Citric acid has been shown to act as an agent for increasing the solubility of aluminum oxyhydroxides in aqueous solutions of high (>2.47 mol/mol) hydroxide-to-aluminum ratios. Conversely, citric acid also colloidally stabilizes particles in aqueous suspensions of aluminum-containing particles. Solutions of aluminum chloride, with and without citric acid added, were titrated with NaOH(aq), The presence and size of particles were determined using quasi-elastic light scattering. In solutions that contained no citric acid, particles formed instantaneously when NaOH(aq) was added but these were observed to rapidly diminish in size, disappearing at OH/Al ratios below 2.5 mol/mol. When the OH/Al ratio was raised beyond 2.5 by adding more NaOH(aq), suspensions of colloidally stable particles formed. Large polycations containing 13 aluminum atoms were detected by Al-27 solution NMR in citric-acid-free solutions with OH/Al ratios slightly lower than 2.5. In comparison, adding citric acid to solutions of aluminum chloride inhibited the formation of large aluminum-containing polycations. The absence of the polycations prevents or retards the subsequent formation of particles, indicating that the polycations, when present, act as seeds to the formation of new particles. Particles did not form in solutions with a citric acid/aluminum ratio of 0.8 until sufficient NaOH(aq) was added to raise the OH/Al ratio to 3.29. By comparison, lower amounts of citric acid did not prevent particles from forming but did retard the rate of growth.
The present invention provides thermal insulator composites based upon nanostructured L3-silica microparticles and polyurethane foam chem. that are both easy to process and have superior insulating properties for use in household and com. refrigeration, construction, and shipping applications. The composite material retains many of the attractive processing characteristics of polyurethane foams such as vol. expansion and shape-filling during polymn. and demonstrates a total thermal cond. between 32% and 44% that of com. available polyurethane foams. [on SciFinder(R)]
The patterning of ceramic thin films is of great interest for use in MEMS and other applications as discussed in the chapters by Maeda et al. and by Muralt and Baborowski. However, the complex chemistries of certain materials make the use of traditional photolithography techniques prohibitive. In this chapter, a number of low-cost, high throughput techniques for the patterrning of ceramic thin films derived from chemical solution precursors, such as sol-gels and ceramic slurries, are presented. Most of these methods are derived from soft lithographic methods using elastomer molds, a method that is categorized as the next generation lithography in the chapter by Alexe et al. Two categories of techniques are discussed: first, the focus is on methods that rely on the principles of confinement within the physical features of the mold to define the pattern on the substracte surface. Then, subtractive patterning techniques that rely on transferring a pattern to a spin-cast large-area continuous thin film are described. Most techniques have been demonstrated with fidelities on the order of 100 nm; however, their inability to precisely register and align the patterns as part of a hierarchical fabrication scheme has hindered their commercial implementation thus far. This chaper has been updated from the original manuscript to reflect the most recent available literature and complements the chapter by Baborowshi on pattern formation by micromachining techniques.
A new patterning technique for the deposition of sol-gels and chemical solution precursors was developed to address some of the limitations of soft lithography approaches. When using micromolding in capillaries to pattern precursors that exhibit large amounts of shrinkage during drying, topographical distortions develop. In place of patterning the elastomeric mold, the network of capillary channels was patterned directly into the substrate surface and an elastomer membrane is used to complete the channels. When the wetting properties of the substrate surfaces were carefully controlled using self-assembled monolayers (SAMs), lead zirconate titanate thin films with nearly rectangular cross-sections were successfully patterned. This technique, called microchannel molding (mu CM), also provided a method for aligning multiple layers such as bottom electrodes for device fabrication.
The modern theory of dispersion forces uses macroscopic dielectric functions epsilon(omega) as a central ingredient. We reexamined the formalism and found that at separation distance < 2 nm the full dielectric function epsilon(omega,k) is needed. The use of epsilon(omega,k) results in as much as 30% reduction of the calculated Hamaker constants reported in the current literature. At larger distances, the theory reduces to the traditional method, which uses dielectric functions in the long-wavelength limit. We illustrate the formalism using the example of interaction between two graphite slabs. This example is of importance for intercalation and exfoliation of graphite and for the use of exfoliated graphite in composite materials. The formalism can also be extended to study anisotropic van der Waals interactions.