EXPERIMENTAL AND NUMERICAL STUDIES OF HEAT TRANSFER THROUGH CONTACT AREAS OF HIGH ELLIPTICITY

This paper describes experimental and numerical studies of heat transfer between surfaces in contact over a highly elliptical area in the presence of a gas or in vacuum. Measurements of the steady-state thermal resistance of an elliptical contact (with a length/width ratio of 16, as predicted by Hertz's theory) formed between cylindrical steel contacting surfaces with widely different radii of curvature (76 cm. vs. 0.95 cm.) are reported and compared to numerical results obtained with a 3-D finite-difference model formulated in the ellipsoidal coordinate system. Very good agreement between experimental and numerical results is obtained. Experimental results are given for contacts with smooth and rough surfaces.     

Transcrystalline Region of Polypropylene: Its Formation, Structure and Mechanical Properties

The question of the dominant factors in the production of a transcrystalline region (TCR), an “interphase,” in crystalline polymers on solidifying in contact with a substrate is investigated for isotactic polypropylene (PP) using polarization microscopy and scanning electron microscopy. Several uncoated substrates, and the same substrates coated with certain vacuum-evaporated metals and carbon, as well as surface replicas of these made with cellulose acetate, are compared as nucleating surfaces. It is concluded that TCR formation is not dependent on the species of substrates or their surface energy but on the geometrical morphology or surface roughness. In fact, the rough surface produced by abrasion with carborundum powder creates a TCR for every substrate used. An all-transcrystalline (TC) film of PP more than 300μm thick is obtained by hot pressing the PP between two sheets of PTFE. Wide-and small-angle X-ray analysis of the film reveals a “cross-hatch” structure of lamellae. Micro-beam X-ray analysis shows that three-dimensional crystal growth occurs up to 20-30μm from the interface and, thereafter, a one-dimensional TCR grows up to the mid-plane of the film from both sides. The stress-strain behavior of the various TC films are compared with spherilitic-crystalline and quenched amorphous films. The TC film is able to undergo up to 800% elongation in spite of its almost perfect crystallinity. The mechanism of deformation is discussed on the basis of X-ray observations.     

Chemical Aging in Epoxies: A Local Study of the Interphases to Air and to Metals

The aging behavior of an epoxy adhesive (DGEBA and DETA) in the bulk surface region and in the contact region to metal substrates (Au, Cu) is studied locally. Therefore, the chemical depth profile during aging is recorded with FTIR microscopy on sample surfaces prepared with low angle microtomy. Two environmental conditions are applied at 60°C for up to 500 days to separate the role of temperature and humidity in aging.

Quantitative evaluation of the IR spectra provides the following results: in the bulk surface region, three aging mechanisms form a gradient region of more than 200 μm. Their intensity and depth profile depend on the environmental conditions. Epoxy-metal contacts are unaffected under dry conditions. Humidity is needed to form a 50 μm thick region where the copper substrate accelerates aging. Based on the experimental results, chemical aging mechanisms are discussed.     

Sequential multiple-laser-assisted polishing of free-standing CVD diamond substrates

Diamond, the best thermal conductor known, is the ultimate choice as a substrate material for the fabrication of denser, smaller and faster electronic packages. Consequently, in recent years, worldwide efforts have focused on the design of manufacturing transparent technologies for post-synthesis processing (polishing, planarization, metallization, die attach,, etc.) of diamond substrates In this study, a manufacturing-transparent, cost-effective, non-vacuum, laser-assisted coarse polishing technique for thick free-standing CVD diamond substrates was investigated [2]. The thickness of the substrates varied from 700 to 1000 μm, with the average grain size ranging from 150 to 200 μm. The average surface roughness (R_a) of the substrates, measured using contact surface profilometry, was between 20 and 30 μm. The substrates were initially irradiated with a Nd-YAG laser (λ = 532 nm) for coarse material removal, followed by an ArF excimer laser (λ = 193 nm) for finer surface finishing. Under optimized conditions, the average surface roughness (R_rma) was reduced from 25 to 5 μm with the Nd-YAG laser, and further to less than or equal to 1 μm with the excimer laser. The technique, which is the fastest processing technique known to the authors, is capable of polishing a 1 cm × 1 cm × 0.07 cm substrate in 50 s.     

WETTABILITY AND SURFACE ENERGETICS OF ROUGH FLUOROPOLYMER SURFACES

Hydrophobic solid surfaces with controlled roughness were prepared by coating glass slides with an amorphous fluoropolymer (Teflon® AF1600, DuPont) containing varying amounts of silica spheres (diameter 48 μm). Quasi-static advancing, θ_A, and receding, θ_R, contact angles were measured with the Wilhelmy technique. The contact angle hysteresis was significant but could be eliminated by subjecting the system to acoustic vibrations. Surface roughness affects all contact angles, but only the vibrated ones, θ_V, agree with the Wenzel equation. The contact angle obtained by averaging the cosines of θ_A and θ_R is a good approximation for θ_V, provided that roughness is not too large or the angles too small. Zisman's approach was employed to obtain the critical surface tension of wetting (CST) of the solid surfaces. The CST increases with roughness in accordance with Wenzel equation. Advancing, receding, and vibrated angles yield different results. The θ_A is known to be characteristic of the main hydrophobic component (the fluoropolymer). The θ_V is a better representation of the average wettability of the surface (including the presence of defects).     

Gas—solid contact in a circulating fluidized bed: The effect of particle size

A recently developed technique for measuring the efficiency of contact between gas and solid phases has been used to investigate the fundamental relationship between particle size and contact efficiency in a circulating fluidized bed. Contact efficiencies have been measured for two grades of sand (140 and 275 μm) and 71 μm FCC catalyst. The measurement technique is based on hot gas as a reacting tracer, with detection by a shrouded rapid-response thermocouple. The rate at which hot gas is thermally equilibrated with the bed is measured and this is interpreted in terms of the degree to which gas and solid come into intimate contact. The results show that, as particles become finer, contact efficiencies improve. However, this improvement is significantly less than the corresponding increase in total gas—solid surface area in the system. Finer particles are likely to form clusters or streamers more readily, and it appears that coarse particles are more surface-efficient in that they expose a greater proportion of external surface to the main gas stream. In other words, the difference in contact efficiency performance between a fine and a coarse powder is reduced by the tendency of the finer powder to aggregate more strongly.     

Atomic force microscope study of the rejection of colloids by membrane pores

An atomic force microscope (AFM) in conjunction with the colloid probe technique has been used to study the electrical double layer interactions between a 0.75 μm silica sphere and a polymeric microfiltration track etch Cyclopore membrane (nominally 1 μm) in aqueous solutions. The silica colloid probe was used to image the membrane surface (using the double layer mode) at different imaging forces in high purity water and at constant imaging force in sodium chloride solutions of different ionic strengths at pH 8. Force-distance measurements show clearly how the sphere detects the membrane surface. Quality of images produced from scanning the 0.75 μm silica particle across the surface deteriorates with increasing distance between the silica sphere and membrane surface. Such images were compared with those obtained from scanning a sharp silicon nitride tip over the membrane surface.     

The effect of bond formation on the tack of polymers

The tack of polymers to be used as adhesives is measured by a two-stage process of bond formation and bond separation. Bond formation is governed by the contact time, the contact force, the roughness of the surfaces, surface and interfacial tensions, and the mechanical or viscoelastic properties of the adhesive and substrate. This paper presents experimental studies of the contact formation of various model polymers on steel surfaces with well-defined and different degrees of roughness. The tack was measured with an instrument of the probe tack type, which determines the adhesive (interfacial) fracture energy per unit of interface as a measure of the tack and by means of which the most important parameters during bond formation and separation, such as the contact time, contact force, rate of separation, and temperature, can be adjusted and measured over sufficiently wide ranges. In the typical time interval for the contact time, the polymers are found in the plateau range of their viscoelastic spectrum. This means that entanglements strongly affect their bonding behaviour. Good agreement was found between the experimental results presented in this study and a model of contact formation on rough surfaces, published recently by Creton and Leibler [1], especially concerning the dependence of the adhesive fracture energy on the contact force and the contact time for smooth and rough substrate surfaces. The influence of the surface roughness becomes significant at low contact forces, where full contact is not yet developed on a rough substrate surface, and for polymers with comparatively high moduli. The fracture energy increases with the contact time and shows the same time dependence as the reciprocal modulus.     

Catalytic behaviour of M1, M2, and M1/M2 physical mixtures of the Mo–V–Nb–Te–oxide system in propane and propene ammoxidation

Essentially pure orthorhombic M1 and pseudo-hexagonal M2 phases were prepared using the precursor method. Consistent with literature the M1 phase was shown to be effective for propane ammoxidation to acrylonitrile while the M2 phase was essentially inert for propane activation. Both phases convert propene efficiently to acrylonitrile. Both phases show a significant selectivity dependence on the ammonia and oxygen concentrations in the feed, revealing thereby additional insights into the reaction mechanism.

Physical mixtures of the two separately prepared phases exhibited symbiosis in the ammoxidation of propane when finally divided (5 μm), thoroughly mixed and brought into intimate contact with each other. Acrylonitrile yields significantly higher than those obtained with the M1 phase alone were demonstrated with a 50 wt.% M1/50 wt.% M2 physical mixture having a corresponding surface area ratio of about 4:1. The phase cooperation effect is particularly large at high propane conversions and non-existent when the particle size of the phases is too large (e.g. >250 μm) and the inter-particle contact is poor.     

NOTE: EFFECT OF HEATING ON THE STRUCTURE OF AN ADHESIVE JOINT, AS INDICATED BY ELECTRICAL RESISTANCE MEASUREMENT

Heating from 20°C to temperatures as low as 25°C was found to cause a partly reversible effect on the structure of an adhesive (epoxy) joint involving steel adjoining (bonded) components with a surface roughness of 120 μm, as shown by monitoring the contact electrical resistivity of the joint during heating and cooling. The reversible portion was due to thermal expansion of the adhesive. The irreversible portion was due to an irreversible microstructural change in the adhesive. These structural changes decreased the extent of electrical contact between the adjoining surfaces at asperities, thereby increasing the contact resistivity.     

Particle-size properties of oilwell cements

Particle size distribution (PSD) was measured by a sedimentation method for 27 oilwell cements that met API physical and compositional specifications. Cumulative mass distribution (CMD) curves had very similar shapes when plotted as a function of particle diameter from 0.4 to 100-μm. Particles <8-μm diameter accounted for from 19 to 40% of the total mass, depending upon the class of cement, and for from 63 to 76% of the total surface area. Detection of multimodal distributions was not possible because of the magnitude of the experimental error. A very high linear correlation was found between Blaine fineness (190 to 440 m²/kg) and surface area calculated from CMD.     

Thermophoretic particle deposition efficiency in turbulent tube flow

This study investigated the thermophoretic particle deposition efficiency numerically. The critical trajectory was used to calculate thermophoretic particle deposition in turbulent tube flow. The numerical results obtained in turbulent flow regime in this study were validated by particle deposition efficiency measurements with monodisperse particles (particle diameter ranges from 0.038 to 0.498 μm) in a tube (1.18 m long, 0.43 cm i.d., stainless-steel tube). The theoretical predictions are found to fit the experimental data of Tsai et al. [Tsai, C. J., J. S. Lin, S. G. Aggarwal, and D. R. Chen, “Thermophoretic Deposition of Particles in Laminar and Turbulent Tube Flows,” Aerosol Sci. Technol., 38, 131 (2004)] very well in turbulent flows. In addition, an empirical expression has been developed to predict the thermophoretic deposition efficiency in turbulent tube flow.     

Excimer Laser Induced Removal of Particles from Hydrophilic Silicon Surfaces

A pulsed KrF excimer laser was used to remove several types of submicron-sized particles from silicon surfaces. Polystyrene latex particles, 0.1 μm and larger, were removed from silicon surfaces by dry laser cleaning (no water layer condensed on the surface) but SiO₂ particles could not be so removed. However, during steam laser cleaning, in which a thin film of water is deposited on the surface as both an energy transfer medium and an adhesion force reduction agent, these 0.1-0.2 μm SiO₂ particles were almost entirely removed. Calculations of the various forces contributing to adhesion indicate that hydrogen bonds are the major contributor to the adhesion of inorganic particles to substrate surfaces. Photoacoustic detection, using piezoelectric transducers, monitored the surface vibrations induced by the laser pulses.     

Thickness effects on structures and electrical properties of lead zirconate titanate thick films

PbZr_0.52Ti_0.48O₃ thick films with thickness of 1–6 μm have been prepared by a polymer-assisted MOD process. The polymer, poly(vinyl acetate) (PVAc) was introduced into PZT precursor solutions. The grain size increased from 30 nm to 100 nm with an increase of the additive amount of PVAc. Meanwhile, the grains grew larger (in a range of 100–500 nm) and the surface of the films became rougher with increasing film thickness. This promotes the structural relaxation and prevents cracking formation. The critical thickness at which the film begins to crack increases significantly. The dielectric constant and remanent polarization (P_r) increased from 1070 to 1490 and from 36.1 μC/cm² to 52.4 μC/cm², respectively, and the coercive field (E_c) decreased from 57.3 kV/cm to 41.3 kV/cm as the film thickness increased from 0.95 μm to 6.02 μm. PZT thick films prepared in this study are promising materials for MEMS applications.     

The short-term fate of droplets of coarse aerosol size in ultra-low-volume insecticide application on to a tropical field crop

An attempt has been made to account for the short-term fate of the entire spray emissions from three portable ultra-low-volume machines using a colorimetric technique to determine recoveries on the leaves and stems of cotton plants, the loss to the ground surface, and the aerial drift over the top of the canopy beyond the confines of the sampling area. With a spray droplet spectrum of 44 μm n.m.d./80 μm v.m.d. produced by a standard rotary atomiser, 78 and 109% of the emission were accounted for (48 and 73% on the crop) under conditions of higher and lower convective turbulence respectively. The measured recovery of a 26 μm n.m.d./42 μm v.m.d. spray from a high-speed rotary atomiser under the former conditions was only 50%, of which, the high proportion of 27% was accounted for as aerial drift beyond a distance of 15 m. It was deduced that the undetected balance of spray either convected above the upper sampling height of 3 m or that evaporation had reduced a significant proportion of the droplets to a diameter below the effective sampling limit of 10 μm. The measured recovery for a 27 μm n.m.d./59 μm v.m.d. spray from a modified mistblower was 93% under the more favourable conditions. The measured aerial drift amounted to only 5% in this case.     

Measurements of the total and regional deposition of inhaled particles in the human respiratory tract

The total and regional deposition of monodisperse aerosols in the human respiratory tract has been measured in 12 healthy subjects breathing through the mouth. Radioactively labelled polystyrene particles in the aerodynamic diameter range 3.5–10.0 μm were employed. The total deposition results are similar to those reported by Stahlhofen et al. (1980), showing only a slight progressive increase with particle size, from a mean fraction of 0.79 of the inhaled aerosol at 3.5 μm, to 0.88 for 10 μm particles. The extrathoracic airways show a very marked deposition at all sizes, predominantly in the throat. The throat values rise rapidly from a mean of 0.09 at 3.5 μm to 0.36 at 10 μm particle diameter. Two intrathoracic fractions were also obtained by the widely accepted method of measuring the relative amounts of activity cleared from the thorax as a function of time. Alveolar deposition was apparently still some 0.06 of the inhaled aerosol at 10 μm particle diameter. Tracheo-bronchial deposition showed little change at any particle size except at 3.5 μm, when it was 0.24 of the inhaled aerosol.     

The Adhesion of Irregularly-shaped 8 μm Diameter Particles to Substrates: The Contributions of Electrostatic and van der Waals Interactions

The forces needed to remove irregularly-shaped, 8 μm diameter, polyester particles from a polyester substrate were measured using an ultracentrifuge. Measurements were also made on a second set of similar particles where nanometer-size silica clusters had been placed on their surfaces. These silica clusters acted as spacers, reducing direct contact between the particle and the substrate. It was found that the separation forces for the bare particles were consistent with predictions of the JKR theory of adhesion, but were much larger than could be accounted for from simple electrostatic interactions associated with either uniformly-charged particles or particles with localized charged patches. It was found, however, that the forces needed to effect separation decreased with increasing silica concentration. For particles with 2% by weight silica clusters on their surfaces, the separation force was only about 5% of the separation forces of the bare particles. At this concentration of silica, the estimates of the separation forces obtained from JKR theory, from the uniformly-charged model, and from the localized-charged-patch model are all about equal. The numerical estimates are consistent with the experimentally-obtained values.     

Formation of low-resistance contact between titanium and lightly doped polycrystalline diamond using highly doped interlayer

A low-resistance ohmic contact between lightly doped polycrystalline diamond (poly-C) and metal was achieved for piezoresistive sensor applications using highly doped poly-C thin interlayer in the contact area for the first time for poly-C. Two Trimethylboron (TMB) doping concentrations were used during the growth of poly-C films using microwave plasma chemical vapor deposition (MPCVD), which yielded a 0.2 μm highly doped layer on top of 1.8 μm lightly doped layer. The resistivities of the highly and lightly doped poly-C layers are 0.022 and 151 Ω cm, respectively. The contacts were defined by partially etching the highly doped poly-C layer beyond the contact area. Kelvin bridges are fabricated to test the contact resistance. It is demonstrated that the contact resistivities are 0.0028 and 0.0083 Ω cm² for contacts with and without interlayer, respectively. This method reduced the contact resistance to one third of the original value and improved the performance of the piezoresistive sensor.     

Preparation and characterization of nano-sized CoMo/Al2O3 catalyst for hydrodesulfurization

CoMo on Al₂O₃ catalyst prepared by spray pyrolysis method was found in the form of spheres of 0.5–1.2 μm, which consisted of tiny primary particles of ca. 10–20 nm diameter. The materials shows comparable activity to those of commercial catalysts in HDS of straight run gas oil, in particular, refractory 4,6-dimethyldibenzothiophene (4,6-DMDBT). Weaker interactions between CoMo and Al₂O₃ are suggested by temperature-programmed reduction (TPR), Raman spectroscopy, to give more active species than those over the impregnated catalysts. This accounts for its comparable activity in spite of its smaller surface area.