Alternating layers

The families of materials discussed in this section have also been deposited from solution to form films having a thickness of the order 1 fim, orientational order being introduced by the application of a strong magnetic field. The diamagnetic nature of these materials is made use of, its effect being very much magnified by their liquid crystalline nature. This topic will be returned to in Chapter 7. 

In order to generate the second harmonic of an electromagnetic wave, one needs to make use of some device which has a non-linear property. In the case we are considering, the non-linear relationship made use of is that between applied electric field and electric polarisation. One can write 

Attempts to produce pyroelectric devices have been less numerous than those aimed at producing second harmonic generators. Clearly a pyroelectric device must be non-centrosymmetric in character and must thus be formed either from an alternate layer structure or from a Z layer structure. 

Many of the papers devoted to the two topics discussed above are essentially technological in character and do not attempt to characterise structure and order and thus are not really relevant to the subject matter of this book. However, a number of such papers have made important contributions to the field of non-linear optics and infrared detection. Thus the remainder of this action will consist of a discussion of those papers whose content is directly relevant to questions of order and structure followed by a catalogue of some of the other more important papers in this general field. 

Recently Cresswell et al. [297] have produced alternating layer structures which they have characterised by SHG and X-ray diffraction and have obtained a partial degree of order. Era et al. [298] have produced thick alternating films which exhibit quadratic dependence of second harmonic intensity on film thickness and which show a reasonable degree of order when studied by X-ray diffraction. 

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Barraud A, Leloup J, Gouzerh A and Palaoin S 1935 An automated trough to make alternate layers Thin Solid Films 133 117-23... 

Presentiy, multilayer capacitors and packaging make up more than half the electronic ceramics market. For multilayer capacitors, more than 20 biUion units are manufactured a year, outnumbering by far any other electronic ceramic component. Multilayer ceramics and hybrid packages consist of alternating layers of dielectric and metal electrodes, as shown in Figures 5 and 6, respectively. The driving force for these compact configurations is miniaturization. 

Multilayer Capacitors. Multilayer capacitors (MLC), at greater than 30 biUion units per year, outnumber any other ferroelectric device in production. Multilayer capacitors consist of alternating layers of dielectric material and metal electrodes, as shown in Figure 7. The reason for this configuration is miniaturization of the capacitor. Capacitance is given by... 

In temperate climates extracts from some plants were found to be excellent preservatives for hides and skins. The hides, with or without hair, were placed in pits in the ground, then covered with alternating layers of bark or leaves and skins. Water was added and later, ie, days or months depending on the thickness of the hide, the hides could be removed, washed, and oiled. The resulting leather is flexible and lasts essentially forever. This procedure was used well into the seventeenth century as the most common method of tanning. In some isolated primitive societies, the method is used in the 1990s. 

Consists of alternative layers of binder and irregularly arranged and partly overlapped spHttings. 

Once purification of the niobium has been effected, the niobium can be reduced to the metallic form. The double fluoride salt with potassium, K2NbFy, can be reduced using sodium metal. The reaction is carried out in a cylindrical iron vessel filled with alternating layers of K NbF and oxygen-free sodium ... 

Built-Up Roofing. Built-up roofing (BUR) is a continuous-membrane covering manufactured on-site from alternate layers of bitumen, bitumen-saturated or coated felts, or asphalt-impregnated glass mats and surfacings. These membranes are generally appHed with hot bitumens or cold apphed bituminous adhesives (qv). 

At red heat, a low carbon ferrous metal, in contact with carbonaceous material such as charcoal, absorbed carbon that, up to the saturation point of about 1.70%, varied in amount according to the time the metal was in contact with the carbon and the temperature at which the process was conducted. A type of muffle furnace or pot furnace was used and the kon and charcoal were packed in alternate layers. 

When a eutectoid steel is slowly cooled from the austenite range, the ferrite and cementite form in alternate layers of microscopic thickness. Under the microscope at low magnification, the diffraction effects from this mixture of ferrite and cementite give an appearance similar to that of a pearl, hence the material is called peadite. 

Alternate-layer LB films (Y-type, ABAB) of long-chain amines and fatty acids maybe used for pyrroelectric appHcations (Fig. 5). Stearylamine, C gH2yNH2, and a series of straight-chain fatty acids, yield a thick film (several hundreds of monolayers) which gave a pyrroelectric coefficient of - 0.05 nC/(cm -K) (72). A coefficient of 0.3 nC/(cm -K) for an 11-monolayer sample of CO-tricosenoic acid and docosylamine C22H4 NH2 has been reported (73). 

Titanium Dichloride. Titanium dichloride [10049-06-6] is a black crystalline soHd (mp > 1035 at 10°C, bp > 1500 at 40°C, density 31(40) kg/m ). Initial reports that the titanium atoms occupy alternate layers of octahedral interstices between hexagonaHy close-packed chlorines (analogous to titanium disulfide) have been disputed (120). TiCl2 reacts vigorously with water to form a solution of titanium trichloride andUberate hydrogen. The dichloride is difficult to obtain pure because it slowly disproportionates. 

An unusual crystal arrangement is exhibited by the isomorphous compounds CrCl and Crl. The close-packed cubic array of Cl or I atoms has two-thirds of the octahedral holes between every other pair of chlorine or iodine planes filled with chromium atoms. Alternate layers of the halogen compounds are held together by van der Waals forces (39,40). 

Multilayer Insulation Miiltilayer insulation consists of alternating layers of highly reflec ting material, such as aluminum foil or aluminized Mylar, and a low-conduc tivity spacer material or insulator, such as fiberglass mat or paper, glass fabric, or nylon net, all under high vacuum. When properly applied at the optimum density, this type of insulation can have an apparent thermal conduc tivity as low as 10 to 50 jlW/m-K between 20 and 300 K. 

Fig. 15. Dynamic stiffness images of alternating layers of polyethylene (PE) of two molecular weights at (a) 105 Hz and (b) 200 Hz. The contrast is due to changes in contact compliance (1/stiffness) of the nanoindenter probe in contact with each of the two polymers. The probe-sample respon.se (1/stiffness) as a function of frequency shown in (c) is consistent with the dynamic stiffness images.

Fig, 9. Polymers containing long alkyl side chains typically form alternating layered structures. 

To achieve pressures intermediate to those achieved by direct contact with a given metal plate, use is often made of alternate layers of various shock impedance materials. Table 3.2 gives a summary of experimental arrangements that have been used in materials studies to achieve pressures from 3 to 80 GPa. 

The crimped metal ribbon arresdng element, shown in Fignre 5-1, is one of die most widely nsed types, especially for detonadon flame arresters. Crimped metal ribbon arresters are made of alternate layers of diin corm-... 

Crimped Metal Ribbon A flame arrester element that is manufactured of alternate layers of thin corrugated metal rihhon and a flat metal rihhon that are wound together on a mandrel to form a cylindrical assembly of many layers to produce a range of different sized triangular cells. The height and width of the triangular cells can he varied to provide the required quenching diameter. 

Figure 6.27 Comparison of the hexagonal layer structures of BN and graphite. In BN the atoms of one layer are located directly above the atoms of adjacent layers with B - N contacts in graphite the C atoms in one layer are located above interstices in the adjacent layer and are directly above atoms in alternate layers only.

Figure 15.18 Comparison of the nickel arsenide structure (a) adopted by many monosulfides MS with the cadmium iodide structure (b) adopted by some disulfides MS2. The structures are related simply by removing alternate layers of M from MS to give MS2.

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