Frederick F. Lange - US grants

This award supports Dr. Frederick F. Lange, a postdoc, and three graduate students from the University of California-Santa Barbara in a collaboration with Fritz Aldinger and Wolfgang Sigmond of the Institute of Non-Metallic Inorganic Materials at the University of Stuttgart. Both groups are working on developing short-range repulsive potentials for silicon nitride (Si3N4) powders to allow consolidated powder bodies to have a clay-like consistency needed for forming reliable engineering shapes. The U.S. group has already shown that short-range potentials can be created for Si3N4. The German group are investigating various chem-adsorption techniques, and are studying the processes involved with state-of-the-art methods and facilities such as Atomic Force Microscopy. Silicon nitride is the most promising structural ceramic for use in advanced heat engines.

0103514
Lange
This proposal was submitted in response to the solicitation "Nanoscale Science and Engineering" (NSF 00-119)
Surfaces can be made either super-hydrophobic or super-hydrophilic by producing a textured surface that resembles hills and valleys, provided that the wetting angle of the liquid on a flat surface of the same material is > 90 degree . The mechanism that produces the effect is based on the fact that the liquid only wets the tops of the hills and gas (air) is trapped within the valleys. Simple analytical functions have been developed for different periodic, textured surfaces to show that the wetting angle is related to the area fraction of wetted 'hills' and the wetting angle for a flat surface.
The lotus leaf is a natural example of a super-hydrophobic surface. In 1997, micron size wax bumps were discovered to produce this effect. Water drops on the lotus leaf are nearly spherical (q ~ 170 degree) and easily roll around collecting dust particles to produce a self-cleansing effect that has made the lotus plant revered for its purity. Synthetic super-hydrophobic surfaces have been produced by coating a surface with a 'rumbled' thin film and with a micromolding method. These surfaces are promising for practical applications that include rain-repellent surfaces, surfaces designed to decrease the resistance to fluid flow, and surfaces designed for selective liquid condensation.
Recently, we demonstrated that super-hydrophobic surfaces could be produced by simply dip-coating a substrate into a slurry containing small, dispersed particles. The particles were attracted to the substrate by to their opposite surface charge, relative to the substrate. We demonstrated that the area fraction and particle size can be systematically controlled and that surfaces can be textured with commerically available particles in the range of 5 nm to 300 nm. By reacting the surface with fluoroalkyltrichlorosilane molecules, a flat surface is rendered hydrophobic, and super-hydrophobic, when textured.
We observed that the super-hydrophobic effect was related to the area fraction of adsorbed silica particles and that the super-hydrophobic effect disappears when the average spacing between the spherical particles exceeds a critical value. When the particles are very small the water droplet shows absolute adherence to the surface. Both of these latter two effects can be predicted with the Laplace equation, which relates the equilibrium curvature of a meniscus to the pressure exerted by the water drop.
Systematic experiments are planned with nano-textured surfaces to study these two new phenomena in relation to specific functions derived with the Laplace equation. This will be accomplished using glass substrates that will be coated with colloidal silica particles that are commercially available in the size range of 5 nm to 300 nm. The prepared nano-textured surfaces will consist of randomly distributed silica sphere on a glass substrate. The silica particles will be fixed to the substrate by sintering. The nano-textured substrates will then be reacted with fluoroalkyltrichlorosilane molecules to ensure optimum hydrophobicity. All experiments will be conducted with deionized water on nano-textured surfaces treated with the same fluoroalkyltrichlorosilane molecules. With these constrains, the surface energy per unit area, g, and the contact angle of a flat surface, q, will be keep constant.
Spontaneous Wetting Experiments: Spontaneous wetting will occur when the Laplace pressure exceeds a critical value to produce an instability; spontaneous wetting is expected to be a strong function of both the area fraction and particle size. Experiments will be designed to determine the validity of this expected result that can be formalized as an analytical equation. These experiments will include contact angle measurements vs. drop size, area coverage and particle size (5 nm to 300 nm).
Adhesion Experiments : Tilting experiments will be carried out to measure the a) advancing and receding contacts angles and b) the critical angle for drop movement, all as a function of the drop size and the characteristics of the textured surface (area fraction and particle size). Experiments will be carried out to determine the critical drop size that can still adhere to the surface when the substrate is held up-side down; these results will be related to an analytical equation that describes the critical pressure for spontaneous de-wetting.
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The International Materials Institute (IMI) at the University of California-Santa Barbara, named the International Center for Materials Science, builds upon the extensive materials expertise at UCSB and the various materials-related Centers at the University, such as the MRSEC, the Institute for Theoretical Physics and the California NanoSystems Institute. The IMI covers a broad spectrum of materials science, including photonic and electronic materials, self-assembled materials, nanomaterials and multifunctional materials. The IMI serves as umbrella for existing and to be developed world-wide networks of collaborations, both at the individual and institutional levels. The IMI international activities focus on Asia and the Americas, with a special emphasis on developing countries in those regions, but the IMI includes participants from across the world. The IMI supports an annual visitors program that convenes leading scientists from the US and abroad for a 3-month period and includes substantial participation from scientists from developing countries in Asia and Latin America. The visitors program focuses on a central materials research theme that changes from year to year and comprises seminars, discussions and active research with UCSB faculty. In addition, the IMI provides fellowships for students, postdoctoral associates and young faculty for overseas research visits. The IMI also supports annual international workshops and an annual international summer school aimed at graduate students, postdoctoral associates, and junior researchers. This award is co-funded by the Division of Materials Research and the Office of International Science and Engineering.

NON-TECHNICAL DESCRIPTION: Zinc oxide (ZnO) single crystal films and periodic structures have potential applications that include energy saving light emitting diodes (LEDs), transparent electrodes for today?s multi-billion LED industry based on GaN alloys, and dye-sensitized solar cells. Due to its relatively high index of refraction (n = 2.05, lambda= 500 nm), and the ability to produce 2D and 3D periodic structures, ZnO has become an important candidate for photonic crystal applications that confine and direct the light used for optical communication. As an optical-electronic material, it has a wide band gap (~3.4 eV), an excitonic binding energy twice that of GaN, a high saturation velocity, a high radiation hardness, and a high optical transparency. In fact, most of these properties of ZnO are either similar or superior to those of GaN, which today is the material used for LEDs that has and will light our world at significant energy reduction. As a wide band gap semiconductor, ZnO faces the same technical hurdle as GaN prior to the innovations made by Shuji Nakamura in the early 1990?s. Due to advantages in properties, raw material availability and cost, ZnO is expected to displace GaN in the multi-billion dollar solid-state lighting industry once this technical hurdle is overcome The research will involve undergraduate interns and a graduate student. Emphasis will be placed on selecting underrepresented students.

TECHNICAL DETAILS: ZnO epitaxial films will be synthesized, under steady-state, equilibrium conditions, in a low temperature (≤ 90°C) continuous, aqueous reactor that has been constructed based on thermodynamic calculations and the retrograde solubility of ZnO. (JJ Richardson and FF Lange, ?Controlling Low Temperature Aqueous Synthesis of ZnO Part I and II,? Crystal Growth & Design, 9 [6], 2570-81 (2009)). The continuous aqueous reactor has also been used to synthesize periodic 2D and 3D nanostructures, with dimensions equivalent to the wavelength of blue to red light, with potential applications as photonic crystals. The research emphasizes the optimization of synthesis parameters, namely, pH, temperature, ammonia concentration, and additions of rate controlling growth agents. The primary goal is to use the continuous reactor to understand the point defects that produce unintentional n-type conductivity, the dislocations at low angle grain boundaries introduced by the coalescence of epitaxial nuclei, and void formation observed during heat treatments at ≥ 250°C. This understanding will, in turn, be used to optimize the optoelectronic properties of aqueous synthesized ZnO so that significant steps can be taken towards band gap engineering and intentional n- and p-type doping.