Computer-aided geometric design of the preform dies for flat film extrusion through 3-D finite element flow analysis

A rational computer-aided design procedure is presented for determining the optimum flow channel geometry of a flat film die, which yields a minimum pressure drop and produces a uniform transverse flow rate distribution. The three-dimensional die surface is generated by analytic expressions that represent a dumbbell-like contour. The die surface may be specified by several geometric parameters. The length of the transition zone turns out to be the controlling parameter. Because of the complicated geometric boundary, it is not possible to optimize the flow channel geometry explicitly; instead, a computer trial procedure is employed. The numerical computation is based on an isothermal three-dimensional flow model, which assumes power law behavior for the polymer melts. The calculated results indicate that for a particular polymer and a particular aspect ratio of slit, there may exist an optimum transition length from which the flow channel geometry of a flat film die may be uniquely defined.     

Comparison of photophysical and colloidal properties of biocompatible semiconductor nanocrystals using fluorescence correlation spectroscopy

A number of different surface chemistries have been developed in recent years to render semiconductor nanocrystals (NCs) stable in water and biocompatible. However, most of these surface modifications affect NCs' photophysical properties, calling for a method to simultaneously monitor colloidal and fluorescence properties. Fluorescence correlation spectroscopy (FCS) combined with ensemble spectroscopic methods and Monte Carlo simulations were used to interpret and derive photophysical as well as colloidal properties of four different NC surface treatments. Using a novel FCS scheme with alternating laser excitation at two different intensities, we first ruled out influences from optical gradient forces (optical trapping). We then compared concentration of emitting particles, brightness per particle, saturation intensity, blinking (intermittency), hydrodynamic radius, and propensity for aggregation of the different bioconjugated NCs. This approach was successfully applied during the development and optimization of peptide-coated NCs.     

EFFECT OF LASER BEAM RADIATION ON FATIGUE CRACK PROPAGATION IN AISI 4150 STEEL

The effect of laser beam radiation on fatigue crack growth in AISI 4150 steel was performed on compact-tension (CT) specimens, in which a composite region (CR) comprised of the hardened zones (HZs) on the top and bottom surfaces and the base metal (BM) in the interior, was aligned either along or normal to the crack growth direction. The microstructure of the HZs consisted of martensite, while lower bainite was present in the 300 °C preheated laser-hardened specimens. When the crack propagated along the laser tracks (LTs), the fatigue crack growth rates (FCGRs) of the laser-hardened specimen were lower than those of the base plate, particularly at low ΔK ranges. On the other hand, for a crack propagating normally to the LTs, decelerated FCGRs in the regions preceding the CR and accelerated FCGRs within the CR itself were found. However, enhanced FCGRs in the CR were not found in preheated specimens with a bainite structure in the HZs as the crack grew normal to the LTs. The enhancement of FCGRs in the CR, which became more accentuated at high ΔK values, was closely related to an embrittled microstructure (martensite) in the HZs.     

Many-impurity effects in a weakly interacting Kondo system: Comparison of theory with experiment

The theory of the many-impurity Kondo system developed in a previous paper is coupled with a statistical model, using a phenomenological relationship between the magnetic susceptibility and the effective Kondo temperature, to obtain the magnetic susceptibility of a very dilute Kondo system well below the single-impurity Kondo temperature. We find that the magnetic susceptibility can be represented as arising from a single-impurity effect proportional to the impurity concentration c, another term proportional to c ², as well as a much smaller c ³ contribution. The coefficient of the c ² term in has an approximate Curie law behavior at sufficiently low temperatures. The detailed temperature and concentration dependence of is in very good quantitative agreement with experiment, and is the first theoretical explanation of the anomalous magnetic susceptibility for Cu-Fe below T _K. The modification of the susceptibility arises in the theory from the inhibition of the spin-compensated state by the impurity-impurity interaction. The latter renormalizes the ln T term even in the region of temperature where the system is magnetically completely disordered.Part of this work was supported by USAFOR, under contract 73-2430 during a summer visit at Yeshiva University.     

Characteristics of heat dissipation from photovoltaic cells on the bottom wall of a horizontal cabinet to ambient natural convective air stream

This study proposes a model to investigate the behaviors of natural convective cooling of photovoltaic cells mounted discretely on the bottom wall of a horizontal cabinet. The effects of Rayleigh number (Ra), dimensionless length of cabinet (C_x), ratio of cabinet wall to air thermal conductivities (K_ef), number of photovoltaic cells (N), emissivity of metal wall (ε_e), and emissivity of glass lens (ε_g) are explored. Furthermore, the importance of thermal interaction between air streams inside and outside the cabinet through conducting wall are examined. The numerical computation domain covers the cabinet and surrounding area, so that the temperature and velocity fields of the combined regions are solved simultaneously. Results show that temperature differences among the photovoltaic cells can be up to 28% for all the investigated cases when 10⁶ ≦ Ra ≦ 10⁸, 5 ≦ C_x ≦ 12.5, 4 ≦ N ≦ 10, 1000 ≦ K_ef ≦ 6300, 0 ≦ ε_e ≦ 0.5 and 0 ≦ ε_g ≦ 0.94. The maximum difference in hot spot temperatures of photovoltaic cells is about 26% among the cases with various K_ef. In addition, the temperatures are rather low for the situation without consideration of thermal interaction between the air streams inside and outside the cabinet. Therefore, without the consideration of the thermal interaction would cause serious under-prediction for the hot spot temperatures of photovoltaic cells in engineering applications.     

Three-dimensional path line tracking and residence time distribution in fishtail dies

The flow kinematics of power-law fluids in fishtail dies is studied. A general isothermal three-dimensional finite element code developed by the authors is used for the flow analysis purpose. The basic geometry of the fishtail die is defined by simple super-elliptical curves, which allows a smooth transition from a circle to a slit. The three-dimensional path line and the residence time distribution (RTD) are calculated from the velocity field obtained from the finite element solution of the conservation equations. The effects of the rheological properties and the die geometry on the path line pattern and the residence time distribution are investigated. The results indicate that as both the length of the transition zone and the fishtail angle increases, the residence time distribution becomes more uniform. However, the power-law index does not affect the residence time distribution significantly.     

The influence of aging treatments on sulfide stress corrosion cracking of PH 13-8 Mo steel welds

Slow displacement rate tensile tests were carried out to study sulfide stress corrosion cracking (SSCC) of PH 13-8 Mo stainless steel welds in a saturated H₂S solution. The welds aged in the temperature range of 482-593 °C were susceptible to SSCC; the fracture surfaces revealed mainly quasi-cleavage fractures after notched tensile tests. However, the SSCC susceptibility in terms of the percentage loss of the notched tensile strength (NTS) of the welds was dependent on the aging treatment. The SSCC resistance and the austenite content of the welds increased with the aging temperatures. The presence of greater amounts of austenite, mainly reverted austenite, in the W1100 specimen (the weld aged at 1100 °F or 593 °C) than that in other aged specimens could account for its lower hardness and better SSCC resistance. On the other hand, the AW (as-welded) specimen containing a small amount of retained austenite films in a soft matrix exhibited a slightly improved SSCC resistance than that in the W1100 specimen. The lower hardness of the AW specimen was owing to the absence of fine coherent precipitates, leading to a reduced local stress and an enlarged plastic zone located in front of the notch in the test. With lower hardness, the local stress would also be lower and less likely to exceed some critical stress for failure in the saturated H₂S solution. For the aged specimens, the hardness/strength level and the amount of reverted austenite were the important factors that affect SSCC susceptibility.     

Magnetic domain imaging of nano‐magnetic films using magnetic force microscopy with polar and longitudinally magnetized tips

Perpendicular or parallel magnetic fields are used to magnetize the tips used in magnetic force microscopy (MFM). In this process, perpendicular or parallel magnetic dipole moments are produced on the tip plane, thus leading to the formation of polar magnetized tips (PM‐tips) or longitudinally magnetized tips (LM‐tips), respectively. The resolution of an MFM image of a magneto‐optic disk is used for calibration of these tips, and the saturated magnetic fields of the PM‐ and LM‐tips are found to be 2720 Oe and 680 Oe, respectively. Because both tips can simultaneously magnetize the sample during the scanning process when measuring a Co thin film, clear MFM images are captured, which enable the identification of magnetizable regions and the distribution of the magnetic domains on the sample surface. These results will be useful for improving the manufacturing processes required for soft nano‐magnetic film production.     

The effect of grain size and aging on hydrogen embrittlement of a maraging steel

Notched tensile tests were conducted under a slow displacement rate to evaluate the influences of grain size and aging on hydrogen embrittlement (HE) of T-200 maraging steel. In addition, an electrochemical permeation method was employed to measure the effective diffusivity (D_eff) and apparent solubility (C_app) for hydrogen of various heat-treated specimens. The results indicated that the aged (482 °C/4 h) specimens comprised of numerous precipitates led to a raised C_app and a decreased hydrogen diffusivity as compared to those of the solution-treated ones. The solution-treated specimens were resistant to gaseous HE, whereas aged specimens were susceptible to it, implying the strength level was the controlling factor to affect the HE susceptibility of the specimens. Nevertheless, all specimens suffered from sulfide stress corrosion cracking (SSCC) severely but to different degrees. The aged specimens were more likely to form intergranular (IG) fractures in H₂S but quasi-cleavage (QC) in H₂. For the solution-treated specimens, a fine-grained structure was susceptible to HE in H₂S and revealed mainly QC that differed from the IG fracture of the coarse-grained one. The fracture mode of the specimens could also be related to the transport path and / or the supply of hydrogen to the plastic zone of notched specimens in hydrogen-containing environments.