A silicon transconductance light emitting device (TRANSLED)

A novel multi-terminal silicon light emitting device (TRANSLED) is described where both the light intensity and spatial light pattern of the device are controlled by an insulated MOS gate voltage. This presents a major advantage over two terminals Si-LEDs, which require direct modulation of the relatively high avalanche current. It is found that, depending on the bias conditions, the light intensity is either a linear or a quadratic function of the applied gate voltage. The nonlinear relationship facilitates new applications such as the mixing of electrical input signals and modulating the optical output signal, which cannot readily be achieved with two terminal Si-LEDs, since they exhibit a linear relationship between diode avalanche current and light intensity. Furthermore, the control gate voltage can also modulate the emission pattern of the light emitting regions, for example, changing the TRANSLED from an optical line source to two point sources.     

Observation of deformation and damage at the tip of cracks in adhesive bonds loaded in shear and assessment of a criterion for fracture

The evolution of damage at the tip of cracks in adhesive bonds deforming in shear was monitored in real time using a high-magnification video camera. Brittle and a ductile epoxy resins were evaluated, with the bond thickness t being an experimental variable. An extensive zone of plastic deformation developed ahead of the crack tip prior to fracture. In the case of the brittle adhesive, for relatively thick bonds tensile microcracks formed within that zone. Increased loading caused the microcracks to grow from the interlayer to the interface, which led to a complete bond separation after interface cracks emanating from adjacent microcracks linked. In contrast, for the ductile adhesive the crack always grew from the tip. Strain gradients tended to develop there when the bond thickness was large.The adhesive shear strain was determined from fine lines scratched on the specimen edge. For both adhesives, the average crack tip shear strain at crack propagation rapidly decreased with increasing t. This effect was attributed to the changing sensitivity of the bond to the presence of flaws; thicker bonds can accommodate larger microcracks or microvoids which cause greater stress concentration. For a given bond thickness, the critical crack tip shear strain agreed well with the ultimate shear strain of the unflawed adhesive previously determined using the napkin ring shear test [12]. This suggests that the ultimate shear strain is a key material property controlling crack growth. The critical distortional strain energy/unit area of the unflawed adhesive W _s was determined from the area under the stress-strain curve in the napkin ring test. Good agreement between W _s and the adhesive mode II fracture energy was found for all joints tested except for relatively thick bonds. For the particular case of an elastic-perfectly plastic adhesive, the agreement above implies % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGak0dh9WrFfpC0xh9vqqj-hEeeu0xXdbba9frFj0-OqFf% ea0dXdd9vqaq-JfrVkFHe9pgea0dXdar-Jb9hs0dXdbPYxe9vr0-vr% 0-vqpWqaaeaabaGaciaacaqabeaadaqaaqaaaOqaaGqaciaa-Deada% WgaaWcbaacbaGaa4xsaiaa+LeacaGFdbaabeaakiabg2da9iaa-Dfa% daWgaaWcbaGaa83CaaqabaGccqGHHjIUcaWF0bGaeqiXdq3aaSbaaS% qaaiaa-LhaaeqaaOGaeq4SdC2aaSbaaSqaaiaa-zgaaeqaaaaa!463A!\[G_{IIC} = W_s \equiv t\tau _y \gamma _f \].     

Multiple models and mental illnesses: A rejoinder to "Failure of a moral enterprise: Attitudes of the public toward mental illness" by T. R. Sarbin and J. C. Mancuso.

Critically analyzes the T. R. Sarbin and J. D. Mancuso (see record 1971-04081-001) report. Data are presented indicating that the public is (a) generally accepting of the medical model of mental illness; (b) optimistic about prognosis; and (c) able to identify the simple schizophrenic, the alcoholic, and the juvenile character disorder as mentally ill and in need of medical care. The public does not place a sizable social distance between themselves and those labeled mentally ill. These data contradict the claims advanced for a unitary social deviancy model. It is argued that the medical model is neither rejected by the public nor discredited by current research. The need is not for the abandonment of medical and psychological models which have already demonstrated formidable heuristic value; what is needed is recognition that there are many different kinds of mental illnesses and that multiple models may consequently have value. (20 ref.) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Shear fracture

The material aspects of mode II and III fracture in adhesively bonded joints were elucidated for a range of adhesives using DCB type test specimens and scanning electron microscopy. The adhesive thickness was varied from as little as a small fraction of the natural crack tip damage zone to a value large enough to expose the bulk fracture behavior.The fracture process in either mode started with the development of tensile microcracks ahead of the crack tip and continued with the spread of this damage through interfacial microcrack linkage and intralaminar adhesive yielding, of which the severity increased with adhesive ductility. Crack growth occurred when the damage zone became fully matured, at which point stable cracking took place with increasing load.Perhaps the most interesting finding was that irrespective of adhesive mechanical properties, energetically and, in essence, morphologically, the forward and anti-plane shear fracture components coincided. The shear fracture work, G _sc , exhibited a plateau followed by a phase of monotonic decline as the adhesive thickness, t, decreased. In the limit t 0 the shearing and opening fracture works converged to a common, intrinsic value, G _c , that was only a small fraction of the bulk material toughness as identified by the plateau value of G _sc .

---

Résumé On a résolu les aspects matériau des ruptures de Modes II et III dans les joints collés, pour une série d'adhésif, en utilisant des éprouvettes d'essai de flexion double cantilever et la microscopie électronique à balayage. On a fait varier l'épaisseur de colle entre une limite minimale correspondant à une faible fraction de la zone d'endommagement à l'extrémité d'une fissure naturelle, et une valeur suffisamment grande pour provoquer une rupture globale.Pour les deux modes, le processus de rupture démarre avec le développement de microfissures de traction en avant de l'extrémité de la fissure, et se poursuit avec l'étalement de cet endommagement au travers des ligaments interfaciaux des microfissures, et avec un cédage intralaminaire de la colle dont l'intensité augmente avec la ductilité de celle-ci. La croissance de la fissure se produit lorsque la zone endommagée atteint une maturité complète, et à ce stade prend place une fissuration stable lorsque s'accroit la charge.Mais la constatation la plus intéressante est peut-être que les composantes de cisaillement de la rupture en amont et antiplanaires coïncident, quelles que soit les propriétés mécaniques de la colle, sous l'angle énergétique et morphologique. Le travail de rupture en cisaillement G _sc fait montre d'un plateau suivi d'une phase de diminution régulière, lorsque décroit l'épaisseur t de l'adhésif. Lorsque t tend vers zero, le travail de rupture en cisaillement et le travail d'ouverture convergent vers une valeur intrinsèque commune G _c , qui n'est qu'une petite fraction de la ténacité globale du matériau, telle qu'elle est identifiée par la valeur plateau de G _sc .

     

Investigation of crystallographic properties of thin germanium crystals grown on silicon substrates by chemical vapor deposition

The quality of germanium crystals deposited from the gas phase by chemical vapor deposition using pyrolytic decomposition of GeH₄ in a helium atmosphere at various growth temperatures on silicon substrates was determined.X-ray diffraction and rocking curve measurements proved to be very useful in determining the degree of preferred orientation of the germanium deposit, since they distinguish clearly between the reflections of the thin germanium deposited layer and those of the thick silicon substrate. Therefore they permit fairly accurate calculations. Laue measurements, in this case, proved to be unsatisfactory because of inadequately low resolution. The results show that a preferred orientation of germanium crystallites in the layer of up to 99.9% was reached.     

The effects of bond thickness, rate and temperature on the deformation and fracture of structural adhesives under shear loading

The deformation and fracture in shear of a structural adhesive undergoing large-scale yielding is studied as a function of bond thickness, h, temperature, T, and strain rate using the Napkin Ring specimen. The lack of edges in this test, and the fact that the strain rate can be locally controlled, allow for a meaningful evaluation of the mechanical response throughout the deformation process. In accord with Airing's molecular activation model, the yield stress linearly decreases with T while logarithmically increasing with the strain rate. The ultimate shear strain, _F, is little sensitive to rate while decreasing with h and increasing with T. Some complementary fracture tests are carried out using the ENF bond specimen in order to explore the relation between the mechanical properties of the nominally unflawed adhesive and the mode II fracture energy, G _IIC. For sufficiently thin bonds, G _IIC/h correlates well with the ultimate energy density (i.e., the area under the stress-strain curve in the Napkin Ring test), given, to a first approximation, by _YF, where _Y is the yield stress in shear. Accordingly, the fracture energy of the bond would be greatly affected by temperature, tending to a small value at the absolute as well as the glass transition temperatures while attaining a maximum in between these two extremes. Because the yield stress does not vary much with h, the variation of G _IIC with the bond thickness reflects that of _F.A large-deformation fracture analysis, based on a cohesive zone like model, is developed to account for the observed variations of _F with h. The analysis assumes that a crack preexist in the bond, either at its center or at the interface. The results suggest that the observed increase of _F with decreasing h is due mainly to two geometric effects. The first is due to the interaction of the bonding surfaces with the stress field generated by the crack and the second has to do with the probability of finding large flaws in the bond to trigger the fracture.     

On the mechanics of edge chipping from spherical indentation

Edge chipping is a basic failure mode in brittle materials which is dictated by a wealth of material and geometric variables. Here we examine the effect of indenter bluntness on chipping load and chip dimensions. Soda-lime glass and YTZP plates are subject to surface-normal loading near an edge by a W/C ball or a Vickers tool. The ball radius r is varied from 0.2 to 8.7 mm while the indent distance h is varied from several millimeters down to a few microns. Although cone cracks are a common feature under spherical indentation, the chipping event is dominated by median-radial cracks formed under the contact. The fracture behavior is characterized by a “large” indent distance regime where the median cracks progress stably up to chipping and a “small” one where they grow unstably to form a chip once initiated. Closed-form relations for chipping load P _F under spherical indentation is developed with the aid of the test data and non-dimensional arguments. While in the “large” distance regime P _F is proportional to h ^3/2 irrespective of tool bluntness, in the “small” regime P _F is proportional to r ^1/2 h ^3/4. Interestingly, the chip dimensions are virtually independent of ball radius, varying linearly with h. Beyond relevance to structural integrity, the chipping test facilitates a simple means for determining fracture toughness K _C as well as the load needed to initiate median cracks in opaque brittle materials. An attempt is made to extend the static analysis to low-velocity impact. The results show that the damage formed during the fracture process has a major influence on dynamic chipping.