Thick-film resistors are electrical composites containing ultrafine particles of ruthenate conductor (Pb,Ru,O, in the present materials) distributed in a highly modified silicate glass. We show that conductor particles remain flocced in the absence of any applied or capillary pressures, but are separated at equilibrium by a nanometer-thick film of glass. Microstructures show evidence for liquid-phase sintering, i.e., contact flattening of particles, under van der Waals attraction alone. Titania addition, which in dilute concentrations markedly increases the resistivity, decreases the temperature coefficient of resistance, and improves voltage stability and noise, is found to increase the equilibrium film thickness between particles by a few angstroms. STEM analyses show that the added titania preferentially concentrates in the silicate-rich grain boundary film, as well as at particle-glass interfaces. The roles of interparticle forces and adsorption on the glass film thickness with and without titania are discussed. The large increase in resistivity caused by titania additions is attributed to the increase in film thickness as well as to local chemical changes of two possible types. Titania enrichment within the glass film itself is expected to decrease the local ruthenium ion solubility, and this along with the possible formation of a more insulating titania-substituted surface layer on ruthenate grains will decrease the tunneling conductivity between conductor grains.
The insulin-like growth factors (IGFs) stimulate the differentiation of skeletal muscle cells. IGF binding proteins (IGFBPs), which are expressed by skeletal muscle cells, may enhance or inhibit IGF actions. To explore the role of skeletal muscle-derived IGFBPs in IGF-induced myogenesis, we compared the differentiation-inducing effects of IGF-I and des(1-3)IGF-I in rat L6E9 skeletal myoblasts. Des(1-3)IGF-I is a naturally occurring IGF-I analog with markedly reduced affinity for IGFBPs but with an affinity for the IGF-I receptor that is comparable to that for native IGF-I. We find that rat L6E9 cells produce principally IGFBP-4 and BP-6, with a minor component of IGFBP-5. Both IGFBP-4 and BP-6 accumulate during differentiation and increase further in response to IGF-I or des(1-3)IGF-I treatment. We find that an IGF-I analog with reduced affinity for IGFBPs is significantly more potent than native IGF-I in stimulating myogenesis (as assessed by myogenin messenger RNA abundance and muscle creatine kinase activity), indicating that IGFBPs expressed by skeletal muscle cells inhibit differentiation induced by IGF-I. In view of the relative abundance of IGFBP-4, its relatively high affinity for IGF-I and the low affinity of IGFBP-6 for IGF-I, it is likely that the inhibitory effect of rat skeletal muscle-derived IGFBPs on IGF-I-induced myogenesis is mediated principally by IGFBP-4.
Polydimethylsiloxane [PDMS]-modified SiO2-TiO2, glasses have been prepared via the sol-gel route. Polymer compositions varied between 17 and 67 wt% PDMS, using PDMS of molecular weights 1,700 and 36,000. Also varied was the Si/Ti ratio for a given polymer content and the nature of the Ti alkoxide.A general synthetic procedure was found which made optically clear samples. Dense monolithic structures were obtained at room temperature for all compositions. The room temperature densification is attributed to relaxation and flow in the sample due to the presence of the polymer. The effects on properties of the overall composition and molecular weight of the polymer are reported, and implications in terms of structural models are considered.
We have studied the densification and dielectric properties of sol-gel derived tantalum oxide thin films as the insulators in MIS capacitors. Hydrolysis of tantalum ethoxide is extremely rapid and goes to completion in ethanol. Condensation is also rapid, and goes to completion in toluene Multiple layers were applied by spin-coating up to thicknesses of 3000 Å before cracking of the coating during drying ensued. Densification occurs from room temperature to 450 C, with the original film thickness decreasing by about half in one hour at 450 C. No additional densification occurs upon heating to 750 C. The dielectric constant decreases from unfired samples to those fired at 450 C, and then increases on firing from 600 to 750 C. The value of the dielectric constant at 1 MHz for samples fired at 750 C for one hour is 20, similar to that of anodically grown Ta2O5. Leakage currents as low as 2 × 10−7 amp cm−2 have been measured for applied fields of 200,000 V cm−1.
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