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METAL HYDRIDES EDITORS AND MAJOR CONTRIBUTORS William M. Mueller AMERICAN SOCIETY FOR METALS METALS PARK, OHIO James P. Bfackledge DENVER RESEARCH INSTITUTE UNIVERSITY OF DENVER DENVER, COLORADO George G. Libowitz LEDGEMONT LABORATORY KENNECOTT COPPER CORPORATION LEXINGTON, MASSACHUSETTS Prepared under the direction of the Division of Technical Information, United States Atomic Energy Commission ® ACADEMIC PRESS New York and London 1968 COPYRIGHT © 1968 BY ACADEMIC PRESS, INC. ALL RIGHTS RESERVED COPYRIGHT ASSIGNED TO THE GENERAL MANAGER OF THE UNITED STATES ATOMIC ENERGY COMMISSION. ALL ROYALTIES FROM THE SALE OF THIS BOOK ACCRUE TO THE UNITED STATES GOVERNMENT. NO REPRODUCTION IN ANY FORM (PHOTOSTAT, MICRO- FILM, OR ANY OTHER MEANS) OF THIS BOOK, IN WHOLE OR IN PART (EXCEPT FOR BRIEF QUOTATION IN CRITICAL ARTICLES OR REVIEWS), MAY BE MADE WITHOUT WRITTEN AUTHORIZATION FROM THE PUBLISHERS. ACADEMIC PRESS, INC. Ill Fifth Avenue, New York, New York 10003 United Kingdom Edition published by ACADEMIC PRESS, INC. (LONDON) LTD. Berkeley Square House, London W.l LIBRARY OF CONGRESS CATALOG CARD NUMBER: 68-26631 PRINTED IN THE UNITED STATES OF AMERICA List of Contributors Numbers in parentheses indicate the pages on which the authors' contributions begin. Richard L. Beck, Collins Radio Co., Newport Beach, California (241) James P. Blackledge, Denver Research Institute, University of Denver, Denver, Colorado (1, 119, 441) Coy L. Huffine, Systems Development Division, IBM Corporation, Rochester, Minnesota (675) George G. Libowitz, Ledgemont Laboratory, Kennecott Copper Corporation, Lexington, Massachusetts (490, 545) Charles B. Magee, Denver Research Institute, University of Denver, Denver, Colorado (165) William M. Mueller, American Society for Metals, Metals Park, Ohio (21, 241, 336, 384) Bernard Siegel, Aerospace Corporation, El Segundo, California (545) Rudolph Spieser, The Ohio State University, Columbus, Ohio (51, 90) Refer requests to: U.S. Dept. of Energy Office of Scientific and Technical Information P.O. Box 62 Oak Ridge, TN 37831 Preface Combinations of hydrogen with metals provide a widely divergent group of materials that run the gamut from extremely unstable compounds to quite stable alloy-like materials. In this book considerable emphasis is placed on the theories of hydride formation as well as on experimental procedures involved in the formation of hydrides, the reactions that occur between hydrides and other media, and the physical and mechanical properties of the several classes of hydrides. The use of metal hydrides in the control of neutron energies is discussed, as are many other immediate or potential uses, e.g., in the production of high-purity hydrogen and in powder metallurgy. It is hoped that this book will serve as a valuable reference to students, research professors, and industrial researchers in metal hydrides and in allied fields. Selected chapters may serve specialists in other fields as an introduction to metal hydrides. We have attempted to cover completely the field of metal hydrides. D. T. Hurd, in An Introduction to the Chemistry of Hydrides, John Wiley & Sons, Inc., New York, 1952, and D. P. Smith, in Hydrogen in Metals, The University of Chicago Press, Chicago, 1948, did this adequately many years ago, but these two books are now outdated. Recent books by G. G. Libowitz (Solid State Chemistry of Binary Metal Hydrides, W. A. Benjamin, Inc., New York, 1965) and K. M. Mackay (Hydrogen Compounds of the Metallic Elements, Barnes & Noble, Inc., New York, 1966) introduce the field of metal hydrides to graduate students and nonexperts but make no attempt to be comprehensive. In addition to the published literature, we have reviewed all appropriate unclassified information from classified documents. Many people were involved, either directly or indirectly, in the completion of this book, and it is impossible to recognize each of them individually. We want to take this opportunity to thank our friends and colleagues for their understanding, their suggestions, and their criti- vii Vlll PREFACE cism, which have done much to sustain us during completion of the manuscript. This book was sponsored jointly by the Division of Technical In- formation, U. S. Atomic Energy Commission, and the University of Denver. We are grateful to James D. Cape, U. S. Atomic Energy Com- mission, for his encouragement and guidance and to Dr. Shirley A. Johnson, Jr., Director, Denver Research Institute, University of Denver, for his patience and understanding of the monumental effort involved. We are most appreciative of the efforts of Marie Fay, Mildred Cain, Linda Hahn, and Frances Haverley, who contributed much to the editing and reproduction of the manuscript. We also express our gratitude to Dr. Charles Magee, Richard L. Beck, Dr. Rudolph Speiser, Dr. Bernard Siegel, and Dr. Coy Hufflne, who contributed significant chapters or sections thereof. Finally, we must acknowledge and thank our families who also contributed, by their understanding, to the completion of this book. It is our hope that the information contained herein will be of lasting and practical value to the metallurgist, inorganic chemist, solid-state physicist, nuclear engineer, and others working with chemical or physical processes involving metal-hydrogen systems. WILLIAM M. MUELLER JAMES P. BLACKLEDGE GEORGE G. LIBOWITZ September, 1968 CHAPTER 1 An Introduction to the Nature and Technology of Hydrides JAMES P. BLACKLEDGE* Recent interest in hydrides stems from their potential application as moderator, reflector, or shield components for high-temperature mobile nuclear reactors. Certain of the hydrided metals and alloys have useful elevated temperature strength and stability and good hydrogen retention and can be fabricated with relative ease. Thus, in part, the limitations of most organic and inorganic hydrogenous materials have been overcome. These important potential applica- tions have tended to overshadow other equally significant uses for metal hydrides, e.g., metathetical reactions, preparation of pure- metal powders, surface coatings, and metal-ceramic seals. The term"hydride" is used in this volume in the generic sense to describe the binary combination of hydrogen and a metal or metalloid. Gibb1 has defined a hydride as a compound in which there is a metal- to-hydrogen bond. The importance of this definition lies in the nature of the bond; some limitation is imposed by use of the term "compound." Often, particularly for metallic hydrides, the reaction between hydrogen and a metal results in a homogeneous phase of somewhat variable composition and with properties (e.g., structure) that are different from those of the metal. Although there has been some con- troversy as to whether such phases are compounds or solid solutions, the definition will be broadly interpreted to include both definite compounds and definable phases of variable composition where an exact stoichiometric ratio does not exist. * Denver Research Institute, University of Denver. 1 2 1 THE NATURE AND TECHNOLOGY OF HYDRIDES There may be several reasons for nonstoichiometry in metal hydrides. Apparent deficiencies in hydrogen may be due to impurities in the metal, to trace impurities in the hydrogen which inhibit the uptake of hydrogen by the metal, or to incomplete hydriding during formation of the hydride. The presence of dislocations also may have an effect on stoichiometry. Variability in composition can be ex- plained2 on the basis of such lattice imperfections as hydrogen vacan- cies or interstitials. The electronic structure of the metal and the electron-to-atom ratio also are major factors in many cases. 3 1-1 CLASSIFICATION OF THE HYDRIDES Generally, hydrides are classified by the nature of the hydrogen bond into three principal categories, covalent or volatile, saline or ionic, and metallic, each of which has distinct properties. Such classification is not always completely descriptive, however. For example, lithium hydride, classified as a saline hydride, actually exhibits some covalent characteristics. The rare-earth hydrides are usually classified with the metallic hydrides; however, they do exhibit some characteristics similar to those of saline hydrides (e.g., high heats of formation and high resistivities at higher hydrogen contents). It is interesting also to observe that europium and ytterbium (as might be expected from comparison of their other properties) form ioniclike dihydrides that are isostructural with the saline-alkaline-earth metal hydrides. The binary hydrides formed by metals (and some of the semi- metals) are shown in Fig. 1.1 in terms of the periodic table. In every instance the theoretical stoichiometric formula is given although in many cases this composition cannot be attained. Many of the transition metals, such as Re, Os, Fe, and Co, form complex hydrides con- taining carbonyl and chelating groups. These are discussed in Chap. 12. 1-1.1 COVALENT HYDRIDES Covalent hydrides may be solid (usually polymeric), liquid, or gaseous. They exhibit considerable similarity in their properties. The bond between hydrogen and the element is of the nonpolar electron- sharing type where valence electrons are shared on a fairly equal basis between the elements held by the bond. Large differences in electric charge do not exist. In general, molecules of covalent hydrides 1-1 CLASSIFICATION OF THE HYDRIDES 3 Saline— —Covalent — 1 IA II A IIIB IV B | Series of Boron Li H (BeH2)n Hydrides Covalent (See Chap. 12) Complex Covalent Series ΝαΗ MgH2 SCehea p. 12 (AlH3)n oHfy dSrii des (See Chap. 12) III A IV A VA VI A v 11 J\/ V 111 Ά \ 1 ±3 11 r> Series TiH2 of Ge KH CaH2 ScH2 (acnudb ic VVHH 2 CCrrHH 2 Mn Fe Co NiH CuH (ZnH2)n (GaH3)n (HSyeder ides tetrag.) Chap. 12) ZrH2 RbH SrH2 YYHH23 (acnudb ic NNbbHH 2 Mo Tc Ru Rh PdH Ag (CdH2)n (ClnnHH)3n) n SSnnH2H4 6 tetrag.) See HfH2 CsH BaH2 REaarreth (acnudb ic Ta H W Re Os Ir Pt Au (HgH2)n ((TTIIHH)3n) n PbH4 Series tetrag.) LaH2_3 CeH2_3 PrH2_3 NdH2_3 P?m SSmmHH32 EuH2 GGddHH32 TTbbHH32 DDyyHH32 HHooHH32 EErrHH32 TTmmHH32 YYbbHH32 (?) LLuuHH32 ThH2 NpH2 PuH2 AmH2 AcH2 Th4H15 PaH3 UH3 NpH3 PuH3 A1T1H3 (?) FiG. 1.1 Binary hydrides formed by metals in the periodic table. are not strongly attracted to each other, and this absence of strong intermolecular forces results in the high degree of volatility and low melting points of the covalent hydrides. The covalent hydrides are generally thermally unstable, such instability increasing with increasing atomic weight of the parent element. These hydrides for the most part are extremely toxic and burn readily in the air or in oxygen with liberation of considerable quantities of heat. Typical covalent hydrides are aluminum hydride, tin hydride, the boron hydrides, and the germanium hydrides. The covalent hydrides are discussed in detail in Chap. 12. 4 1 THE NATURE AND TECHNOLOGY OF HYDRIDES 1-1.2 SALINE HYDRIDES Saline hydrides are formed by the reaction of the strongly electro- positive alkali metals and alkaline-earth metals with hydrogen, which, because of electron transfer, becomes strongly electronegative. In contrast to the covalent hydrides, the bond of the saline hydrides results from the strong electrostatic forces existing between the dis- similar electric charges of the two ions. Therefore, saline hydrides are highly polar. These saltlike hydrides are crystalline, exhibit high heats of formation and high melting points, and are electrical conductors in the molten state. The saline hydrides are more dense than the alkali metals (45 to 75%) and the alkaline-earth metals (20 to 25%). This is due to the strong attraction between the metal and the hydride ions and, for the alkali-metal hydrides, to the more efficient packing of atoms in going from the body-centered cubic structure of the metal to the face-centered cubic structure of the hydride. Hydrides of the alkali metals are isostructural with sodium chloride; hydrides of the alkaline-earth metals have an orthorhombic structure that bears some resemblance to the barium halides. The saline hydrides are discussed in detail in Chap. 6. 1-1.3 METALLIC HYDRIDES Figure 1.1 shows that metallic hydrides are formed by the transition metals. They generally exhibit metallic properties in the accepted sense; these include high thermal conductivity and electrical resis- tivity, hardness, luster, and in some instances useful mechanical properties. Unlike metals, however, they are usually quite brittle. For example, when massive uranium is hydrided, the reaction pro- ceeds rapidly and efficiently at reasonably low temperatures to the formation of finely powdered UH . Yttrium, on the other hand, may 3 be hydrided to form the massive dihydride YH , which possesses 2 useful mechanical properties, or it may be hydrided completely to the trihydride YH , which forms as a finely divided friable material. 3 Because of the wide homogeneity ranges of most of the metallic hydrides, they have sometimes been considered as solid solutions of hydrogen in the metal. That this is not so can be seen in Table 1.1, which shows that the structures of the hydrides are distinctly different from those of the parent metal. Cerium, nickel, actinium, and palla- dium do not change structure on hydriding, but they do undergo dis- continuous increases in lattice parameter. Compound formation in TABLE 1.1 STRUCTURES OF METALS AND CORRESPONDING HYDRIDES 0 Metal Metal sublattice Metal structure' Hydride in hydride0 Ti, Zr, Hf H.c.p. TiH2, ZrH2, HfH2 F.c.c, f.c.t. V B.c.c. VH B.c.t. VH2 F.c.c Nb B.c.c. NbH B.c.c, b.c. orthorhombic NbH2 F.c.c. Ta B.c.c. TaH B.c.t., b.c. orthorhombic Cr B.c.c. CrH Hexagonal CrH2 F.c.c Ni F.c.c. NiH F.c.c (flo = 3.52 Â) («o = 3.72 A) Pd F.c.c. PdH F.c.c (tfo = 3.89 Â) (a0 = 4.03Â) La, Pr, Nd, Sc H.c.p. LaH2, PrH2, NdH2, F.c.c ScH 2 Ce F.c.c. CeH2 F.c.c. (cio = 3.52 Â) (a0 = 5.58A) Sm Rhombohedral SmH2 F.c.c SmH3 Hexagonal Gd through Tm, H.c.p. Dihydrides F.c.c. Lu, T Y Trihydrides Hexagonal Eu B.c.c. EuH2 Orthorhombic Yb F.c.c. YbH2 Orthorhombic («0 = 5.49 Â) YbH2+8 F.c.c (a = 5.19Â) 0 Ac F.c.c. AcH2 F.c.c. («0 = 5.31 Â) (fl0 = 5.67Â) Th F.c.c. ThH2 F.c.t. Th H B.c.c. (complex) 4 15 Pa B.c.t. PaH ß-W (cubic) 3 U Orthorhombic jS-UH j8-W (cubic) 3 a-UH B.c.c. 3 Pu Monoc linic PuH3 F.c.c. PuH Hexagonal 3 Li, Na , K, B.c.c. LiH, NaH, KH, F.c.c Rb, «C s RbH, CsH Ca, Sr F.c.c. CaH2, SrH2 Orthorhombic Ba B.c.c. BaH2 Orthorhombic a From G. G. Libowitz, The Solid State Chemistry of Binary Metal Hydrides, p. 45, W. A. Benjamin, Inc., New York, 1965. b H.c.p., hexagonal close-packed; b.c.c, body-centered cubic; f.c.c, face-centered cubic; f.c.t., face-centered tetragonal; j8-W, ß-tungsten structure; b.c.t., body-centered tetragonal. 5