Contact twins

Figure 7.1. Various growth twins, (a) Contact twin (albite) (b) inclined twin (quartz) (c) elbow twin (rutile) (d) cyclic twin (chrysoberyl) (e) lamellar twin (albite) ...

Japanese twins of quartz have attracted interest since ancient times because they exhibit a remarkably platy V-shape, in contrast to the hexagonal prismatic morphology of coexisting single crystals. Since they grow on substrate, the V-shape was assumed to represent the upper half of an X-shape [12], which implies that the Japanese twins are penetration twins. The platy form of Japanese twins has been explained as being due to preferential growth at a re-entrant corner formed by two individuals. If the platy form is indeed simply due to the re-entrant corner effect, we should expect a variation of forms, from V-shape to fan-shape, as the effect proceeds, as shown in Fig. 10.9. If, however, it represents the upper half of an X-shape, Japanese twins should be penetration twins, not contact twins. 

If two individuals conjugate on an r or a z face, a nucleus of a Japanese twin with 1122 as the composition plane is formed. This indicates that a Japanese twin is not a penetration twin (i.e. not the upper half of an X-shape), but a contact twin (i.e. the upper half of a Y-shape). Horizontal banding in geode agate (see Section 10.9) appears through grain size variation due to gravitational sedimentation, and con-... 

Figure 1.2 Japan law contact twin quartz crystal. This type of twinning was discovered in 1829 by C. S. Weiss in quartz crystal from the La Gardette mine in France. However, because of the abundance of these specimens in Japan, they are now known as Japanese twins.

Figure 6.5. Evolution of a micro-contact twin that has been created on a step. The dashed lines are grain boundaries [100].

Signal processing in mechanical impedance analysis (MIA) pulse flaw detectors by means of cross correlation function (CCF) is described. Calculations are carried out for two types of signals, used in operation with single contact and twin contact probes. It is shown that thi.s processing can increase the sensitivity and signal to noise ratio. 

Here (j(t) consists of a few llrst periods only. The s(t) pulses are typical for single contact probes, q(t) - for twin contact probes. 

To increase the sensitivity, direction of amplitude variation of probe output signal in defective area must coincide with the one after CCF processing. If the defect decreases the probe signal (single contact probe) A((/should be set Ai// = 0, in the opposite case (twin contact probe) it should be set Aif/= n. So the instrument should be supplied with a device to adjust A((/ and to sustain it constant. 

The maximum bubble pressure method is good to a few tenths percent accuracy, does not depend on contact angle (except insofar as to whether the inner or outer radius of the tube is to be used), and requires only an approximate knowledge of the density of the liquid (if twin tubes are used), and the measurements can be made rapidly. The method is also amenable to remote operation and can be used to measure surface tensions of not easily accessible liquids such as molten metals [29]. 

At the instant of contact between a sphere and a flat specimen there is no strain in the specimen, but the sphere then becomes flattened by the surface tractions which creates forces of reaction which produce strain in the specimen as well as the sphere. The strain consists of both hydrostatic compression and shear. The maximum shear strain is at a point along the axis of contact, lying a distance equal to about half of the radius of the area of contact (both solids having the same elastic properties with Poisson s ratio = 1/3). When this maximum shear strain reaches a critical value, plastic flow begins, or twinning occurs, or a phase transformation begins. Note that the critical value may be very small (e.g., in pure simple metals it is zero) or it may be quite large (e.g., in diamond). 

In this microcalorimeter, the heat sink is not a massive metal block but is divided into several parts which are mobile with respect to each other. Each thermoelectric element (E) and a cell guide (D) are affixed to a fluxmeter holder (C). The holder (C) is mobile with respect to a massive arm (B) which, in turn, rotates around a vertical axle (A). All parts of the heat sink are made of brass. Surfaces in contact are lubricated by silicone grease. Four thermoelectric elements (E) are mounted in this fashion. They enclose two parallelepipedic calorimetric cells, which can be made of glass (cells for the spectrography of liquids are particularly convenient) or of metal (in this case, the electrical insulation is provided by a very thin sheet of mica). The thermoelectric elements surrounding both cells are connected differentially, the Petit microcalorimeter being thus a twin differential calorimeter. 

Twins are intergrown crystals such that the crystallographic directions in one part are related to those in another part by reflection, rotation, or inversion through a center of symmetry across a twin boundary. Twinned crystals are often prized mineralogical specimens. When twins are in contact across a well-defined plane (which is not always so), the boundary is generally called the composition plane. The only twins that are considered here will be reflection twins, where the two related parts of the crystal are mirror images (Fig. 3.22). The mirror plane that relates the two components is called the twin plane. This is frequently, but not always, identical to the plane along which the two mirror-related parts of the crystal join, that is, the composition plane. Repeated parallel composition planes make up a polysynthetic twin (Fig. 3.23). 

Fibres reacted with pre-heated MAPP always exhibited larger contact angles compared with fibres reacted with as-received MAPP. FTIR studies showed that there was a higher level of ester linkages when the fibres were reacted with the pre-heated MAPP. Wood polymer composites were made by shear mixing in a twin screw extmder, and the mechanical properties of these composites were found to be superior when pre-heated MAPP was used as the coupling agent. 

The antistatic properties of the step 2 product were evaluated by preparing tapes of blends with polystyrene by extruding in a twin-screw extruder using a flat die at 200°C. The contact angle of the tapes was measured using the sessile drop method and water as the measuring liquid. Testing data not supplied by author. 

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