The structure factor of the lyotropic liquid-crystal cesium perflouro-octanoate--water after the material has been quenched form the isotropic phase to the nematic phase was found to display non-Porod power-law behavior in the limit of large wave vectors. Our measurements were consistent with the scaling law S(q,t)\ensuremath{\sim}${\mathit{t}}^{\mathrm{\ensuremath{-}}3/2}$${\mathit{q}}^{\mathrm{\ensuremath{-}}6}$ for bulk samples and S(q,t)\ensuremath{\sim}${\mathit{t}}^{\mathrm{\ensuremath{-}}1}$${\mathit{q}}^{\mathrm{\ensuremath{-}}4}$ for thin samples exhibiting two-dimensional behavior. Dimensional crossover was also observed.
The majority of printed circuit boards are copper clad laminates composed of fiberglass cloth impregnated with FR4 epoxy. An important factor affecting the reliability of these assemblies is the integrity of the epoxy/glass fiber interface. The goal of this work is to investigate mechanisms for the loss of adhesive strength between E-glass and FR4 epoxy upon humidity and temperature conditioning. In this paper the authors discuss the distribution of moisture between the interface region and the bulk epoxy examined by neutron reflection, and the relationship of this data to adhesive strength.
Abstract The interaction of water with a common commercial glass cloth silane finish is explored by neutron reflection. The silane coating is applied to the oxide surfaces of polished silicon wafers. Detailed profiles of D2O within the ∼80 Å silane finish layers are measured after exposure for 48 hours to a saturated D2O atmosphere at either 22°C or 80°C. The nature of the interaction of D2O with the finish layer is probed by exposing the samples to vacuum following adsorption. In both samples, the profile of adsorbed D2O is composed of at least two distinct layers: a thin (< 30 Å) D2O -rich layer adjacent to the interface, and the bulk of the film in which only a low level of D2O is present. The amount of adsorbed D2O is greater for the sample conditioned at 80°C than for the sample conditioned at 22°C. In addition, adsorbed D2O within the interfacial layer is removed more slowly during evacuation for the sample conditioned at 80°C than for the sample conditioned at 22°C. These latter two results are interpreted as indicating increased hydrolysis of siloxane bonds for the samples conditioned at 80°C. Surprisingly, after several months in vacuum the remaining D2O redistributes within the layer, accumulating within a very thin layer at the interface in both samples. The nature of this redistribution is not understood.
We studied the coarsening behavior of two lyotropic liquid-crystal systems by static light scattering. The samples were quenched from the isotropic phase into either the nematic phase or a region of coexistence between nematic and isotropic phases. In the coexistence region, we observed, in both two and three dimensions, Porod power-law tails of the scattering intensity. Such a behavior is described by S(q)\ensuremath{\sim}${\mathit{q}}^{\mathrm{\ensuremath{-}}(\mathit{d}+1)}$ in the limit of large wave vectors q, where S is the scattering intensity, q is the wave vector, and d is the dimension of the system. In addition, the nematic phases displayed novel power-law scaling behavior at large q, namely, S(q)\ensuremath{\sim}${\mathit{q}}^{\mathrm{\ensuremath{-}}\mathit{u}}$, where u=4 in two dimensions and u=6 in three dimensions. These results will be compared to recent theoretical predictions.
We have studied the freezing and melting of a number of cryogenic fluids (hydrogen, neon, oxygen, and argon) confined in porous glasses (Vycor and a silica xerogel). ac heat-capacity measurements show broadened latent-heat peaks associated with both freezing and melting at temperatures substantially below the bulk melting temperatures. Thermal cycling shows pronounced hysteresis, with melting occurring at a higher temperature than freezing. Also, the latent heat of freezing appears to be much smaller than that of melting. The hysteresis in the argon-Vycor system was studied in detail using high-resolution ultrasonic techniques which directly probe the shear modulus of the material in the pores. We found that the onset of freezing is extremely sharp, despite the random pore geometry, and that freezing continues over a range of temperatures. The freezing process is extremely irreversible, in the sense that, once the solid appears, subsequent warming does not cause it to melt until a much higher temperature. This is true even if only a small fraction of the fluid is initially frozen. The melting branch of the hysteresis loop is more nearly reversible. In order to correctly measure the latent heat of freezing in the presence of such hysteresis, a technique should be used in which cooling is performed monotonically, for example, thermal relaxation or differential scanning calorimetry.
