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Thermogravimetric Study of Barium Titanate James Nicholas Lingscheit Iowa State University

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Thermogravimetric Study of Barium Titanate James Nicholas Lingscheit Iowa State University Iowa State University Capstones, Theses and Retrospective Theses and Dissertations Dissertations 1973 Thermogravimetric study of barium titanate James Nicholas Lingscheit Iowa State University Follow this and additional works at: https://lib.dr.iastate.edu/rtd Part of the Chemical Engineering Commons Recommended Citation Lingscheit, James Nicholas, "Thermogravimetric study of barium titanate " (1973). Retrospective Theses and Dissertations. 5029. https://lib.dr.iastate.edu/rtd/5029 This Dissertation is brought to you for free and open access by the Iowa State University Capstones, Theses and Dissertations at Iowa State University Digital Repository. It has been accepted for inclusion in Retrospective Theses and Dissertations by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected]. INFORMATION TO USERS This material was produced from a microfilm copy of the original document. While the most advanced technological means to photograph and reproduce this document have been used, the quality is heavily dependent upon the quality of the original submitted. The following explanation of techniques is provided to help you understand markings or patterns which may appear on this reproduction. 1. The sign or "target" for pages apparently lacking from the document photographed is "Missing Page(s)". If it was possible to obtain the missing page(s) or section, they are spliced into the film along with adjacent pages. This may have necessitated cutting thru an image and duplicating adjacent pages to insure you complete continuity. 2. When an image on the film is obliterated with a large round black mark, it is an indication that the photographer suspected that the copy may have moved during exposure and thus cause a blurred image. You will find a good image of the page in the adjacent frame. 3. When a map, drawing or chart, etc., was part of the material being photographed the photographer followed a definite method in "sectioning" the material. It is customary to begin photoing at the upper left hand corner of a large sheet and to continue photoing from left to right in equal sections with a small overlap, if necessa./, sectioning is continued again — beginning below the first row and continuing on until complete. 4. The majority of users indicate that the textual content is of greatest value, however, a somewhat higher quality reproduction could be made from "photographs" if essential to the understanding of the dissertation. Silver prints of "photographs" may be ordered at additional charge by writing the Order Department, giving the catalog number, titie, author and specific pages you wish reproduced. 5. PLEASE NOTE: Some pages may have indistinct print. Filmed as received. Xerox University Microfilms 30Q North Zeeb Road Ann Arbor, Michigan 48106 74-557 LINGSCHEITj James Nicholas, 1941- THERMOGRAVIMETRIC STUDY OF BARIUM TITANATE. Iowa State University, Ph.D., 1973 Engineering, chemical University Microfilms, A XEROX Company, Ann Arbor, Michigan THIS DISSERTATION HAS BEEN MICROFILMED EXACTLY AS RECEIVED. Thermogravimetric study of barium titana te by James Nicholas Lingscheit A Dissertation Submitted to the Graduate Faculty in Partial Fulfillment of The Requirements for the Degree of DOCTOR OF PHILOSOPHY Major: Ceramic Engineering Approved: Signature was redacted for privacy. In Charge of Major Work Signature was redacted for privacy. For the Major Department Signature was redacted for privacy. For the Graduate College Iowa State University Ames, Iowa 1973 ii TABLE OF CONTENTS Page INTRODUCTION 1 REVIEW OF LITERATURE 4 Physical Characteristics of BaTiO^ 4 Crystal Structure of BaTiOg 5 The Ferroelectric Nature of BaTiO^ 11 Applications of Barium Titanate 17 THEORY 26 Pure Stoichiometric Compounds 26 Nonstoichiometrie Compounds 30 Defect Models for Nonstoichiometric Barium Titanate 32 Oxygen deficit 34 Oxygen excess 38 Impurities 39 Ba:Ti ratio not unity 40 Corroborating research 44 Thermogravimetrie Analysis of Defect Structures 45 THE EXPERIMENT 56 Cahn Vacuum Microbalance 56 Temperature Specification 60 Atmosphere Control 63 Specimen Preparation 68 Experimental Procedure 77 Errors 81 RESULTS AND DISCUSSION 85 Preliminary Work 85 iii Page Final Analysis 97 Average behavior of nonstoichiometric BaTiO^_^ 97 Anomalous behavior of nonstoichiometric BaTiO^_^ 109 Reduction behavior of nonstoichiometric BaTiO^_^ 121 Oxidation behavior of nonstoichiometric BaTiO^_^ 127 CONCLUSIONS 133 LITERATURE CITED 138 ACKNOWLEDGMENTS 147 APPENDIX A 149 APPENDIX B 159 1 INTRODUCTION Barium titanate has been an important electronic ceramic material for over thirty years. This highly versa­ tile ceramic compound came into existence as a result of some early (circa 1938-1945) product-oriented research carried out on divalent metal titanates, zirconates, and stannates by the Titanium Alloy Manufacturing Company (1). These experi­ ments produced the first of the high dielectric constant ceramics. As a result of the concentrated search for a World War II replacement for capacitor-grade mica, barium titanate received much experimental attention. This wartime research revealed that barium titanate and the barium-strontium titanate solid solutions were a new class of ferroelectric materials (2). This new material, barium titanate, had the unique property of being a ferroelectric at room temperature with a high relative dielectric constant. As more investigators turned their attention to this coiiçxjund, more new and interesting properties were discovered, until barium titanate and its solid solutions emerged as the pivotal materials in the field of electronic ceramics. Originally, the high relative dielectric constant of barium titanate made it a suitable material with which to fabricate capacitors. As a room temperature ferroelectric, barium titanate may find applications utilizing its remnant 2 polarization in switching circuitry and memory hardware. In a reduced or doped form^ barium titanate is a semiconductor with a large positive teimerature coefficient of resistance over the tenterature range near the Curie point. This effect, coupled with the change in the Curie point accompanying the substitution of various ions for the barium and/or titanium ions, has resulted in thermistor devices. The very high piezoelectric coupling coefficient has prompted the use of barium titanate as a transducer material. Barrier layer capacitors (doped barium titanate which has been reduced and subsequently had its surface layers reoxidized) are of more current interest. Due to its chemical and mechanical stability, its unique electrical properties, and its ease of fabrication as a poly- crystalline product, barium titanate is of interest from the viewpoint of practical applications. In addition, barium titanate is also interesting to basic scientists, as it has by far the most simple and open structure of any known ferro­ electric material. This feature has resulted in barium titanate being the most extensively investigated of the ferro­ electric materials (3). It has, to date, become the focus of both theoretical and experimental studies of ferro- electricity. ^ Much interest has been expressed in the behavior of non- stoichiometrie barium titanate with various investigators employing a variety of research schemes to elucidate the 3 properties of the doped or reduced material. The purpose of this work is to investigate the nons to ichiometr ic behavior of barium titanate as evidenced by the minute changes in weight accompanying the nonstoichiometrie state. Specifi­ cally, the author will investigate a high-purity barium titanate utilizing a vacuum microbalance to discern the changes in stoichiometry resulting from changes in sairç>le temperature and changes in the oxygen partial pressure of the gaseous atmosphere surrounding the specimen. This work will attempt to delineate the temperature and oxygen pressure dependence of the deviation from stoichiometry of pure barium titanate as well as to demonstrate the feasibility of the use, to high precision, of a vacuum microbalance under dynamic gas flow conditions at elevated temperatures. The defect structure of barium titanate will be subject to interpreta­ tion in light of these thermogravimetric data. 4 REVIEW OF LITERATURE Physical Characteristics of BaTiO^ Barium titanate is unique from the isomorphous, earlier- known ferroelectric materials in that it is a ceramic material exhibiting typical ceramic characteristics. Barium titanate has a high melting point of 1618®C (3), is insoluble in water, and has a theoretical density of 6.20 gm/cm^ in the cubic structure and 6.017 gm/cm^ in the tetragonal structure (4). In fired form, barium titanate is brittle, but not so brittle as to preclude forming into widely applicable shapes. Barium titanate has a translucent yellow appearance in pure form with an index of refraction of 2.40 at room temperature which increases with temperature to go through a poorly- defined maximum of 2.46 at the 120°C transition temperature (5). Barium titanate which contains iitç>Tirities e^diibits a phototropic darkening which is reversible upon heating. Since "purity" is always a matter of degree, this effect is present in some degree in all barium titanate powders. Of interest is the return of the normally creamy-white colored powder to a "pure" yellow upon slight heating. The author observed this behavior for his hi^ purity powder in a pressed and fired specimen. Near 200®C the color of this material was "canary" yellow, a color
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