Crystal, habit simple

Changes in the solid state can inLuence dissolution rate through the surface area term or th< solubility term. Surface area differences can arise from simple particle-size effects between different crystal forms and also from shape factors. Different crystal habits and shapes can alter the exposei surface area without a change in median particle-size measurements, since these are often calculati... 

It is known that the relative growth rates on 100 and 111 planes Vjoo/v,ii determine the crystal habit of diamond, and the appearance or disappearance of crystallographic planes in diamond films depends on the growth velocities of the corresponding planes. The facets that appear on a crystal are those for which the normal gro vth velocity is the slowest. On the basis of the simple Wullf criterionl O for crystal habit, the most stable growth planes in diamond are the octahedral 111 planes, followed by the... 

If the growth of an ice crystal from the vapour were a simple near-equilibrium process, then the resulting crystal habit could be determined by Wulff s theorem (Wulif, 1901) which states that, in equilibrium, the distance of any face from the centre of the crystal... 

Another important discovery that changed the field of chemical sensing was the synthesis of single crystal one-dimensional oxide nanowires. These are created by the simple evaporation of the desired commercial metal oxide powders at high temperatures and their condensation at lower temperatures on the substrates. These materials have great potential thanks to their reduced lateral dimensions and single crystal habits, both for fundamental study and for potential nanodevice applications, i.e. the third generation of metal oxide gas sensors. 

A tailored modifier does not always have to be deliberately added to a crystallizing system it may already exist, e.g., as a synthesis by-product of a chemical reaction. If its presence causes a crystal habit problems, it must be removed or deactivated. On the other hand, it may have a beneficial effect. These are both commonly encountered cases in the manufacture of organic chemicals. A simple, but industrially important, example is the effect of biuret on the crystallization of urea (Davey, Fila and Garside, 1986). In the synthesis of urea (NH2CONH2) from ammonia and carbon dioxide a small amount of biuret (NH2CONHCONH2), a condensation dimer, is formed. The presence of biuret is actually beneficial because from pure aqueous solution urea crystals form as elongated [001] needles that are difficult to process. Biuret retards growth in the... 

The simplest method is optical microscopy, in which visible light (photons) is used to observe a sample. It has a resolution limit around 0.25-0.5 pm, which is on the order of 2/2, where 2 is the wavelength of incident light. From a strict colloid science point of view, it lies near the upper limit of colloid particle sizes and appears to be of limited utility. However, it is of great help in the identification of minerals, because it allows observation of crystal habits (the shape and size of crystals, which are determined by their internal symmetry). With experience, many minerals can be identified in a soil sample under a microscope, even from simple inspection. A unique feature of optical microscopy is the availability of polarized light, which is handy in distinguishing minerals or even different crystal types of the same compound (Bullock et al. 1985 Cady, Wilding, and Drees 2010). 

Why do we get differences in crystal shape or habit This may be a matter of directional rates of growth. Factors affecting directional rates will then affect the habit. Directional rates of growth can be illustrated with a relatively simple crystal structure, that of sodium chloride. 

Because the rate of growth depends, in a complex way, on temperature, supersaturation, size, habit, system turbulence and so on, there is no simple was of expressing the rate of crystal growth, although, under carefully defined conditions, growth may be expressed as an overall mass deposition rate, RG (kg/m2 s), an overall linear growth rate, Gd(= Ad./At) (m/s) or as a mean linear velocity, // (= Ar/At) (m/s). Here d is some characteristic size of the crystal such as the equivalent aperture size, and r is the radius corresponding to the... 

In dairy products, crystallization is more complex. The impurities (e.g., other milk components), as far as lactose is concerned, may interfere with the crystalline habit. As a result, the crystals tend to be irregularly shaped and clumped, instead of yielding the characteristic crystals obtained from simple lactose solutions. In some instances, the impurities may inhibit the formation of nuclei and thus retard or prevent lactose crystallization (Nickerson 1962). 

The calculation of the absorption factor in practice demands use of a computer with a sophisticated program and requires also accurate knowledge of the shape and dimensions of the crystal specimen. The problem of measuring the shape and dimensions of an object as small as a diffraction specimen with sufficient accuracy is by no means trivial, and some workers prefer to retain the simple parallelepipedal habit possessed by some specimens naturally, rather than to grind the crystal to a somewhat rough sphere with a somewhat ill-defined radius. The problem of measurement is especially severe for chips or fragments... 

Ice crystals can grow in two simple distinct ways either by the freezing of liquid water or by direct sublimation from the vapour phase. In each case the mechanisms which determine the rate and habit of growth are the transport of water molecules to the point of growth and their accommodation into the growing interface, together with the transport of latent heat away from this interface. Many different physical situations can occur, of course, but they are all controlled by these basic mechanisms. 

The habits of an ideal quartz crystal (Fig. 1.1.) has five simple forms rhombo-hedrons, R, and trapezohedron, x, hexagonal, prism, and bipyramidal. The crystal has its main optical vertical axis of third-order and three horizontal axes of second-order directed to ribs of the prism. These axes are so called "electric ones". Examination of the positions of the faces of a crystal reveals the sign of its optical rotation. The crystal is a (/-quartz if x and s faces are situated at the top of the face of the right side prism. The crystal is an /-quartz... 

There is no simple or generally accepted method of expressing the rate of growth of a crystal, since it has a complex dependence on temperature, supersaturation, size, habit, system turbulence, and so on. However, for carefully defined conditions crystal growth rates may be expressed as a mass deposition rate Rq (kgm s ), a mean linear velocity v(ms ) or an overall linear growth rate G (ms ). The relationships between these quantities are... 

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