Type II

In the case of type II silicon ribbons, the heat is removed from the liquid-solid interface through the solidified wafer into the cold substrate. In contrast to the type I technologies, heat removal through the thin wafer with a large cross section is more effective, resulting in a much higher growth rate. 

In this case, ribbon growth speed can be expressed as 

In general, crystal growth is more complex than the system outlined above due to the behaviour of the To (temperature at the bottom of the solidified wafer), which in general is not constant, the temperature dependence of the material characteristics, and the often turbulent flow in the liquid silicon melt. The variable growth speed results in thickness dependent material characteristics due to processes like velocity dependent effective segregation of metallic impurities. 

In contrast to type I crystal growth, where a relatively large temperature gradient in the solidified silicon is the driving force for crystallisation, the temperature gradient through the solidified silicon in type II processes can 

The crystal growth speed can be controlled by the heat extraction capacity of the substrate material. During cooling, the difference in thermal expansion coefficient between substrate material and Si ribbon causes a separation of the silicon ribbon from the substrate and allows the re-use of the substrate material. 

Althongh these are not common in cereals, type II resistant starches are highly crystalline native granules, such as those present in raw bananas (Musa acuminata) or potatoes (Solanum tuberosum), and resist digestion in the small intestine. 

These have previously been obtained by electrophilic attack on ene-yl complexes [equation (a) Y = CH(C02Me)2, OMe ch = 1J-C5H5 diene = 1,5-cyclooctadiene]1 or by reaction of the compounds (diene)MBr2 with 57-C5H6Fe(CO)2Br (diene = 1,5-cyclooctadiene or 1,2,3,4-tetraphenyl-l,3-cyclobutadiene).2 An example of the former method is given in which the methoxy-cyclooctenyl derivative is used as the substrate and tetrafluoro-boric acid as the electrophile. The substrate is conveniently prepared and used without isolation, and in this way the reaction takes only a few hours, starting with dichloro(l,5-cycloocta-diene)palladium, prepared as described above. 

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In ternary systems, we distinguish between two common types. In type II, two binaries are partially miscible and the third binary is completely miscible in type I, only one binary is partially miscible. (A third type, where all three binaries are only partially miscible, is relatively rare and not considered here.)... 

For systems of type II, if the mutual binary solubility (LLE) data are known for the two partially miscible pairs, and if reasonable vapor-liquid equilibrium (VLE) data are known for the miscible pair, it is relatively simple to predict the ternary equilibria. For systems of type I, which has a plait point, reliable calculations are much more difficult. However, sometimes useful quantitative predictions can be obtained for type I systems with binary data alone provided that... 

To illustrate, predictions were first made for a ternary system of type II, using binary data only. Figure 14 compares calculated and experimental phase behavior for the system 2,2,4-trimethylpentane-furfural-cyclohexane. UNIQUAC parameters are given in Table 4. As expected for a type II system, agreement is good. 

Figure 4-14. Predicted liquid-liquid equilibria for a typical type-II system shows good agreement with experimental data, using parameters estimated from binary data alone.

Many well-known models can predict ternary LLE for type-II systems, using parameters estimated from binary data alone. Unfortunately, similar predictions for type-I LLE systems are not nearly as good. In most cases, representation of type-I systems requires that some ternary information be used in determining optimum binary parameter. 

Adsorption isotherms are by no means all of the Langmuir type as to shape, and Brunauer [34] considered that there are five principal forms, as illustrated in Fig. XVII-7. TVpe I is the Langmuir type, roughly characterized by a monotonic approach to a limiting adsorption at presumably corresponds to a complete monolayer. Type II is very common in the case of physical adsorption... 

This description is traditional, and some further comment is in order. The flat region of the type I isotherm has never been observed up to pressures approaching this type typically is observed in chemisorption, at pressures far below P. Types II and III approach the line asymptotically experimentally, such behavior is observed for adsorption on powdered samples, and the approach toward infinite film thickness is actually due to interparticle condensation [36] (see Section X-6B), although such behavior is expected even for adsorption on a flat surface if bulk liquid adsorbate wets the adsorbent. Types FV and V specifically refer to porous solids. There is a need to recognize at least the two additional isotherm types shown in Fig. XVII-8. These are two simple types possible for adsorption on a flat surface for the case where bulk liquid adsorbate rests on the adsorbent with a finite contact angle [37, 38]. 

Because of their prevalence in physical adsorption studies on high-energy, powdered solids, type II isotherms are of considerable practical importance. Bmnauer, Emmett, and Teller (BET) [39] showed how to extent Langmuir s approach to multilayer adsorption, and their equation has come to be known as the BET equation. The derivation that follows is the traditional one, based on a detailed balancing of forward and reverse rates. 

The very considerable success of the BET equation stimulated various investigators to consider modifications of it that would correct certain approximations and give a better fit to type II isotherms. Thus if it is assumed that multilayer formation is limited to n layers, perhaps because of the opposing walls of a capillary being involved, one... 

Equation XVII-78 turns out to ht type II adsorption isotherms quite well—generally better than does the BET equation. Furthermore, the exact form of the potential function is not very critical if an inverse square dependence is used, the ht tends to be about as good as with the inverse-cube law, and the equation now resembles that for a condensed him in Table XVII-2. Here again, quite similar equations have resulted from deductions based on rather different models. 

There is little doubt that, at least with type II isotherms, we can tell the approximate point at which multilayer adsorption sets in. The concept of a two-dimensional phase seems relatively sterile as applied to multilayer adsorption, except insofar as such isotherm equations may be used as empirically convenient, since the thickness of the adsorbed film is not easily allowed to become variable. 

Greenfield S R and Wasielewski M R 1995 Near-transform-limited visible and near-IR femtosecond pulses from optical parametric amplification using Type II p-barium borate Opt. Lett. 20 1394-6... 

Manna A et al 1997 Synthesis and oharaoterization of hydrophobio, approtioally-dispersible silver nanopartioles in Winsor Type II mioroemulsions Chem. Mater. 9 3032... 

A simple VB approach was used in [75] to describe the five structures. Only the lowest energy spin-pairing structures I (B symmehy) of the type (12,34,5 were used (Fig. 21). We consider them as reactant-product pairs and note that the transformation of one structure (e.g., la) to another (e.g., Ib) is a thr ee-electron phase-inverting reaction, with a type-II transition state. As shown in Figure 22, a type-II structure is constructed by an out-of-phase combination of... 

Figure 22. Ad out-of-phase combiDatioD of two type-I (Bi symmetry) structures yields a type-II structure (A2 symmetry),...

Figure 6-1. Different forms of representation of a chemical graph a) labeled (numbered) graph b) adjacency matrix c) connectivity table, type I d) connectivity table, type II f) line notations g) structural index.

The thiazole ring can be obtained directly by other methods, but they have limited application. An example is the synthesis of Cook and Heilbron using a-aminonitriles or a-aminoamides and carbon disulfide (or thioacid derivatives) as reactants of type II. 

A Type II isotherm indicates that the solid is non-porous, whilst the Type IV isotherm is characteristic of a mesoporous solid. From both types of isotherm it is possible, provided certain complications are absent, to calculate the specific surface of the solid, as is explained in Chapter 2. Indeed, the method most widely used at the present time for the determination of the surface area of finely divided solids is based on the adsorption of nitrogen at its boiling point. From the Type IV isotherm the pore size distribution may also be evaluated, using procedures outlined in Chapter 3. 

The physical adsorption of gases by non-porous solids, in the vast majority of cases, gives rise to a Type II isotherm. From the Type II isotherm of a given gas on a particular solid it is possible in principle to derive a value of the monolayer capacity of the solid, which in turn can be used to calculate the specific surface of the solid. The monolayer capacity is defined as the amount of adsorbate which can be accommodated in a completely filled, single molecular layer—a monolayer—on the surface of unit mass (1 g) of the solid. It is related to the specific surface area A, the surface area of 1 g of the solid, by the simple equation... 

To obtain the monolayer capacity from the isotherm, it is necessary to interpret the (Type II) isotherm in quantitative terms. A number of theories have been advanced for this purpose from time to time, none with complete success. The best known of them, and perhaps the most useful in relation to surface area determination, is that of Brunauer, Emmett and Teller. Though based on a model which is admittedly over-simplified and open to criticism on a number of grounds, the theory leads to an expression—the BET equation —which, when applied with discrimination, has proved remarkably successful in evaluating the specific surface from a Type II isotherm. 

If plotted as n/n against p/p°. Equation (2.12) gives a curve having the shape of a Type II isotherm so long as c exceeds 2. From Fig. 2.1 it is seen... 

The Type II isotherms obtained experimentally often display a rather long straight portion (BC in Fig. 2.9), a feature not strictly compatible with the properties of the BET equation which, as we have seen, yields a point of... 

Fig. 2.9 A typical Type II isotherm, showing Point A and Point B .

The kind of results adduced in the present section justify the conclusion that the quantity n calculated by means of the BET equation from the Type II isotherm corresponds reasonably well to the actual monolayer capacity of the solid. The agreement lies within, say, +20 per cent, or often better, provided the isotherm has a well defined Point B. 

As will be demonstrated in Chapter 4, however, the presence of micropores distorts the Type II isotherm in a sense which is reflected in a much increased value of the constant c. In such cases the value of c is no guide at all to the course of the isotherm on the external surface. Consequently the appropriate criterion for choosing the correct f-curve for a particular system is the similarity in chemical properties and not in c-values l>etween the solid under test and the reference solid. 

If micropores are introduced into a solid which originally gave a standard Type II isotherm, the uptake is enhanced in the low-pressure region and the isotherm is correspondingly distorted. The effect on the t-plot is indicated in... 

As will be demonstrated in Chapter 4, an isotherm which is reversible and of Type II is quite compatible with the presence of micropores. If such pores are present, the isotherm will be distorted in the low-pressure region, the value of c will be greatly enhanced, and the specific surface derived by the BET procedure will be erroneously high. In particular, a BET specific surface in excess of - 500m g" should be taken as a warning that... 

Fig. 3.1 A Type IV isotherm. The corresponding Type II isotherm follows the course ABCN (cf. dashed line).

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