Showing papers on "Sessile drop technique published in 1991"

Wettability of SiO2 and oxidized SiC by aluminium

Abstract: The silica layer grown naturally or artificially on the surface of SiC fibres or particles used in alumina-based matrix composites is supposed to have two functions: protection of the SiC from aluminium attack and improvement of the wettability of SiC by aluminium which would result from the reaction between aluminium and SiO 2 . The effective role of silica in the wetting of aluminium on SiC was studied using the sessile drop method and the immersion-emersion tensiometric technique. Aluminium contact angles were measured first on amorphous SiO 2 and then on thermally oxidized SiC monocrystals (silica layers of 10–50 nm), between 933 K and 1173 K, and under a dynamic vacuum of 10 −4 −10 −5 Pa. In the two systems it appeared that silica acts as an oxygen source which causes oxidation of liquid aluminium. As a result the wetting kinetics was slowed down and even blocked: the apparent contact angle at 973 K is very high (above 150°). At higher temperatures (above 1073 K) deoxidation of aluminium by evaporation of the alumina layer allowed a real interface to be established between the solid and the liquid. However, as the silica reduction reaction occurred before the wetting, the stationary contact angle of aluminium on SiO 2 was found to be that of aluminium on alumina, and the steady contact angle of aluminium on oxidized SiC was that on alumina (at temperatures less than 1073 K) or on SiC (at temperatures higher than 1173 K). The strong reactivity between aluminium and SiO 2 cannot be used to improve the wetting of this metal on SiC. Consequently, silica layers on SiC cannot help the incorporation of particles or the infiltration of fibres by aluminium.

Contribution to the study of reactive wetting in the CuTi/Al2O3 system

Abstract: The wetting (kinetics of spreading and stationary contact angles) of CuTi alloys on monocrystalline alumina under high vacuum, at a temperature of 1373 K, by the sessile drop technique was investigated. The morphological and chemical characteristics of the metal-ceramic interface were determined by scanning electron microscopy and microprobe analysis. When the results are analysed, three distinct effects of the Ti solute on wetting can be identified and evaluated semi-quantitatively: (a) a reduction in the solid-liquid interfacial tension by adsorption into the liquid side of the interface; (b) a reduction in this tension by formation of a TiO metallic-like oxide layer in the solid side of the interface; (c) a contribution to the wetting driving force due to the free energy released at the interface by the reaction between Ti and Al2O3.

Interfacial phenomena in the solidification processing of metal matrix composites

Abstract: Capillary phenomena pertinent to solidification processing of metal matrix composite materials are reviewed. Recent research on the design of wettability measurement methods that are pertinent to metal matrix composite fabrication and overcome limitations of the sessile drop technique is presented. The discussion also addresses non-chemical aspects of wetting of a reinforcement by liquid metal, with emphasis on the infiltration process, and reviews engineering approaches for improved wettability and control of interface microstructure.

Wetting and interfacial chemistry in liquid metal-ceramic systems

Abstract: Wetting data for liquid metal-ceramic systems are reviewed, with emphasis on experimental results obtained on oxides by the sessile drop technique. Examples are given in order to illustrate the different wetting behaviours of reactive and non-reactive pure metal-ceramic pairs. The effect of oxygen on the wettability is presented both when it acts as a dissolved element and when it causes the formation of an oxide film on the liquid metal. The influnece of alloying elements is illustrated by numerous results and discussed using a thermodynamic model. Using some examples it is shown that the general behaviours holding for wettability in metal-oxide systems are also valid for the other families of ceramics.

Comparison of different methods for contact angle measurement

Abstract: The contact angle of water on several polymer films was determined by three different methods; telescopic sessile drop, laser beam goniometry, and the Wilhelmy plate technique. The telescopic sessile drop method is the simplest, but the least accurate; whereas the laser beam goniometry compares favorably with the Wilhelmy plate in terms of accuracy, but cannot easily provide information on contact angle hysteresis.

High-Temperature Wetting of Sapphire by Aluminum

Abstract: Sessile drop studies of molten aluminum on single-crystal sapphire substrates were conducted to investigate the effects of atmosphere on contact angle, substrate reactions, and interfacial crystal growth. Unlike previous investigations performed briefly in a vacuum environment in a temperature range within 600°C of the aluminum melting point, these experiments were conducted at higher temperatures (1200° to 1600°C) and at 1-atm total pressure over longer experimental times to more closely approach equilibrium conditions. A continuously flowing buffered gas system utilizing high-purity metered mixtures of hydrogen and helium in combination with a thoria ceramic electrolyte sensor were employed to achieve variations of the oxygen partial pressure from 10−19 to 10−15 atm while continuously maintaining the total pressure at 1 atm. At constant temperature, it was found that neither the oxygen partial pressure nor the crystallographic orientation of the sapphire substrate had a significant effect on the observed contact angles. A continuous decrease of acute contact angles and a single reaction ring characterized the 8-h experiments without the alternating spreading and contracting behavior repeatedly reported in the literature. This phenomenon can be attributed to the lower rate of metal evaporation and interfacial reaction at the higher total gas pressure and yet extremely low oxygen partial pressure of these experiments. Profilometric analysis of sapphire substrates subsequent to the removal of the quenched sessile drops indicates a reduction in metal–solid interaction due to the closer approach to equilibrium than in previous studies. An epitaxial orientation with respect to the substrate was observed in α-alumina crystallite formation at the metal–ceramic interface. Experimental evidence suggests that it was formed by a nucleation and growth process during the cooling period.

Effect of corrugations of the three-phase line on the drop size dependence of contact angles

Abstract: Conflicting observations of the drop size dependence of contact angles have been reported in literature: while some investigators reported an increase of contact angle with an increase in drop size, others reported a decrease. The former was discussed in terms of negative line tension, and the latter was considered as an effect of positive line tension. While some investigators accept the possibility of positive as well as negative line tension, the present authors suspect that line tension, at least for solid—liquid—fluid systems, is positive. In the present paper, by developing a model to calculate the mean contact angle of a sessile drop with a corrugated three-phase line, we show that the observed increase of contact angles with the increase in drop size does not necessarily imply a negative line tension; it may well be a consequence of the corrugations of the three-phase line caused by heterogeneity of the solid surface.

The K2ZrF6 wetting process: Effect of surface chemistry on the ability of a SiC-Fiber preform to be impregnated by aluminum

Abstract: SiC-ceramic materials, either as flat substrates or porous fiber preforms, are spontaneously wetted by aluminum at 700 °C to 800 °C when they have been pretreated with an aqueous solution of K2ZrF6. The wetting enhancement effect results from exothermic chemical reactions occurring at the SiC/Al interface. The first phenomenon thought to occur is a disruption of the alumina film covering liquid aluminum due to a reaction of A12O3 with K2ZrF6. Then alumina is totally dissolved at low temperatures by potassium/aluminum mixed fluorides, giving rise to a very fluid cryolitic liquid spreading out on the surface of liquid aluminum and to the true SiC/Al interface. Simultaneously, a large evolution of heat occurs, mainly due to the reduction of K2ZrF6 by aluminum and the formation of Al3Zr. The impregnation of 2D-SiC/SiC preforms by aluminum is modeled, and the effect of both the contact angle decrease and local temperature rise on the impregnation of the preforms, e.g., by gravity casting, is established.

Wettability of silicon carbide by gold, germanium and silicon

Abstract: Knowledge of the wetting behaviour of metals with ceramics is necessary for the understanding of adhesion between metals and ceramics. Among many possible metal -ceramic combinations, the meta l -oxide systems are the most studied [1]. Silicon carbide, SiC, on the other hand, is one of the silicon-base ceramics. In this letter the "sessile drop" method was used to determine the contact angle, 0, of gold, germanium and silicon on singlecrystal SiC between their melting points and 1430 °C. From these measurements of 0 and the values of the surface tension, aLv, of the metals, the work of adhesion, W, is calculated from

The wetting, reaction and bonding of silicon nitride by Cu-Ti alloys

Abstract: The wettability and reactivity of pressureless sintered Si3N4 by powdered Cu-Ti alloy were investigated using sessile drop tests conducted in a vacuum. Bonding of Si3N4 to itself was also carried out and joint strength was evaluated by compressive shear testing. The correlation of wetting behaviour with reaction and bond strength was interpreted. The wettability of Cu-Ti alloys on Si3N4 was improved greatly by addition of titanium up to 50 wt%. However, the reaction-layer thickness was increased up to 10 wt% and thereafter decreased up to 50 wt%. We propose the dovetail model which describes the reaction-layer growth behaviour with titanium. As the titanium content was increased, it tended to form a continuous thin reaction layer which greatly improved the wettability. From metallographic and XRD analyses, TiN and Ti suicide were found in the reaction layer. The thermodynamic reaction for TiN formation was suggested to be Si3N4(s) + 4Ti (1 − sol) = 4TiN(s) + 3Si(s). Ti-silicide might be formed during cooling by the reaction with Ti and Si which had been decomposed from Si3N4, diffused to and dissolved in the liquid Cu-rich alloy. The reaction layer growth was controlled by diffusion of nitrogen or titanium in the reaction layer according to the titanium concentration. The shear strength of Si3N4 to Si3N4 was affected by the morphology and thickness of the reaction layer rather than the wettability. As the titanium content increased, shear strength also increased rapidly up to 5 wt% and then slowly up to 50 wt%. As the reaction temperature and time were increased, shear strength was lowered due to the greater thickness of the reaction layer despite improved wettability.

The Effect of Melt Composition on the Wetting Angle Between Silicate Melts and Olivine

Abstract: The wetting angle between silicate melts containing Ca, Li, Na, or K and olivine single crystals have been measured as part of an investigation of the dependence of the solid-liquid interfacial energy on melt composition and olivine orientation. The wetting angle increases with increasing silica content of the melt on (100) surfaces, but decreases with increasing silica content on (010) and (001) surfaces. For a given silica content, the wetting angle on (100) decreases in going from Ca to Li to Na to K, while the wetting angle on (010) and (001) increases in going from Ca to K-bearing melts. Based on published values for liquid-vapor interfacial energies, the observed changes in wetting angle with changes in melt composition indicate that the solid-liquid interfacial energy increases with increasing silica content of the melt for the (100) surface. However, for (010) and (001) surfaces, the variation of the solid-liquid interfacial energy with silica content depends upon whether Ca or K is present in the melt. In addition, the solid-liquid interfacial energy depends upon the orientation of the olivine in the following manner: γ sl (010) ⩽γ sl (001) <γ sl (100) .

Wettability and spreading kinetics of liquid aluminium on boron nitride

Abstract: The wettability and spreading kinetics of liquid AI on CVD-BN were investigated by the sessile drop method in a vacuum of about 1.1×10−3 Pa at 1070 to 1430 K. The wettability on the Al-BN system was different from that of the Al-SiC system reported in the literature. The wetting angle of Al-BN linearly decreased with an increase of temperature in high temperature range, and tended towards 15° at 1430 K. Complete wetting could be obtained at about 1470 K. The interface energy between liquid Al and the reaction layer and the surface energy of CVD-BN were calculated by means of Warren thermodynamics analysis. The surface of CVD-BN was examined by XRD. The results show that the surface of CVD-BN was chiefly composed of those low energy planes parallel to (001). According to the relationship of the spreading kinetics from the experiments, the apparent activation energy of the liquid Al spreading on the surface of CVD-BN was calculated. The result shows that the activation energy of interfacial reaction accounts for about 54% of the spread activation energy. The spread of liquid Al on CVD-BN was affected by the interfacial reaction, diffusion and liquid movement on the surface.

Shape of a small sessile drop and the determination of contact angle

Abstract: Free liquid surfaces in equilibrium are described by the Laplace capillary equation with suitable boundary conditions generally given in terms of the contact angle. By a fortuitous formulation in the axisymmetric case, the second order ordinary differential equation can be reduced to a pair of coupled first-order equations. For the case of a small liquid drop, the present formulation allows perturbation solutions to second order to be derived in closed form. Furthermore the solutions obtained can be used to calculate contact angles, if the height and maximum width of the drop is known, the method being equally simple whether the contact angle is less than or greater than 90°.

Wetting characteristics and interfacial reaction of liquid aluminium on hot-pressed boron nitride substrate

Abstract: The wettability and interfacial reaction between liquid aluminium and hot-pressed (HP) boron nitride were investigated by the sessile drop method, electron probe microanalysis and X-ray diffraction. The results show that the interfacial reaction and diffusion of liquid aluminium on the ceramic surface promote wetting of the ceramic. Interface analysis shows that liquid aluminium can react intensely with HP boron nitride at high temperature, and there exists a nitrogen-rich layer near the aluminium and a pile-up of boron near the ceramic. After the experiment at 1435 K, the surface of the reaction layer was completely covered by aluminium nitride.

The wettability of carbon/TiB2 composite materials by aluminum in cryolite melts

Abstract: Both pure TiB2 and carbon/TiB2 composites are potential cathode materials for aluminum reduction cells. An important requirement for this application is that the material be wetted by aluminum in cryolite melts. A sessile drop technique combined with X-ray radiography was used to measure the contact angle formed between aluminum and pure hot-pressed TiB2, carbon/TiB2 composite, graphite, and a carbonaceous cement in cryolite melts. Pure hot-pressed TiB2 was found to be completely wetted by aluminum in cryolite melts. Graphite and the carbonaceous cement were nonwetted by aluminum in cryolite melts, the contact angles being in the range of 144 to 158 deg. The contact angle formed by aluminum on the carbon/TiB2 composite in cryolite melts exhibited time dependency. It was proposed the time dependency was due to (1) removal of contamination from the composite surface and (2) removal of aluminum from the drop. The wettability of the composite material increased as the TiB2 content increased.

Kinetics of gas-to-liquid transfer of particles in metal matrix composites

Abstract: The distribution of reinforcing particles in metal matrix composites produced by casting techniques depends on the interaction between various solid and liquid phases during transfer of particles from gas to liquid, transfer of particles from liquid to solid and during particle-particle interaction in the liquid. Only the first issue will be addressed in this paper. In order to predict particle behavior during gas-to-liquid transfer thermodynamic and kinetic models have been proposed. The total free energy change involved in the transfer of a particle from gas to liquid must be negative for this transfer to occur spontaneously. A new model based on the force balance on the particle has been developed. The governing equation includes a surface energy component which can be calculated from sessile drop experiments assuming equilibrium surface thermodynamics. The critical acceleration, acr required for particle incorporation may then be evaluated. Nevertheless it must be emphasized that sessile drop data are for vacuum, not for atmospheric environment. In the latter case, oxide films forming on the melt will influence the transfer and alter the value of the required force. In order to try to include the influence of oxide films forming on the surface of the liquid metal in the calculation of the critical acceleration, an approach involving experimental work with a two-bucket centrifugal apparatus was used. From these experiments a critical angular speed for which incorporation occurs can be determined. Thus another critical acceleration for incorporation (ωcr2R) can be calculated. If ωcr2R is equal to the previous acceleration acr the role of oxide films is negligible. If, on the contrary, there are differences they can be attributed to the presence of oxide films on the surface of the metal. Validation of the model has been carried out using data collected from sessile drop experiments and from experiments consisting of centrifuging spheroidal ceramic particles in aluminum melts. During the experiments alumia, zirconia, SiC-coated zirconia, and graphite-coated zirconia particles have been used. The basic criterion for selection of these materials was ϱP >ϱL. Calculation of the incorporation acceleration requires knowledge of wetting angles. Accordingly, sessile drop experiments were also performed to determine the wetting angles for some various metal-ceramic systems that were not available in the literature.

An automated apparatus for interfacial tension measurements by the sessile drop technique

Abstract: A fully computer controlled apparatus for interfacial tension measurements at the mercury/solution or solution/solution interface is described. The apparatus is based on the drop‐shape technique and employs a solid state video camera to take the drop profile; measurements can be accomplished as a function of interface polarization potential, temperature and solution concentration. Overall data reproducibility is better than 0.06%, with an acquisition time of about 45 s for each experimental data point.

Wettability of Pyrolytic Boron Nitride by Aluminum

Abstract: The wetting of pyrolytic boron nitride by molten 99.9999 percent pure aluminum was investigated by using the sessile drop method in a vacuum operating at approximately 660 micro-Pa at temperatures ranging from 700 to 1000 C. The equilibrium contact angle decreased with an increase in temperature. For temperatures at 900 C or less, the equilibrium contact angle was greater than 90 deg. At 1000 C a nonwetting-to-wetting transition occurred and the contact angle stabilized at 49 deg.

Surface modification of corneal contact lens with phosphoryl choline by glow discharge.

Abstract: Polymers like poly(methylmethacrylate) (PMMA) and poly(2-hydroxyethylmethacrylate) (PHEMA) are widely used in the development of hard and soft contact lenses. Cell adhesion and deposition of chemicals such as calcium, lipoproteins and mucin on the lens surface cause visual acuity which is the main problem in extended uses of contact occular lenses. Inorder to minimise the cell adhesion and other type of depositions, a method of surface modification of lens involving the use of phosphoryl choline, a phospholipid and the glow discharge technique has been described. The power variation of the lenses after modification has been checked using Topcon lensometer. The possible power changes of the modified samples due to the exposure to the normal light in the laboratory, darkness, ultraviolet (U. V.) light or saline have been investigated by taking the visible and ultra violet spectra using Beckman spectrophotometer. Surface energy variations after modifications of the samples have been checked by sessile drop w...

Determination of Surface Properties of Porous Solids

Abstract: The surface area of porous solids was determined from the heat of immersion method using calorimetry. The heat of immersion of different solid powders in n-hexadecane was measured. These data agreed with the BET method. The surface structure of solids (glass, teflon) and porous solids (glass spheres, catalysts) have also been analyzed by liquid evaporation method. The liquid in a cylinder which contained fine solid powders was found to evaporate at three distinct different rates. The first evaporation rate of liquid molecules is for the bulk liquid, the second rate is for liquid molecules which are present between particles and the third rate is from the liquid molecules which are present in the pore volume of particles. In addition, the effect of some detergents on evaporation rate of sessile drop have been studied.