Habit, crystal


Sodium Perborate. The common commercial practice for the manufacture of sodium perborate tetrahydrate involves the reaction of sodium metaborate and hydrogen peroxide and subsequent crystallization of sodium perborate tetrahydrate. Borax and sodium hydroxide are added to recycled mother Hquor to make a strong (150—600 g/L) metaborate solution at an elevated (40 to 90°C) temperature. Insoluble impurities ate removed by settling and filtration. Metaborate Hquor cooled to room temperature is mixed with hydrogen peroxide in an agitated crystallizer and the exothermic heat of reaction is removed by evaporation or refrigerative cooling. The sodium perborate tetrahydrate is separated by filtration or centrifugation. The product crystals ate very sensitive to trace impurities such as siHca, carbonate, magnesium, calcium, organic compounds, and transition metals that affect properties such as the crystal habit, crystal strength, caking, and stabiHty. Sodium perborate monohydrate is commercially prepared by thermal dehydration of the tetrahydrate or reaction of a sodium metaborate and hydrogen peroxide solution in a fluidized-bed dryer at 45—85°C (106).  [c.202]

Even if perfect cleavage planes are present, those that actually predominate may be determined by experimental conditions. Selective adsorption of some constituent may change the crystal habit by retarding the growth of certain planes (see Section IX-4) for example, sodium chloride crystallizes from urea solution in the form of octahedra instead of cubes and the shape of silver crystals in air is different from those annealed in nitrogen, due to the influence of adsorbed oxygen [39]. In addition to the well-known effects of oxygen on crystal growth, a recent study by Madey and co-workers shows the influence of thin metallic films on W(lll) and Mo(ll 1) crystals [96, 97]. The adsorption of a monolayer of a metal, such as Pd and Pt, having electronegativities greater than some critical value, causes W(lll) and Mo(lH) to restructure to a highly faceted pyramidal surface with the primary facets in the (211) direction. The dependence on electronegativity indicates the influence of electronic effects on the surface energies.  [c.272]

Referring to Fig. VII-2, assume the surface tension of (10) type planes to be 400 ergs/cm. (a) For what surface tension value of (11) type planes should the stable crystal habit just be that of Fig. Vll-2a and (b) for what surface tension value of (11) type planes should the stable crystal habit be just that of Fig. VII-2i> Explain your work.  [c.285]

An enlarged view of a crystal is shown in Fig. VII-11 assume for simplicity that the crystal is two-dimensional. Assuming equilibrium shape, calculate 711 if 710 is 275 dyn/cm. Crystal habit may be changed by selective adsorption. What percentage of reduction in the value of 710 must be effected (by, say, dye adsorption selective to the face) in order that the equilibrium crystal exhibit only (10) faces Show your calculation.  [c.285]

In Section 1.3 it was noted that the energy of adsorption even for a perfect crystal differs from one face to another. An actual specimen of solid will tend to be microcrystalline, and the proportion of the various faces exposed will depend not only on the lattice itself but also on the crystal habit this may well vary amongst the crystallites, since it is highly sensitive to the conditions prevailing during the preparation of the specimen. Thus the overall behaviour of the solid as an adsorbent will be determined not only by its chemical nature but also by the way in which it was prepared.  [c.18]

The number and kind of defects in a given specimen, as well as the crystal habit and with it the proportion of different crystal faces exposed, will in general depend in considerable degree on the details of preparation. The production of a standard sample of a given chemical substance, having reproducible adsorptive behaviour, remains therefore as much an art as a science.  [c.20]

In dealing with experimental thermodynamics, one of the criteria that a true equilibrium has been established is to approach the state of interest from opposite directions. Accordingly, we are in the habit of thinking of the equilibrium melting point of a crystal or the equilibrium freezing point of the corresponding liquid as occurring at the same temperature. In dealing with polymer crystals, unfortunately, we are not so lucky as to observe this simple behavior. The transition liquid crystal is so overshadowed by kinetic factors that some workers even question the value of any thermodynamic discussion of the transition in this direction. Furthermore, because of the kinetic complications occurring during th,e formation of the crystal, the transition crystal liquid also becomes more involved.  [c.204]

Crystallization conditions such as temperature, solvent, and concentration can influence crystal form. One such modification is the truncation of the points at either end of the long diagonal of the diamond-shaped crystals seen in Fig. 4.11b. Twinning and dendritic growth are other examples of such changes of habit.  [c.240]

The study of the chemical behavior of concentrated preparations of short-Hved isotopes is compHcated by the rapid production of hydrogen peroxide ia aqueous solutions and the destmction of crystal lattices ia soHd compounds. These effects are brought about by heavy recoils of high energy alpha particles released ia the decay process.  [c.216]

Monohydrate Process. In this process (21), trona is ground and calcined (150—300°C) to a cmde soda ash using rotary gas-fired and coal grate-fired calciners. The calciner product is then leached with hot water and the clear, hot Hquor sent to evaporative recirculating crystallizers (40—100°C) where sodium carbonate monohydrate is produced. Both multiple-effect and mechanical vapor recompression (MVR) crystallizers are used. Some producers send the Hquor through activated carbon beds before crystallization to remove trace organics from the solution. These organics, solubilized from the oil shale, can affect crystallizer performance by foaming and changing crystal growth rates and habit. Other trace contaminants are removed from the system by purging small amounts of mother Hquor. Weak Hquors and insolubles are removed by clarifiers and subsequent filtration. The insolubles are washed again to recover any additional alkaH.  [c.525]

Pressure precipitation involves the separation of metals, metal oxides, or other inorganic materials by taking advantage of the retrograde solubiUty of a number of common compounds. Products are often of a weU-defined form, eg, particle size, crystal stmcture, habit, and of relatively high purity, despite origination from a relatively low grade source. A combination of pressure leaching and precipitation cycles can result in the conversion of a low grade ore or inorganic waste material to a commercially valuable product.  [c.497]

Many biological systems exhibit the properties of Hquid crystals. Considerable concentrations of Hquid crystalline compounds have been found in many parts of the body, often as sterol or Hpid derivatives. A Hquid crystal phase has been impHcated in at least two degenerative diseases, atherosclerosis and sickle ceU anemia. Living tissue, such as muscle, tendon, ovary, adrenal cortex, and nerve, show the optical birefringence properties that are characteristic of Hquid crystals. The Hquid crystal state has been identified in many pathological tissues, particularly in areas of large Hpid deposits. Massive deposits of Hquid crystalline cholesterol derivatives have been found in the kidneys, Hver, brain, spleen, marrow, and aorta walls. Certain living sperms possess a Hquid crystalline state. Solutions of tobacco mosaic vims (TMV), coUagen, hemoglobin of sickle ceU anemia, native protein, nucleic acid genetic material, and fibrinogen also show resemblances to the Hquid crystal state.  [c.202]

Chemically, inorganic pigments ate quite simple materials and include elements, their oxides, mixed oxides, sulfides, chromates, siUcates, phosphates, and carbonates. Laboratory preparation of these materials can be quite simple, but their large-scale production is complex and demands attention to every detail of the manufacturing process. This is because the appHcation usefulness of inorganic pigments is determined by physical as well as chemical properties. Particle size, shape, and surface properties ate as important in the pigment performance as chemical composition. For inorganic pigments that can exist in several crystal stmctures, controls must be exercised to make sure that the proper crystal habit having the optimum coloristic properties is produced.  [c.3]

Ammonium sulfate [7783-20-2], (NH 2 U4, is a white, soluble, crystalline salt having a formula wt of 132.14. The crystals have a rhombic stmcture d is 1.769. An important factor in the crystallization of ammonium sulfate is the sensitivity of its crystal habit and size to the presence of other components in the crystallizing solution. If heated in a closed system ammonium sulfate melts at 513 2° C (14) if heated in an open system, the salt begins to decompose at 100°C, giving ammonia and ammonium bisulfate [7803-63-6], NH HSO, which melts at 146.9°C. Above 300°C, decomposition becomes more extensive giving sulfur dioxide, sulfur trioxide, water, and nitrogen, in addition to ammonia.  [c.367]

In the case of Cu—Zn and other systems that have relatively complex ordered martensite stmctures (9R or 18R), the reverse transformation is crystaHographicaHy restricted. This means that although there are many variants which can form on transformation from parent to martensite, only a single parent orientation is possible in the reverse, or shape-recovery, transformation. This process is illustrated in Figure 2a the shape recovery effect is shown in Figure 2b. A detailed discussion of the crystallography of martensitic transformations and lattice-invariant shear is available (3,4). When a martensite group is deformed to coalesce into a single orientation, the dominant mechanism is twinning, where each twin is actually an alternate variant of the martensite crystal. Thus, for the four variants that cluster about the (110) habit plane, each orientation is a twin of another, and by the degenerate variant-twin relationship, a group of martensite plates from a single parent crystal can, on deformation, coalesce to a single crystal (single variant) of martensite. The parent phase, usually ordered B2 or 1 0, transforms to one of the martensite crystal forms that exhibit shape memory. Examples are 2H, Cu—Al—Ni,  [c.463]

Compared to calcium chloride [10043-32-4], which is commonly used as stiffening promoter, these sulfamates do not contain chloride and are not alkaline. Therefore highly durable concrete is made using these sulfamates (60,61). These sulfamates and sulfamic acid are also used with the ground injection material the main component of which is water glass, to help easy adjustment of gelation time and increasing firmness of ground (62—64). Crystallisation of ammonium sulfate [7783-20-2] from its mother Hquor with addition of guanidine sulfamate [51528-20-2] as crystal habit modifier produces bigger and spherically shaped granular crystals (65,66). Sulfamates of alkaline metals are used as additives in chromium tanning of hides (67).  [c.65]

Over the years emphasis has been placed on obtaining greater uniformity in silver haUde crystal size and habit in the grain population, in the behef that the chemical sensitization process can then yield a higher average imaging efficiency. One way of doing this is to adjust the nucleation conditions so  [c.468]

Along with operating variables of the crystallizer, nucleation and growth determine such crystal characteristics as size distribution, purity, and shape or habit.  [c.342]

Both supersaturation and temperature can have different effects on the growth rates of different faces of the same crystal. Such occurrences have implications with respect to crystal habit, and these are dealt with in a later section.  [c.345]

The morphology (including crystal shape or habit), size distribution, and purity of crystalline materials can determine the success in fulfilling the function of a crystallization operation.  [c.345]

The general shape of a crystal is referred to as its habit. The appearance of the crystalline product and its processing characteristics (such as washing and filtration) are affected by crystal habit. Relative growth rates of the faces of a crystal determine its shape faster-growing faces become smaller than slower-growing faces and, in the extreme, may disappear from the crystal altogether. Growth rates depend on the presence of impurities, rates of cooling, temperature, solvent, mixing, and supersaturation. Furthermore, the importance of each of these factors may vary from one crystal face to another. For example, consider Figure 9 which shows that the (111) face grows between 1.6 and 2.2 times as fast as the (110) face at the conditions examined. These results account for the elongated crystal shape exhibited by magnesium sulfate heptahydrate crystals. In addition, the effects of supersaturation and temperature are different on the growth rates of the two faces studied. Such behavior leads to changes in habit as the temperature and/or supersaturation are changed in a crystallizer.  [c.346]

A number of studies have shown that various additives can be included in a process stream to alter crystal habit (5). Prediction of such behavior is difficult and extensive laboratory or bench-scale experiments may be required to evaluate the effectiveness of habit modifiers. More recently, some measure of success has been achieved with altering the habit of organic crystals based on the molecular stmcture and characteristics of the crystallizing species. One category of additives affecting crystal habit is surfactants (43). Should an additive enhance the properties of a crystalline material, for example, by making it easier to filter, the expense associated with its use may be warranted. Significant efforts toward tailoring additives so that they have specific effects on crystal habit have been made by a number of research groups (44,45).  [c.346]

Purity. Although crystallization has been employed extensively as a separation process, purification techniques using crystallization have become increasing important. Mechanisms by which impurities can be incorporated into crystalline products include adsorption of impurities on crystal surfaces (50), solvent entrapment in cracks, crevices and agglomerates, and inclusion of pockets of Hquid (51). An impurity having a stmcture sufficiendy similar to the material being crystallized can also be incorporated into the crystal lattice by substitution or entrapment (52,53). Among these mechanisms, inclusion formation has been extensively studied (54—58). It has also been suggested that the purity may be direcdy linked to size and habit of product crystals, but the interaction does not appear to be simple. It has been noted (59) that the key to producing high purity crystals was to maintain the supersaturation at a low level so that large crystals were obtained. Others have found that reducing the size of ammonium perchlorate crystals resulted in a substantial decrease in moisture due to inclusion (58).  [c.347]

Internal structure can be different in crystals that are chemically identical, even though they may be formed at different temperatures and have a different appearance. This is called polymorphism and can be determined only by X-ray diffraction. For the same internal structure, very small amounts of foreign substances will often completely change the crystal habit. The selective adsorption of dyes by different faces of a crystal or the change from an alkaline to an acidic environment will often produce pronounced changes in the crystal habit. The presence of other soluble anions and cations often has a similar influence. In the crystallization of ammonium sulfate, the reduction in soluble iron to below 50 ppm of ferric ion is sufficient to cause significant change in the habit of an ammonium sulfate crystal from a long, narrow form to a relatively chunky and compact form. Additional information is available in the patent hterature and Table 18-4 lists some of the better-known additives and their influences.  [c.1656]

In Section 1.3 it was noted that the energy of adsorption even for a perfect crystal differs from one face to another. An actual specimen of solid will tend to be microcrystalline, and the proportion of the various faces exposed will depend not only on the lattice itself but also on the crystal habit this may well vary amongst the crystallites, since it is highly sensitive to the conditions prevailing during the preparation of the specimen. Thus the overall behaviour of the solid as an adsorbent will be determined not only by its chemical nature but also by the way in which it was prepared.  [c.18]

The number and kind of defects in a given specimen, as well as the crystal habit and with it the proportion of different crystal faces exposed, will in general depend in considerable degree on the details of preparation. The production of a standard sample of a given chemical substance, having reproducible adsorptive behaviour, remains therefore as much an art as a science.  [c.20]

Since the crystal shape, or habit, can be determined by kinetic and other nonequilibrium effects, an actud crystal may have faces that differ from those of the Wulff construction. For example, if a (100) plane is a stable or singular plane but by processing one produces a plane at a small angle to this, describable as an (xOO) plane, where x is a large number, the surface may decompose into a set of (100) steps and (010) risers [39].  [c.261]

Since the polymer crystal habit is characterized by plates whose thickness is small, surface phenomena are important. During the early development of the crystal, the lateral dimensions are also small and the effect is even more pronounced. The key to understanding this fact lies in the realization that all phase boundaries possess surface tension and that this surface tension measures the Gibbs free energy stored per unit area of the phase boundary. To get a preliminary feel for the importance of this, suppose we consider a spherical phase of radius r, density p, and surface tension 7. The total surface free energy associated with a particle such as this is given by the product of 7 and the area of the sphere, or y(4-nr ). The total mass of material in the sphere is given by the product of the density and the volume of the sphere, or p(4m 13). The ratio of the former to the latter gives the Gibbs free energy arising from surface considerations, expressed per unit mass that is, the surface Gibbs free energy per unit mass is Sy/pr. Since 7 is small compared to most other chemical and physical contributions to free energy, surface effects are not generally considered when, say, the AG° of formation is quoted for a substance. The above argument shows that this becomes progressively harder to justify as the particle size of the material decreases. The emergence of a new phase implies starting from an r value of zero in the argument above, and the surface contribution to the energy becomes important indeed. Since two phases with their separating surface must already exist for 7 to have any meaning, we are spared the embarrassment of the surface free energy becoming infinite at r = 0. Nevertheless, it is apparent from the foregoing that the effect of the surface free energy contribution is to increase G. Inspection of Fig. 4.3a shows that an increase in the G value for the crystalline phase arising from its small particle size has the effect of shifting Tj to lower temperatures. The smaller the particle size, the bigger the effect. This is the origin of all superheating, supercoohng, and supersaturation phenomena An equilibrium transition is sometimes overshot because of the difficulty associated with the initiation of a new phase. Likewise, all nucleation practices-cloud seeding, bubble chambers, and the use of boiling chips-are based on providing a site on which the emerging phase can grow.  [c.212]

The properties of fillers which induence a given end use are many. The overall value of a filler is a complex function of intrinsic material characteristics, eg, tme density, melting point, crystal habit, and chemical composition and of process-dependent factors, eg, particle-si2e distribution, surface chemistry, purity, and bulk density. Fillers impart performance or economic value to the compositions of which they are part. These values, often called functional properties, vary according to the nature of the appHcation. A quantification of the functional properties per unit cost in many cases provides a vaUd criterion for filler comparison and selection. The following are summaries of key filler properties and values.  [c.366]

The crystal stmcture of fluorite gives its name to the fluorite crystal type. The lattice is face-centered cubic (fee), where each calcium ion is surrounded by eight fluoride ions situated at the comets of a cube, and each fluoride ion lies within a tetrahedron defined by four calcium ions (3). The bonding is ionic. The unit cell (space group can be pictured as made up of eight small cubes, each containing a fluoride ion, and the eight forming a cube with a calcium ion on each comer and one in the center of each face (Fig. 1). The lattice constant is 0.54626 nm at 25°C (4). The habit is usually cubic, less frequendy octahedral, rarely dodecahedral. Cleavage on the [111] planes is perfect. The crystals are britde with dat-conchoidal or splintery fracture. Luster is vitreous, becoming duU in massive varieties.  [c.172]

Liquid crystal accumulations have been noted ia pathological Hpid and cholesterol deposits ia some rare metaboHc diseases, eg, cholesterol ester storage disease, Tangiers, Farbers, Neimaim-Pick, Gauchers, Krabbes, Fabrys, and Tay Sachs diseases, and ia gallstone formation (42).  [c.203]

Various organic compounds can be added to the reaction vessel during crystal growth to modify or control the resulting crystalline morphology. For example, silver chloride, which under most conditions grows with cubic stmcture (45), can be prepared with octahedral and dodecahedral morphologies if habit modifiers are used (46). Figure 5 shows electron micrographs of cubic, octahedral, and tabular silver bromide grains. Figure 6 shows the atomic arrangement within a silver haUde unit cell and orientation of three important crystallographic surfaces silver haUde cubes are bounded by < 100 > surfaces, dodecahedra are bounded by < 110 > surfaces, and octahedra are bounded by < 111 > surfaces. The arrangement of lattice ions and the associated intedattice site distances (47—49) on perfect surfaces are exhibited in Figure 7. Newly developed analytical techniques, such as surface extended x-ray absorption fine stmcture (sexafs) and atomic force microscopy (afm), suggest that the stmcture and interionic separations on the surface of silver hahde microcrystals are the same as in the bulk for < 100 > AgBr surfaces, but that some reconstmction may occur on < 111 > surfaces because of electrostatic effects (50,51). In spite of the presence of gelatin during the growth of silver haUde microcrystals, and coulombic forces driving reconstmction, the surfaces can be remarkably smooth. Atomically smooth < 100 > AgBr surfaces for thin-film epitaxial vapor deposition on cleaved NaCl substrates have been observed using afm (52) (see Trim films).  [c.443]

Gibbsite (oc-Aluminum Trihydroxide, Hydrargillite). Gibbsite, commonly associated with bauxite [1318-16-7] deposits of tropical regions, is the most important aluminum compound. Tire gibbsite lattice consists of double layers of hydroxide ions, and aluminum occupies two-tliirds of the interstices within the layers. Tire hydroxyls of adjacent layers are situated directly opposite each other. Tire layers are somewhat displaced in the direction of the a-axis and the hexagonal symmetry (iDnicite t yDe) is lowered to monoclinic. Tire particle size of gibbsite varies from 0.5 to nearly 200 llm depending on the method of preparation. Tire smaller crystals are composed of plates and prisms whereas the larger particles appear as agglomerates of tabular and prismatic crystals. Tire basic crystal habit is pseudo-hexagonal tabular.  [c.168]

Calcium carbonate occurs naturally in three crystal stmctures calcite [13397-26-7] aragonite [14791-73-2] and, although rarely, vaterite. Calcite is thermodynamically stable, aragonite is metastable and irreversibly changes to calcite when heated in dry air to about 400°C. Vaterite is metastable to calcite and aragonite under geological conditions but is found during the high temperature precipitation of calcium carbonate (1). The crystal forms of calcite are in the hexagonal system with 32/m symmetry the crystals are varied in habit and over 300 different forms have been described. Aragonite is orthorhombic with 2 / m2 / m2 / m symmetry and three crystal habits are common acicular pyramidal, tabular, and pseudohexagonal (2).  [c.410]

The thermochemical McKenna process proceeds from tungsten—mineral concentrates to produce tungsten carbide crystals and crystal fragments ranging in size from about 840 through 44 p.m. Macrocrystalline WC, as grown in a menstmum alloy, forms weU-developed, angular crystals having a triangular habit. The crystals always contain a perfectly stoichiometric bound-carbon content and are monocrystalline. Both in initially coarse form, and after size-reduction by milling, macrocrystalline WC has comparatively low specific surface and is entirely free of W2C. Powders are prepared over a wide range of particle sizes, from granular screen-sized ranges to micron-sized powders.  [c.449]

Pelletizing Precipitation. Typical, cold, soda lime water softening generates a 5—15% soupy calcium carbonate sludge, which has been dumped into lagoons for disposal. By controlling reaction conditions of lime and hard water and providing for recirculating seeds of sand or calcium carbonate, precipitation of the carbonate can be controlled to form on the sand and to grow to 1.5-mm tight pellets. The pellets dewater by draining to less than 10% moisture (80,81). Control of crystal growth and crystal habit is used to improve dewatering in the production of phosphoric acid and in scmbbing of flue gas with lime (82,83). In each process, a precipitate of gypsum is formed. The most frequent appHcations of crystal growth regulation are to prevent scaling and to control freezing, for example, of trainloads of coal. The chemical principles are similar.  [c.24]

From the industrial point of view, the term crystal habit or crystal morphology refers to the relative sizes of the faces of a crystal. The crystal habit is determined by the internal structure and external influences on the crystal such as the growth rate, solvent used, and impurities present during the crystallization growth period. The crystal habit of commercial products is of very great importance. Long, needlehke crystals tend to be easily broken during centrifugation and drying. Flat, platelike crystals are very difficult to wash during filtration or centrifugation and result in relatively low filtration rates. Complex or twinned crystals tend to be more easily broken in transport than chunky, compact crystal habits. Rounded or spherical crystals (caused generally try attrition during growth and handling) tend to give considerably less difficulty with caking than do cubic or other compact shapes.  [c.1656]

Hartman and Perdok (1955) predicted that crystal habit or crystal morphology was related to the internal structure based on energy considerations and speciilated it should be possible to predict the growth shape of crystals from the shce energy of different flat faces. Later, Hartman was able to predic t the calculated attachment energy for various crystal species. Recently computer programs have been developed that predic t crystal morphology from attachment energies. These techniques are particularly useful in deahng with organic or molecular crystals and rapid progress in this area is being made by companies such as Molecular Simulations of Cambridge, England.  [c.1656]

In addition to the impurities within the crystal structure itself, there is normally an adhering mother-hquid film left on the surface of the crystal after separation in a centrifuge or on a filter. Typically a centrifuge may leave about 2 to 10 percent of the weight of the crystals as adhering mother hquor on the surface. This varies greatly with the size and shape or habit of the crystals. Large, uniform crystals precipitated from low-viscosity mother liquors wih retain a minimum of mother liquor, while nonuniform or small crystals precipitated from viscous solutions will retain a considerably larger proportion. Comparable statements apply to the filtration of crystals, although norm ly the amounts of mother liquor adhering to the crystals are considerably larger. It is common practice when precipitating materials from solutions which contain appreciable quantities of impurities to wash the crystals on the centrifuge or filter with either fresh solvent or feed solution. In principle, such washing can reduce the impurities qriite substantially. It is so possible in many cases to reslurry the cryst s in fresh solvent and recentrifuge the product in an effort to obtain a longer residence time during the wasning operation and better mixing of the wash liquors with the crystals.  [c.1656]

The following information regarding the product, properties of the feed solution, and required materials of construction must be available before a ciystallizer apphcation can be properly evaluated and the appropriate equipment options identified. Is tne ciystaUine material being produced a hydrated or an anhydrous material What is the solubility of the compound in water or in other solvents under consideration, and how does this change with temperature Are other compounds in solution which coprecipitate with the product being ciystaUized, or do these remain in solution, increasing in concentration until some change in product phase occurs What will be the influence of impurities in the solution on the crystal habit, growth, and  [c.1668]

Pfann has verbally described what led up to his invention, and his account is preserved in the Bell Laboratory archives. As a youth, he was engaged by Bell Laboratories as a humble laboratory assistant, beginning with duties such as polishing samples and developing films. He attended evening classes and finally earned a bachelor s degree (in chemical engineering). He records attending a talk by a famous physical metallurgist of the day. Champion Mathewson, who spoke about plastic flow and crystal glide. Like Rosenhain before him, the youthful Pfann was captivated. Then, while still an assistant, he was invited by his manager, E.E. Schumacher, in the best Bell Labs tradition, to take half your time and do whatever you want . Astonished, he remembered Mathewson and chose to study the deformation of lead crystals doped with antimony (as used by the Bell System for cable sheaths). He wanted to make crystals of uniform composition, and promptly invented zone-levelling. (He took it for granted that this idea was obvious to everyone, but was wrong .) Pfann apparently impressed the Bell Director of Research by another piece of technical originality, and was made a full-fledged member of technical staff, though innocent of a doctorate. When William Shockley complained that the available germanium was nothing like pure enough, Pfann, in his own words, put my feet up on my desk and tilted my chair back to the window sill for a short nap, a habit then well established. I had scarcely dozed off when I suddenly awoke, brought the chair down with a clack I still remember, and realised that a series of molten zones, passed through the ingot of germanium, would achieve the aim of repeated fractional crystallisation. Each zone swept some impurity along with it, until dissolved impurities near one end of the rod are reduced to a level of one in hundreds of millions of atoms. Pfann described his technique, and its mathematical theory, in a paper (Pfann 1954) and later in a book (Pfann 1958, 1966). Incidentally, the invention and perfection of zone-refining was one of the factors that turned solidification and casting from a descriptive craft into a quantitative science.  [c.261]


See pages that mention the term Habit, crystal : [c.117]    [c.215]    [c.190]    [c.132]    [c.462]    [c.508]    [c.196]    [c.146]   
Physical chemistry of surfaces (0) -- [ c.261 ]