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The Effect of LiF Addition on the Sintering Mechanism of Spinel, MgAl2O4 Hans-Joachim Kleebe, Mathis Müller, Keith Rozenberg 1 , Ivar E. Reimanis 1 1 Colorado School of Mines, Metallurgical and Materials Engineering Dept., Golden, CO 80401, USA MgAl2O4 is widely regarded as one of the most promising optical ceramics [1,2]. It has excellent transmissivity in the near infrared frequency range. There is an excellent body of research on the sintering mechanisms of pure spinel for optical applications. Unfortunately hot pressing with LiF doped spinel, which is one of the most promising processes for producing optical quality spinel ceramic, is also one of the least understood. Many compounds have been proposed or used as sintering aids for MgAl2O4 spinel (hereafter called spinel). For example, Na3AlF6, AlCl3, CaCO3, and LiF have all been shown to promote sintering in spinel, though the detailed mechanisms have remained elusive. LiF has been used to speed up the sintering kinetics of several other systems including SrTiO3, and BaTiO3, and MgO. LiF, is the only compound considered for use as a sintering aid in the production of low porosity, transparent spinel via hot pressing. The effect of LiF on the sintering of spinel has been only qualitatively studied. An understanding of the mechanism for removal of LiF from the system is essential as any residual LiF or related species could destroy the optical properties of the material. It has been shown that LiF reduces the sintering temperature of spinel by nearly 200°C, and decreases reaction temperatures during reactive sintering. More recent studies on dense spinel were performed to determine the location of LiF subsequent to sintering, with the objective of better understanding its role in sintering. LiF was not identified in any of the studies, suggesting that it may react with spinel during sintering and/or ultimately was removed from the system. It is known that LiF is active at the interface between spinel grains at higher temperatures. Furthermore, it has been reported that spinel sintered in the presence of LiF has a substantially higher Al content. Large amounts of LiF have also been observed to act corrosively with spinel. All of these observations indicate that LiF does chemically react with the spinel at elevated temperatures. A recent study indicates that LiF reacts with Al in spinel, forming LiAlO2, and leaving Mg-rich regions in the matrix. This, however, leaves the question of what becomes of the fluoride species. This spinel project addresses the issue of how LiF can promote the sintering performance of magnesium aluminate. In addition to HRTEM investigations, model experiments with high volume fraction of LiF addition are performed. [1] R. J. Bratton, “Translucent Sintered MgAl2O4”, J. Am. Ceram. Soc., 57 [7] 283-86 (1974). [2] C.-J. Ting and H.-Y. Lu, “Hot Pressing of Magnesium Aluminate Spinel; Part-II. Microstructural Development”, Acta mater. 47 [3] 831- 40 (1999). Fig. 2: TEM image of the spinel sample, doped with 5 wt% LiF and pre-sintered at 900 o C. In close proximity of the intrinsic pores, an amorphous LiFphase was observed (transient phase). - 152 -
Microstructure Characterization of Boron Suboxide, B6O Hans-Joachim Kleebe, Stefan Lauterbach, Mathias Herrmann 1 , Jack Sigalas 2 1 Fraunhofer Institut für Keramische Technologien und Systeme, Dresden 2 University of the Witwatersrand, Johannesburg, Gauteng, South Africa Hexaboron monoxide (B6O), commonly termed boron suboxide, is known as a superhard material, constituted of the light elements boron and oxygen. The extraordinary hardness of such a low-density material has been attributed to the intrinsic strong and directional covalent bonds between B and O, leading to a tight, threedimensional network with an excellent resistance towards external shear. The average Vickers hardness (Hv) of B6O is about 45 GPa, which is next to diamond (Hv: 70–100 GPa) and cubic boron nitride, c-BN (Hv: 45–50 GPa). Similarly, the average fracture toughness of B6O (4.5 MPa m 1/2 ) is higher as compared to c-BN (2.8 MPa m 1/2 ) and comparable to diamond (5.0 MPa m 1/2 ). Due to its strong covalent bonding, B6O materials demonstrate exceptional physical and chemical performance such as high hardness, low density, high thermal conductivity, chemical inertness and good wear resistance. Its thermal stability, even at temperatures above 1000 °C, and its chemical inertness with ferrous alloys makes it in some instances even more suitable for industrial applications as for example diamond. In general, B6O materials have potential applications as abrasives, due to their high hardness, and are also considered as potential candidates for high-temperature semiconductors (with an estimated band gap of approximately 2.4 eV) and for thermo-electrics. Despite various potential applications, densification and processing of B6O materials was shown to be rather cumbersome. Therefore, a detailed microstructure characterization was performed in order to gain insight in the submicron structure that formed upon sintering. ACerS – NIST Phase Equilibria Diagrams Fig. 02339—System Al2O3-B2O3. P. J. M. Gielisse and W. R. Foster, Nature (London), 195 [4836] 69-70 (1962). The main objectives of the TEM study were (i) to verify as to whether an amorphous intergranular phase is present at the B6O grain boundaries (doped versus undoped sintered samples), (ii) what type of crystalline grain-boundary phases are present in the sintered, coated B6O sample: Al4B2O9 and/or Al18B4O33, and (iii) the overall defect structure in the materials was to be characterized (in particular, the question whether stacking faults in B6O can be eliminated via long-term annealing was addressed). - 153 -
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ANNUAL REPORT 2 0 0 6 FACULTY OF MA
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Annual Report 2006 Faculty 11 Mater
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Preface The year 2006 of the Depart
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difference of numerical and analyti
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Research Projects Bulk Amorphous Al
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el Dsoki, C.; Landersheim, V. ; Kau
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Das, J.; Kim, K. B.; Xu, W.; Wei, B
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Staff Members Head Prof. Dr. Jürge
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Zhou L.; Rixecker G.; Aldinger F.;
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concerned with the synthesis and ch
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• Surface analysis The UHV-surfac
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T. Mayer, U. Weiler, E. Mankel and
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Thin films The Thin Film division w
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Berky, W.; Gottschalk, S.; Elliman,
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Hesse, S.; Zimmermann, J.; von Segg
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Structure Research The research in
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Dincer, I.; Elerman, Y.; Elmali, A;
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hexakis(imidazole)nickel(II)bis(for
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Collaborations The above mentioned
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Hoffmann, P.; Non-Invasive Identifi
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Ferroelectrics For this class of ma
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Materials Modelling The research ac
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Materials for Renewable Energies In
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Collaborative Research Center (SFB)
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B7 P.I.: Prof. H. v. Seggern / Dr.
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transition allows to investigate at
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screening effects. Gold coating of
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constituents. The requirement of va
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