Compression-strained channel

Figure 4.20. Cross-section HRTEM image of a transistor containing a compression-strained channel. Reproduced with permission from Intel Corporation (http //www.intel.com/research/downloads/Strained-Si-paper-IEDM-1203.pdf).

Whereas the positive effects of tensile strain of a MOSFET channel is easy to visualize (increases the lattice parameter the free mean path of electrons), the effects of channel compression strain seems counter-intuitive. Explain how compression strain increases the conductivity of the channel region. Why is this type of strain not preferred for n-type Si ... 

PP bead foams of a range of densities were compressed using impact and creep loading in an Instron test machine. The stress-strain curves were analysed to determine the effective cell gas pressure as a function of time under load. Creep was controlled by the polymer linear viscoelastic response if the applied stress was low but, at stresses above the foam yield stress, the creep was more rapid until compressed cell gas took the majority of the load. Air was lost from the cells by diffusion through the cell faces, this creep mechanism being more rapid than in extruded foams, because of the small bead size and the open channels at the bead bonndaries. The foam permeability to air conld be related to the PP permeability and the foam density. 15 refs. 

Fig, 19. (a) Sketch of the channel-die apparatus used for the deformation experiment. Dimensions are in millimetres. The compression stamp is moved along the deformation direction D. The flow of the sample is constrained by the rigid walls of the die in the direction C, and free flow is possible in the direction F. (b) Stress (cr)-strain(e) diagram resulting from channel-die extrusion of bisphenol-A polycarbonate at 300 K and a strain rate of e = 0.01 s l. (c, d) Dipolar DECODER spectra of 13C-labelled bisphenol-A polycarbonate before and after deformation. The spectra exhibit a characteristic star-like ridge pattern. Each of three types of corners (C, D, F) in the pattern corresponds to vectors oriented along a particular direction in the channel-die used for the experiment, (e, f) The anisotropy caused by the deformation becomes readily visible in the difference spectrum (deformed minus undeformed). For clarity, the negative (f) and positive contours (e) have been drawn separately. (Reproduced from Utz et al. with permission.)... 

Indeed, as fluid flows, foam channels closed above grow in thickness at the bottom, thus creating an increasing counteraction to the gravitational force, which slows down the outflow until equilibrium is attained [214]. It should be noted that this effect is possible only in closed deformable channels with negative curvature, which are typical of foam. According to [324], the capillary rarefaction is a characteristic of the foam compressibility and determines its elastic resistance to the strain caused by the liquid redistribution. 

Fig. 9.5 A sketch of a channel-die compression device used in producing highly textured semi-crystalline polymers in plane-strain compression.

Fig. 9.7 Small-angle X-ray scattering (SAXS) patterns of HDPE after several levels of plane-strain compression in a channel die (a) unstrained, (b) after = 0.9, (c) after Ce = 1.14, and (d) after e = 1-86. A new long period of restructuring occurs after Ce = 1.14 (from Gal ski et al. (1992) courtesy of the ACS).

The deformation, starting from an initial compression-molded rectangular plate, equilibrated by annealing at 170 °C in vacuum, was performed in an environment of relative humidity 60% at 20 °C to large plastic strains in a channel die similar to the one described for the work on HDPE in Section 9.3.3. The end result of the plane-strain compression history of the Nylon at a CR of 4.0 (se = 1.39), with a similar complement of intermediate-structure probes of TEM, WAXS, and SAXS, was a texture of similar perfection to that of HDPE, with orthotropic symmetry, but incorporating a dual symmetrical set of intermixed monoclinic components of indeterminable scales and form of special aggregation, as depicted in Fig. 9.14. As with the HDPE, the principal direction of molecular alignment... 

The book concentrates heavily on research conducted at the Massachusetts Institute of Technology from the mid 1980s to the mid 2000s by the author and a group of collaborators. It reports on extensive experimental studies and related computational simulations. In the latter there is much emphasis on development of mechanistic models ranging from unit plastic relaxation events to the evolution of deformation textures in channel die compression flow to large plastic strains. At every level the experimental results are compared in detail with predictions from the models. 

Lamellar orientation in thin films of a model diblock copolymer with symmetric poly(styrene)- -PLLA (PS-PLLA) was investigated by Chen et al. [62] in the molten state on silicon wafer supported surfaces. Stretching and compression were apt to induce orientation of PLA. Pluta and Galeski [63] studied the plastic deformation of amorphous and thermally noncrystallizable 70/30 PLA/PDLLA induced by plane strain compression in a channel die. The results revealed that plastic deformation transformed an amorphous PLA or PDLLA (thermally noncrystallizable) into a crystalline fibrillar texture oriented in the flow direction. 

Recently, Randall and Doyle have studied the electrophoretic collision of DNA with an obstacle in a microfluidic channel, both experimentally and theoretically [6]. In that case, large deformations due to field gradients and hydrodynamic strain near the obstacle quickly stretch and compress the molecule and cause configuration-dependent hooking interactions, as shown in Fig. 2b. (See also di-electrophoretic motion of particles and cells.)... 

Plane strain compression hi a channel die is kinematically very similar to drawing the sample is extended and its cross-section decreases accorduigly. However, the possibility of void formation is limited due to the compressive component of stress. It means that the differences in true stress-true strain dependencies of drawn and plane strained polymers should be attributed to the formation and development of cavities. Slopes of the elastic region of true stress-true strain curves are similar in tension and in plane strain compression. The difference in mechanical properties of polymers sets in at yielding in tension. The scale of difference depends on the particular polymer the yield in drawing for POM, PA 6, PP and HDPE takes place at a much lower stress than in plane strain compression. For polymers with low crystal plastic resistance, such as LDPEs and ethylene-octene copolymer (EOC), the stresses at selected deformation... 

The mechanisms associated with plastic deformation of crystalline polymers can be explained better on the basis of two examples of isotactic polypropylene subjected to drawing eind to plane strain compression in a channel die. The envisioned differences between defiarmation with and without cavitation are summarized in Figure 1.21. 

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