The Preparation and Characterization of Materials
Aaron Wold Brown University Providence, RI 02912
Several examples which illustrate different aspects of materials problems encountered hy this writer during the past twenty years will be presented in this paper. These examples will be used to demonstrate a number of different svnthetic approaches as well as to relate the "quality" of the ~'oduct to the type of measurement being studied. Unfortunately, much of what will he presented does not appear in inorganic chemistrv textbooks. but the information is commonlv accepted among solid st& chemists. At an academic institution, such as Brown University, one person or a small group who synthesizes a new material also carries out the entire sequence of events from X-ray analysis and chemical analysis to the measurement of the specific properties desired. In addition, they may also study the relationships between material composition, crystal structure, and the properties determined. This work can be illustrated in Figure 1.
value because it is carried out without a clear understanding of the need to distinguish between property studies on illdefined or well-defined materials. Some Problems Associated with Solld-Solid Reactions T h e preparation of polycrystalline solids usually involves chemical reactions between two solids where a melt is not formed.The process is usually a complexone which depends upon the surface area as well a3 the deiert structure. Cnfort;nately, the formation of a product tends to reduce the area of contact between the reactants and also to reduce the rate of the reaction. The extent of product formation is influenced by the area of interfacial contact and the ease of diffusion throueh a nroduct laver. A much smaller amount of ~ r o d u c t ~~~is obtained when particles of small surface area are in contact than when a "comnact" of a fine powder. hiah surface area reaction mixture is reacted. The difrusion i f r&ctants through a product layer depends on temperature, defect structure of prodnct layers, grain boundary contacts, presence of imnurities and effectiveness of nhase boundam contacts. Usuallv continued grinding between heat cycles facilitates chemical reactivitv between solid ~ h a s e sNot . onlv is the surface area of the reaction mixture maintained but also fresh surfaces of the reactants are hrouaht into contact. form as a result of the reaction When only solid of two solids, studies of the reaction become experimentally very difficult to follow. Semi-qnantitative measurement of the product with X-ray diffraction is probably the most widely used method for the determination of product formation. However, this method gives low accuracy for poorly crystallized substances or those which are formed with hiehlv - .defective structures. For many single-phase products one can nevertheless follow the cell dimensions as a function of composition. It is possible also to compare the observed changes of densitv with those calculated from X-rav data. ~
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Figure 1. Preparation, structure, and properties of a new phase
In an industrial laboratorv. .. there are usually many different specialists who carry out these steps in separate locations and erouns. - . One obvious conseauence of this s~ecializationis that the "research team" must he well coordinated and directed in order to expedite communication among.the -Preparers, analyzers, measurers, interpreters, appliers, fabricators, and eventual users of the prodnct. The hope of many chemists is that someone else will discover the merit of their new compound. The property investigators make measurements.and attemnt to internret their results wishine that better or other materials were available. Last, hut notleast, the users are frustrated because nractical Droeress . .. is so unpredictable and sometimes unrepeatable. What therefore seems to be most urgent is tu train students who ran help t'orge a chain of quantitative understanding that links preparatiw ingredients and methodsw~thknowledge of thesignificant featuresof the resultant material. The descriution of the procedure rhosen for the preparation of a materiaishould be given far more attentionthan it has received in the past. The quality of the starting materials as well as the subtleties of the technique(s) used will determine in large measure the quality of the final product. The complete control of preparative conditions is absolutely essential for an understanding of properties and their ability to be varied. In addition. the more that measurements are used to analvze for specific features of composition and structure, the more correlation there will he between com~osition,structure, and properties. Much solid state research, today, is concerned with the effort to understand properties in terms of a particular composition and structure, but much of this effort is of little
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'Intering and Cry4a1 Orowth When a solid is heated, it may he observed to undergo physical and/or chemical changes. These include sintering, melting, and thermal decomposition. The process of sintering ~ r o h a b l yresults from the crystal growth a t the contact area between crystals which are touching. This process proceeds at the expense of one ofthem. As a result thr crystallitcs hecome bonded together and the average size 1s observed to i n crease. At ele\,ated temperatures, the ions hecome mobile and melting mcurs. Thi, ordered lattice array is replaced hy rhe long-range order oithe liquid state. Cr)stalli?ation may proceed by several different paths:
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1. Vapor solid, condensation 2. Solution solid, precipitation 3. Melt solid, freezing 4.
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solid B, transformation.
All of these processes have in common the condition that Presented at the ACS National Meeting, March 25, 1980,as part of the State-of-the-ArtSymposium on Solid State Chemistry in the Undergraduate Curriculum sponsored by the Division of Chemical Education. Volume 57, Number 8, August 1980 / 531
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for crystallization to proceed, the final crystals must have lower free energy than the initial state of the system. The nrocess of crvstal formation involves two steps: First, the Formation o f a new nucleus and second, the growth of this nucleus to form a particle of appreciahle size. Crystals may contain many imperfections or "defects" other than impurity atoms and these defects are most important in determining the properties of the crystal. In addition to defects, distortion of the lattice through departures from perfect alignment of repetitive units may exist. This type of imperfection is called a "dislocation." Hence, dislocations are regions within the solid a t which the regularly repeating lattice array shows a discontinuity or a distortion from the ideal alignment of units within the crystal. Many properties of crystals are dependent on the number and type of dislocations present; for example, a dislocation represents a region of weakness. References related to the problems associated with defects and dislocations will he found in the bibliography at the end of this paper. In order to illustrate many of the problems associated with the preparation of suitable materials for further study either by solid state physicists or engineers, the following format has been chosen. The next section will he devoted to some aspects dealing with the characterization of materials and will focus upon a number of important problems which have to he solved for all materials. This section will he followed by a comparison of the preparation of polycrystalline transition metal oxides crystallizing with the spinel structure. By then examining the Cu-S system we should he able to appreciate many of the difficulties associated with multiphase formation and variable composition. Finally, the preparation of pure magnetite single crvstals will he chosen to illustrate the difficulties associated with the selection of an appropriate method for growing crystals where purity plays an enormous role in the understanding of the physical properties. Characteriratlon of Materials' Characterization of the Major Phase
For elements and for manv. simple . com~oundsthe matter of identification is trivial. However, in the case of complex compounds, phase identification is prerequisite to detailed characterization. Indeed, problems frequently arise through imnroner . . identification of phases present and through the failure to investigate homogeneity. The methods used for phase identification are principally X-ray diffraction and microscopic examination. X-ray identification using most powder techniques is capable of detecting approximately 2-5s of a foreign phase under favorable circumstances. X-ray diffraction and observation of optical characteristics by standard petrographic techniques are used to identify the phase present, while petrographic techniques alone are often capable of greater subtlety in detecting traces of foreign phases. However, ordinary microscopic observations including examination between crossed polarizers are often neglected. Such examination combined with etching and similar metallographic techniques is often capahle of shedding light on phase homogeneity in both metallic and nonmetallic crystalline solids. In addition to determining the presence of impurities, one of the major areas of difficulty is the study of the distribution of such impurities. The importance of knowing how the impurities are distributed, whether completely statistically a t the atomic level or in a single "inclusion," is vital to every interpretation of properties measured on the phase. In this regard the introduction of the electron microprohe has brought about a large advance in our ability to he able to determine impurity d&ribution. Unfortunately, its spatial resolution ~
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The format for this section was based on a report prepared by the Committee on Characterization of Materials, Materials Advisory Board, Sciences National Academy of Engineering,Characterization of Materials, MAB-229-M,U.S. Clearinghouse for Federal Scientific and Technical Information, Springfield,Virginia (1967). 532 1 Journal of Chemical Education
is limited to about a micron and its sensitivity and precision (about 0.1%) are poor. However, in some cases, use of the lathodoluminescence in the proheprovides a tool for detecting inhomogeneities at the ppm level with micron resolution. The scanning electron microscope can provide in favorable case some data on impurity distrihution at the 0.1 p level. For single crystal materials, X-ray topography (rocking curve, Lana, anomalous transmission photographs) is a powerful quaEtative mapping method for observing homogeneity. For the determination of concentration and stoichiometry of the maior phase. . the classical techoiaues are still being used. In this connection mainly wet chemical analysis, inciuding gravimetric, volumetric and electrochemical techniques are used. Some instrumental techniques, e.g. X-ray fluorescence, atomic absor~tion,and neutron activation are becoming preferred met'hodsfor certain types of compounds. There is certainly the possibility of wider use of automating wet chemical techniques for analysis of solid materials in the way that routine clinical analysis has been automated. The precision ordinarily expected for stoichiometric determinations is probably in the range f0.01-0.1% a t best in the case of comnounds like KC1 and MnFe7Oa. he exact determination of %ichiometry when precise results are reauired almost always depends upon indirect measurementsand upon a "logic2 chain of reasoning" which is often not tenable. Conductivity and mobility measurements together with assumptions as the origin of donors or acceptors sometimes allow estimates of stoichiometry to be made in semiconductors. Similarly, precise lattice parameter and pycnometric density determinations provide some means of es&ating vacancy concentration and in some cases stoichiometry. Once the stoichiometrv has been determined, it is conventional to assign valencesof multivalent elements in a comnound in order to preserve electrical neutrality. This procebure presents dificulties when more than one mult&alent element is present in the same compound or where several valency assignments are possible for a given element in the compound. If it is possihle to get the compound into solution without altering the valence, the classical wet chemical and electrochemical methods which are quite precise (0.01%), permit the derermination of \.aleme. For inm and some uther elements, \Ihsl,auer spwtrus~opyi i applicahl~with a precision of in the precision of .-.--~- nhout ~.l%.~Imnrovements Mlissbauer measurements are likely. High resolution X-ray spectra also give information on valence which is applicable to most elements. In actual practice the valence is often deduced from the color of the compound. Absorption spectroscopy is useful in particular circumstances, hut it will probably he limited to the determination of valence of trace elements. Similarly, electron spin resonance is applicable to elements present in trace concentrations. The location of the atoms of the principal phase is determined by X-ray and neutron diffraction, but the sensitivity is only of the order of 0.1% at best. Other quantitative techniques for site location have not been developed, although ESR, NMR, Atomic Absorption, and Mossbauer spectroscopy are useful in obtaining qualitative information in some circumstances.
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Characterization of Minor Phases and Impurities
Most inorganic impurities can he detected down to about 10 nglg, though the limit of quantitative determination is generally higher. The sensitivity of quantitative analysis by most instrumental methods is frequently limited by a lack of suitable standards. Emission spectroscopy is the most generally applicable of the survev methods. It is used widelv for characterization of solids and has the capability of detecting up to 70 elements hv d.c. arc excitation. Determination of nonmetallic elements is also possible with emission spectroscopy, hut this requires special techniques which are rarely used.
Spark-source mass spectrometry can detect all elements, with sensitivities often as good as 10 nglg. Residual gases in the vacuum system restrict detection limits for oxygen and nitroeen. X-ray spectroscopy can he employed also for survey analysis of imnurities and has the advantane of heina non-destructive. I.oatr lirnirn uf deterthn are rarcl) herter than 10 to 100 pprn, and the element5 of the first ueriod cannot he detected st lwv concentrations. Electrical measurements are useful for determining the total content of electrically active impurities in conductors and semiconductors. A number of techniques are available for the determination of vacancy and interstitial concentrations. A comparison of the microscopic density of a crystal with the density calculated by the X-ray determined lattice constant, molecular or atomic weieht. - . and Avoeadro's numher reveals,. in ~rincinle. . . . the presence of lattice vacancies or interstitials. There are numerous uncertainties which limit the technique especially the fact that the vacancy or interstitial concentritioni in crystals near room temperature are frequently of the order of 5 ppm or less and too small to produce an observable density difference ( I ). Ionic conductivitv"of nola . crvstals can he used to determine vacancy and interstitial concentrations ( 2 )and a few examples exist such as the anion vacancv in the alkali halides where the vacancy or interstitial is responsible for an optical absorption hand whose magnitude is a measure of the concentration (3). When deviations from stoichiometric proportions are associated with vacancies or interstitials, chemical analysis suffices to determine the defect concentrations. An example of such a system is titanium carbide where the sodium chloride structure is stable over the carbon range from TiCo.6 to TiC1.o. The lower carhon concentration represents 50%vacancies in the f.c.c. carbon suhlattice (4). In the case of tantalum carbide. vacancies in the carhon suhlattice result in deviations from stoichiometry, and the X-rav lattice constant is sufficientlv sensitive to the vacancv content to be a useful measure of the vacancy concentration (5). Characterization of Surfaces Undoubtedly, among the least understood and most poorly characterized features of a solid is the surface. Defect solidstate theory, useful for interpreting and predicting the electronic behavior of semiconductors and insulators, has been utilized with good results to analyze and interpret chemisomtive processes occurrina on such solids (6). Despife the recent sophi
