on improvements of primary parameters like total and in-line transmission, scattering and absorption losses. The present paper directs the attention to previously neglected issues such as the quantified representation of remaining visible defects and the diversity of optical quality criteria for different groups Of applications. (C) 2014 Elsevier B.V. All rights reserved.

At the Stockholm Conference of the European Ceramic Society 2011 it was shown that, together with the particle size of raw powders and the homogeneity of particle coordination in the green bodies, the real structure of the lattices is a third major influence on sintering, which may differ significantly even at fixed stoichiometry. Antisite defects govern the whole defect chemistry of MgAl2O4 and should, therefore, affect diffusion and sintering. This possibility is investigated here with solid state (27)AlMAS nuclear magnetic resonance measurements of the occupancy of octahedral and tetrahedral sites. Based on previous results with model powders synthesized in order to identify lattice effects at constant particle size distribution, homogeneity and composition, the study has now been extended to commercial spinel powders supplied by Asian, European, and American manufacturers. The results confirm a correlation of increasing cation disorder with improved sintering densification.

Contrary to the moderate ballistic advantage of Al2O3 ceramics over MgAl2O4 polycrystals, the present study shows a reverse ratio of the ballistic strength of alumina and spinel single crystals: Spinel single crystals outperform sapphire and exhibit a similarly high stability as submicrometer Al2O3 ceramics. The results correlate with different cleavage of single crystalline spinel and sapphire, changing the fragmentation on ballistic impact.

It is demonstrated that a complete elimination of pores on sintering is governed not only by the size of the ceramic powder particles and by the homogeneity of their mutual coordination but similarly strongly by the state of the crystal lattice: with different cation disorder at fixed stoichiometry (n=1) the sintering temperatures may differ by as much as 200°C at constant powder particle size and equal homogeneity of the green bodies. Additionally, the impact of stoichiometry was investigated over the range between n=1 and n=3 with retarded reactive sintering at moderately increased Al2O3 concentrations but promoted densification of alumina-rich compositions. Taking advantage of the observed effects, sintered spinel ceramics were derived by reactive sintering of undoped MgO/Al2O3 mixtures resulting in an in-line transmittance which equals spinel single crystals of similar composition from 200nm wave length up to the IR range.

Reference EPFL-ARTICLE-180443doi:10.1016/j.jeurceramsoc.2012.05.017View record in Web of Science Record created on 2012-08-03, modified on 2017-05-10

Abstract Calcium carbonate precipitation in sea ice is thought to potentially drive significant CO2 uptake by the ocean. However, little is known about the quantitative spatial and temporal distribution of CaCO3 within sea ice, although it is hypothesized that high quantities of dissolved organic matter and/or phosphate (common in sea ice) may inhibit its formation. In this quantitative study of hydrous calcium carbonate as ikaite, sea ice cores and brine samples were collected from pack and land fast sea ice between September and December 2007 during two expeditions, one in the East Antarctic sector and the other off Terre Adélie. Samples were analysed for CaCO3, salinity, dissolved organic carbon/nitrogen, inorganic phosphate, and total alkalinity. No relationship between these parameters and CaCO3 precipitation was evident. Ikaite was found mostly in the uppermost layers of sea ice with maximum concentrations of up to 126 mg ikaite per litre melted sea ice being measured, although both the temporal and horizontal spatial distributions of ikaite were highly heterogeneous. The precipitate was also found in the snow on top of the sea ice at some of the sampling locations.

This chapter contains sections titled: Introduction Experimental Results and Discussion Conclusions Acknowledgement