Threshold layer compression

C-A transition induced by layer compression Ribotta, Meyer and Durand observed that a compression applied normal to the layers of induces a transition to S. when the stress exceeds a threshold value. The effect is particularly easy to observe very near T ca-If 5 is the strain normal to the layers, the energy density may be written as... 

An even wider range of wavelength, toward the infrared, can be covered with quantum well lasers. In the Al Ga As system, compressively strained wells of Ga In As are used. This ternary system is indicated in Figure 6 by the line joining GaAs and In As. In most cases the A1 fraction is quite small, X < 0.2. Such wells are under compressive strain and their thickness must be carefully controlled in order not to exceed the critical layer thickness. Lasers prepared in this way are characterized by unusually low threshold current density, as low as ca 50 A/cm (l )-... 

For the c-BN formation a stress threshold was observed in the deposited layers. The h-BN intermediate layer shows a preferred orientation, where the c-axis of the h-BN is parallel to the substrate. Both effects are explained by the compressive biaxial stress induced by the ion bombardment. The mechanism for the conversion of h-BN into c-BN is explained by rather high temperatures originated during thermal spikes (direct h-BN —> c-BN transformation). The stress caused by the bombardment with high energetic ions is considered to be a reason for the growth of the c-BN crystals [190, 191]. A stress within the layer of up to 10 GPa has been observed. This biaxial stress causes a hydrostatic pressure up to the values usual in HP-HT synthesis. 

Figures 322(a) and (b) show again the conditions in a pelleting machine with concave die. Figure 322(a) depicts the mechanisms of compression and extrusion in the work area , the material volume wedged in between the press roller and die. Figure 322 explains the phenomena. Feed deposited in a layer on the die is pulled into the space between the roller and die and compressed. Neither the roll force nor the force from the die resisting extrusion (flow) are constant. The roll force increases with progressing densification while the flow-resisting force remains constant until a threshold pressure, defined by the static friction in the die holes, is surpassed. After extrusion (movement in the die holes) has started, both the resisting and the roll forces decrease.

Beyond a threshold displacement of order A, a tension (but not a compression) on the layers causes an undulation of period y/Xd, where d is the total thickness of the preparation. The period here can easily be comparable to, or slightly less than, the wavelength of light. The state obtained strongly scatters light. Some display applications have made indirect use of this instability. 

As equation (5.91) shows, the threshold load can be estimated from a knowledge of Hy, K, and all of which are found by the indentation hardness technique. In the case just considered, with Kc = 0.75 MN Hy = 6 GN m, ( c = 130 MN m , and A = 0.076, an estimate for P is 0.43 N. This small load over the area represented by a, is of course a large local stress, but the estimate does emphasize that small particles can generate critical flaws beneath the surface. The effect of the surface compression layer can be seen by removing the -1.6critical load decreases to 0.068 N for this glass. 

Ribotta and co-workers were the first to observe that a uniaxial pressure applied normal to the smectic A layers induces a transition to the smectic C phase when the stress exceeds a threshold value [130]. The behavior is much more pronounced when the stress is applied at a temperature close the SmA-SmB transition. The finite tilt angle induced can be directly related to B, the elastic constant of compression. Further studies have shown that the temperature dependence of the critical stress and strain re-... 

---