The MN+1AXN phases: A new class of solids

This paper reviews the crystal chemistry, synthesis, densification, microstructure, mechanical properties, and oxidation behavior of zirconium diboride (ZrB2) and hafnium diboride (HfB2) ceramics. The refractory diborides exhibit partial or complete solid solution with other transition metal diborides, which allows compositional tailoring of properties such as thermal expansion coefficient and hardness. Carbothermal reduction is the typical synthesis route, but reactive processes, solution methods, and pre-ceramic polymers can also be used. Typically, diborides are densified by hot pressing, but recently solid state and liquid phase sintering routes have been developed. Fine-grained ZrB2 and HfB2 have strengths of a few hundred MPa, which can increase to over 1 GPa with the addition of SiC. Pure diborides exhibit parabolic oxidation kinetics at temperatures below 1100°C, but B2O3 volatility leads to rapid, linear oxidation kinetics above that temperature. The addition of silica scale formers such as SiC or MoSi2 improves the oxidation behavior above 1100°C. Based on their unique combination of properties, ZrB2 and HfB2 ceramics are candidates for use in the extreme environments associated with hypersonic flight, atmospheric re-entry, and rocket propulsion.

Refractory Diborides of Zirconium and Hafnium

We present, as a progress report, a revised and much enlarged version of the thermodynamic dataset given earlier (Holland & Powell, 1985). This new set includes data for 123 mineral and fluid end-members made consistent with over 200 P–T–XCO2–fO2 phase equilibrium experiments. Several improvements and advances have been made, in addition to the increased coverage of mineral phases: the data are now presented in three groups ranked according to reliability; a large number of iron-bearing phases has been included through experimental and, in some cases, natural Fe:Mg partitioning data; H2O and CO2 contents of cordierites are accounted for with the solution model of Kurepin (1985); simple Landau theory is used to model lambda anomalies in heat capacity and the Al/Si order–disorder behaviour in some silicates, and Tschermak-substituted end-members have been derived for iron and magnesium end-members of chlorite, talc, muscovite, biotite, pyroxene and amphibole.



For the subset of data which overlap those of Berman (1988), it is encouraging to find both (1) very substantial agreement between the two sets of thermodynamic data and (2) that the two sets reproduce the phase equilibrium experimental brackets to a very similar degree of accuracy. The main differences in the two datasets involve size (123 as compared to 67 end-members), the methods used in data reduction (least squares as compared to linear programming), and the provision for estimation of uncertainties with this dataset. For calculations on mineral assemblages in rocks, we aim to maximize the information available from the dataset, by combining the equilibria from all the reactions which can be written between the end-members in the minerals. For phase diagram calculations, we calculate the compositions of complex solid solutions (together with P and T) involved in invariant, univariant and divariant assemblages. Moreover we strongly believe in attempting to assess the probable uncertainties in calculated equilibria and hence provide a framework for performing simple error propagation in all calculations in thermocalc, the computer program we offer for an effective use of the dataset and the calculation methods we advocate.

An enlarged and updated internally consistent thermodynamic dataset with uncertainties and correlations: the system K2O–Na2O–CaO–MgO–MnO–FeO–Fe2O3–Al2O3–TiO2–SiO2–C–H2–O2

and Perspectives Sophie Carenco,†,‡,§,∥,⊥ David Portehault,*,†,‡,§ Ced́ric Boissier̀e,†,‡,§ Nicolas Meźailles, and Cleḿent Sanchez*,†,‡,§ †Chimie de la Matier̀e Condenseé de Paris, UPMC Univ Paris 06, UMR 7574, Colleg̀e de France, 11 Place Marcelin Berthelot, 75231 Paris Cedex 05, France ‡Chimie de la Matier̀e Condenseé de Paris, CNRS, UMR 77574, Colleg̀e de France, 11 Place Marcellin Berthelot, 75231 Paris Cedex 05, France Chimie de la Matier̀e Condenseé de Paris, Colleg̀e de France, 11 Place Marcellin Berthelot, 75231 Paris Cedex 05, France Laboratory Heteroelements and Coordination, Chemistry Department, Ecole Polytechnique, CNRS-UMR 7653, Palaiseau, France

Nanoscaled metal borides and phosphides: recent developments and perspectives

The Binary Rare Earth Oxides.

Samples of a TiB2 ceramic containing 0 to 10 wt% Ni were fabricated by hot-pressing. Several properties, including fracture strength, indentation fracture toughness, and thermal expansion (between 25° and 1000°C) were measured. Resulting data were correlated with sample microstructure and composition.

Effect of Microstructure on the Properties of TiB2 Ceramics

The recent results of leach tests performed under hydrothermal conditions have shown that borosilicate glasses are physically and chemically unstable at elevated temperatures and pressures (1). These results emphasize the need for investigations of alternate high level waste forms which offer the potential of both increased resistance to leaching and stability under various possible metamorphic geological conditions. Although many of the previous (and present) studies of high level waste forms emphasize the containment of fission products, in fact, the dominant long term (i.e., after 300 to 500 year) radiation hazard is posed by the α-emitting actinides. Accordingly, the object of the present investigation is to evaluate an alternate means of actinide containment which is potentially superior to encapsulation in borosilicate glass. The mineral monazite, which is a mixed lanthanide-actinide orthophosphate [i.e., (Ce, La, Nd, Th, U)PO4] of varying composition, is a crystalline material with established chemical and physical stability when subjected to natural geological conditions over extremely long time periods. Additionally, since monazite is a natural source of the actinides Th and U, it has been subjected to radiation damage (including α-particle damage) during its natural lifetime, and the continued maintenance of its physical and chemical integrity is evidence of its intrinsic resistance to radiation. In view of these obviously desirable characteristics, a series of investigations of the physical and chemical properties of both natural monazite and the related, synthetically produced lanthanide orthophosphates has been undertaken. These investigations have included the usual applied studies such as leach testing under various conditions and the consolidation of precipitated orthophosphate powders by hot pressing as well as more fundamental determinations of the valence states and site symmetry of the actinide impurities. The details of this work are presented in the following discussion.

Monazite and other lanthanide orthophosphates as alternate actinide waste forms

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Heat capacity and entropy of fayalite (Fe2SiO4) between 5.1 and 383 K: comparison of calorimetric and equilibrium valus for the QFM buffer reaction.

The EPR spectra of Er3+, Yb3+, and Gd3+ have been observed in the host single crystals ZrSiO4, HfSiO4, and ThSiO4 which were grown by a flux‐gradient technique. For most systems, spectra were observed which exhibited both tetragonal and orthorhombic symmetry. The tetragonal spectra were characterized by a principal axis system which coincided with the principal crystallographic axes. The observed orthorhombic spectra were of two types. For Gd3+ in ZrSiO4 and HfSiO4, orthorhombic spectra due to eight inequivalent sites were observed and had principal axis systems which were not simply related to the host crystallographic axes. All other orthorhombic spectra resulted from four inequivalent sites whose z and x axes lay in {110} planes with equal angles between the crystal tetragonal symmetry axis (c axis) and each z axis. For both types of orthorhombic spectra, the corresponding EPR transitions from the different sites were super‐imposed with the applied magnetic field parallel to the c axis. Spin‐Hamiltonian parameters have been determined for the observed tetragonal and orthorhombic spectra. Powder pattern analysis was employed for the Gd3+ orthorhombic spectra, due to the complexity which resulted from the large number of inequivalent sites. The implications of these results regarding the nature of the various sites are examined in conjunction with the results obtained previously by others for impurities in zircon‐structure and scheelite‐structure hosts.

EPR Investigations of Er3+, Yb3+, and Gd3+ in Zircon‐Structure Silicates

The compound TiB2 has numerous exceptional properties including high hardness, high melting temperature, high electrical conductivity, and nonreactivity with various liquid metals. These make it an attractive candidate for technological applications, such as cutting tools, valve trim for erosive environments, and cathodes in Hall-Heroult cells for aluminum smelting. In general, such applications require fabrication of TiB2 into various shapes, and the latter requirement dictates the production of high-density polycrystalline ceramic bodies. The preparation and characterization of TiB2-based materials have already been the subject of numerous previous works, exemplified by the references (1–9). These include data on the phase equilibria in TiB2 containing systems,1,3,9 the wettability of TiB2 by various metals,1,3,4 and the results of property determinations on TiB2-based specimens.

C. B. Finch

Papers

Effect of Microstructure on the Properties of TiB2 Ceramics

Monazite and other lanthanide orthophosphates as alternate actinide waste forms

Heat capacity and entropy of fayalite (Fe2SiO4) between 5.1 and 383 K: comparison of calorimetric and equilibrium valus for the QFM buffer reaction.

EPR Investigations of Er3+, Yb3+, and Gd3+ in Zircon‐Structure Silicates

Structure-Property Correlations for TiB2-Based Ceramics Densified using Active Liquid Metals