[신간의 별자리x] 우리/미술, 그리고 ‘슬픔의 박물관’

Molecular self-assembly is the spontaneous association of molecules under equilibrium conditions into stable, structurally well-defined aggregates joined by noncovalent bonds. Molecular self-assembly is ubiquitous in biological systems and underlies the formation of a wide variety of complex biological structures. Understanding self-assembly and the associated noncovalent interactions that connect complementary interacting molecular surfaces in biological aggregates is a central concern in structural biochemistry. Self-assembly is also emerging as a new strategy in chemical synthesis, with the potential of generating nonbiological structures with dimensions of 1 to 10(2) nanometers (with molecular weights of 10(4) to 10(10) daltons). Structures in the upper part of this range of sizes are presently inaccessible through chemical synthesis, and the ability to prepare them would open a route to structures comparable in size (and perhaps complementary in function) to those that can be prepared by microlithography and other techniques of microfabrication.

Molecular self-assembly and nanochemistry: A chemical strategy for the synthesis of nanostructures

In the laser treatment of a workpiece (9), e.g. for surface hardening, melting, alloying, cladding, welding or cutting, the adverse effect of reflectivity of the surface, when a pure, monochromatic laser (6) is used, is remedied by the simultaneous application of a relatively shorter wavelength beam (1). The two beams (1)(5) may be combined by a beam coupler (4) or may reach the workpiece (9) by separate optical paths (not shown). The shorter wavelength beam (1) improves the coupling efficiency of the higher- powered laser beam (5).

Laser Material Processing

Additive manufacturing (AM) is a technology which has the potential not only to change the way of conventional industrial manufacturing processes, adding material instead of subtracting, but also to create entirely new production and business strategies. Since about three decades, AM technologies have been used to fabricate prototypes or models mostly from polymeric or metallic materials. Recently, products have been introduced into the market that cannot be produced in another way than additively. Ceramic materials are, however, not easy to process by AM technologies, as their processing requirements (in terms of feedstock and/or sintering) are very challenging. On the other hand, it can be expected that AM technologies, once successful, will have an extraordinary impact on the industrial production of ceramic components and, moreover, will open for ceramics new uses and new markets.

Additive Manufacturing of Ceramics: Issues, Potentialities, and Opportunities

Outstanding mechanical and physical properties like high thermal resistance, high hardness and chemical stability have encouraged use of structural ceramics in several applications. The brittle and hard nature of these ceramics makes them difficult to machine using conventional techniques and damage caused to the surface while machining affects efficiency of components. Laser machining has recently emerged as a potential technique for attaining high material removal rates. This review paper aims at presenting the state of the art in the field of laser machining of structural ceramics and emphasizes on experimental and computational approaches in understanding physical nature of the complex phenomena.

Laser machining of structural ceramics—A review

A low-toxicity gel system based on the polymerization of low-toxicity 2-hydroxyethyl methacrylate (HEMA) was successfully developed In order to obtain high solid loading ceramic slurry with low viscosities, a polyelectrolyte dispersant was selected The results based on alumina suggested that the polyelectrolyte dispersant was more effective for the new HEMA system than tri-ammonium citrate (TAC), which was widely used for dispersing alumina powders The green alumina bodies derived through the HEMA exhibited a mechanical strength as high as 18 MPa SEM photos revealed that the green body also had a high homogeneity Dense complex-shaped ceramic parts were produced through the new gelcasting system

Alumina gelcasting by using HEMA system

Carbon nitride films were synthesized by electrodeposition from an organic liquid on silicon substrates at temperatures lower than 80 \ifmmode^\circ\else\textdegree\fi{}C. The N/C ratio in the films is about 0.25. The x-ray diffraction measurement indicates that the film contains some ${\mathrm{C}}_{3}{\mathrm{N}}_{4}$ polycrystalline phases. The Fourier-transform infrared spectroscopy, x-ray photoelectron spectroscopy, and Raman spectroscopy analyses suggest the presence of both single and double carbon-nitrogen bonds. The deposition mechanisms are also discussed. Our paper shows that the electrodeposition technique may hold some promise for synthesizing the metastable compound ${\mathrm{C}}_{3}{\mathrm{N}}_{4}$ in the liquid phase.

Attempt to deposit carbon nitride films by electrodeposition from an organic liquid

This article presents a new lost mold rapid prototyping method which combines selective laser sintering (SLS) and gelcasting techniques for fabricating piezoelectric ceramics. SLS was used to fabricate sacrificial molds of the desired structure of the ceramic part. Then aqueous PZT (lead zirconate titanate) suspension was cast in the mold and solidified in situ through formation of a three-dimensional network gel. Because the polymer mold can be easily removed at the initial stage of sintering and the gelcast PZT body has a high green strength, the desired geometry of the PZT part can be completely retained after sintering of the ceramics. Complex-shaped PZT parts were successfully fabricated after using concentrated PZT suspension with low viscosity. Densities and electrical properties, such as the d33, the relative permittivity e, the dielectric loss tgδ and the electromechanical coupling factor Kp of the gelcast PZT parts were also compared with those of the die-pressed PZT samples. The results indicated that the gel-forming process did not deteriorate the electrical properties of the samples, if proper dispersant was selected in developing concentrated ceramic slurry.

Rapid Prototyping of Piezoelectric Ceramics via Selective Laser Sintering and Gelcasting

Environment-friendly polyvinylidene fluoride (PVDF) based composite films were fabricated by using a high permittivity lead-free Ba(Sn,Ti)O3–(Ba,Ca)TiO3 (BCTS) ceramic filler with a special ‘tricritical’ phase structure. The dielectric and energy storage performance and the microstructure evolution of the samples were investigated. Infrared spectroscopy, X-ray diffraction (XRD) and synchrotron radiation XRD results showed that the plate-shaped BCTS nanopowders induced a distinct mutual phase transition but not the usual nonpolar to polar one. Consequently, a largely increased dielectric constant (er), a higher energy density (∼4 J cm−3 at 200 kV mm−1) and a dielectric loss even lower than that of neat PVDF were simultaneously achieved with a very small loading (0.59 vol%) of BCTS. Theoretical simulations showed that er was well predicted by the effective medium theory and the modified Lichtenecker model, implying the important role of the size and shape of filler particles in performance. A special transitional stratification layer observed around the BCTS particles implies heterogeneous nucleation of the polymer. A mechanism due to the interplay of crystallization and possible stress and electrostatic interaction was proposed to explain the phase evolution. The results may provide new insight into the understanding of the structural characteristics of the composites and a new strategy for tailoring their performance.

A distinct mutual phase transition in a new PVDF based lead-free composite film with enhanced dielectric and energy storage performance and low loss

Gelcasting is a novel method for the molding of ceramic powder based on a synthesis of concepts derived from traditional ceramics and polymer chemistry. Although a wealth of literature has been published describing the application of the technology to the fabrication of structural ceramics, little has been reported about its application to the fabrication of functional ceramics. In order to extend the technology to the fabrication of piezoelectric ceramics in this article the colloidal chemistry of PZT powders in aqueous solution, the rheological behavior of the PZT suspension and the influence of dispersants on the rheology of the suspension and on the electrical properties of the gelcast PZT samples are investigated. Because the electrical properties of PZT are very sensitive to the chemical composition of the sample, unsuitable selected dispersants can result in the deterioration of the electric properties of the material although concentrated PZT slurry with low viscosity can be obtained by different dispersants.

Kai Cai

Papers

Alumina gelcasting by using HEMA system

Attempt to deposit carbon nitride films by electrodeposition from an organic liquid

Rapid Prototyping of Piezoelectric Ceramics via Selective Laser Sintering and Gelcasting

A distinct mutual phase transition in a new PVDF based lead-free composite film with enhanced dielectric and energy storage performance and low loss

Application of gelcasting to the fabrication of piezoelectric ceramic parts