Kathy Lu

Freeze casting of porous materials: review of critical factors in microstructure evolution

Abstract Freeze casting is a promising technique to fabricate porous materials with complex pore shapes and component geometries. This review is aimed to elaborate the fundamental principles of the porous microstructure evolution and critical factors that influence the fundamental physics involved in freeze casting of particulate suspensions. The discussion separately analyses homogeneous and directional freeze casting for both aqueous and non-aqueous systems. The effects of additives, freezing conditions, suspension solids loading and particle size on pore shape, size and morphology evolution are discussed. Special techniques based on modified freeze casting, such as freeze tape casting, double sided freeze casting and field directed freeze casting, are also included.

Formation mechanism of TiO2 nanotubes and their applications in photoelectrochemical water splitting and supercapacitors.

Structural observations of the transition of TiO2 nanopores into nanotubes by increasing the OH(-) concentration in the electrolyte challenge the validity of existing formation mechanisms of anodic TiO2 nanotubes. In this study, dehydration of titanium hydroxide in the cell wall is proposed as the mechanism that leads to the separation of neighboring nanotubes. Based on this understanding, bamboo-type TiO2 nanotubes with large surface area and excellent interconnectivity are achieved by cycling high and low applied potentials. After thermal treatment in a H2 atmosphere, the bamboo-type TiO2 nanotubes show large photoelectrochemical water splitting efficiency and supercapacitors performace.

Sintering of nanoceramics

Abstract There are two challenges in nanoceramic sintering: fully densifying the sintered body and maintaining the sintered grains at <100 nm size. This review examines the fundamental factors underlying nanoceramic sintering and the approaches to effectively utilise the sintering factors to advantage. Nanoceramic sintering techniques are divided into four categories: pressureless sintering, pressure sintering, electrically assisted sintering, and other sintering related techniques. Pressureless sintering has mainly evolved around modifying sintering schedules, improving nanoparticle packing characteristics, and using additives to tailor the diffusion rates. Pressure sintering, which includes hot pressing, hot isostatic pressing, and sinter forging, can effectively achieve full densification for nanostructured ceramics but microstructural inhomogeneity and sintered shape limitation are difficult to overcome. For electrically assisted sintering, many nanoceramics have been sintered to full density with spark plasma even though the atomic diffusion process is not well understood; microwave sintering can achieve fast heating but has limited ability in reaching full density or controlling grain growth. Plasma spray forming and dynamic compaction are drastically different from the mainstream sintering concepts and so are briefly reviewed. Finally, the remaining issues in nanoceramic sintering are summarised and the future directions are projected.

Hierarchically branched titania nanotubes with tailored diameters and branch numbers.

Over the past decade, electrochemical anodization of self-organized TiO(2) nanotubes has been studied intensively with the main focus being on uniform diameters along the TiO(2) nanotube depth direction. In the present work, hierarchically branched TiO(2) nanotubes with tailored diameters and branch numbers are successfully achieved by adjusting the anodization voltage. Reducing the applied voltage by a factor of 1/√n causes a one trunk nanotube to diverge into n-branched TiO(2) nanotubes, whose diameters are 1/√n of the trunk nanotube diameter (n is an integer). Multiple layers of branched TiO(2) nanotubes are also obtained by further dividing the branched nanotubes when the applied voltage is further reduced step-by-step with a 1/√n factor. Enlargement and termination of TiO(2) nanotubes occur when the anodization voltage increases by √n times. Alternating increase and decrease in the applied voltage lead to a more sophisticated hierarchical structure of TiO(2) nanotubes. The fundamental understanding of these processes is discussed.

Network structure and thermal stability study of high temperature seal glass

High temperature seal glass has stringent requirement on glass thermal stability, which is dictated by glass network structures. In this study, a SrO–La2O3–Al2O3–B2O3–SiO2 based glass system was studied using nuclear magnetic resonance, Raman spectroscopy, and x-ray diffraction for solid oxide cell application purpose. Glass structural unit neighboring environment and local ordering were evaluated. Glass network connectivity as well as silicon and boron glass former coordination were calculated for different B2O3:SiO2 ratios. Thermal stability of the borosilicate glasses was studied after thermal treatment at 850 °C. The study shows that high B2O3 content induces BO4 and SiO4 structural unit ordering, increases glass localized inhomogeneity, decreases glass network connectivity, and causes devitrification. Glass modifiers interact with either silicon- or boron-containing structural units and form different devitrified phases at different B2O3:SiO2 ratios. B2O3-free glass shows the best thermal stability a...

Influence of Patterned Concave Depth and Surface Curvature on Anodization of Titania Nanotubes and Alumina Nanopores

Vertically aligned TiO(2) nanotube and Al(2)O(3) nanopore arrays have been obtained by pattern guided anodization with uniform concave depths. There are some studies about the effect of surface curvature on the growth of Al(2)O(3) nanopores. However, the surface curvature influence on the development of TiO(2) nanotubes is seldom studied. Moreover, there is no research about the effect of heterogeneous concave depths of the guiding patterns on the anodized TiO(2) nanotube and Al(2)O(3) nanopore characteristics, such as diameter, growth direction, and termination/bifurcation. In this study, focused ion beam lithography is used to create concave patterns with heterogeneous depths on flat surfaces and with uniform depths on curved surfaces. For the former, bending and bifurcation of nanotubes/nanopores are observed after the anodization. For the latter, bifurcation of a large tube into two smaller tubes occurs on concave surfaces, while termination of existing tubes occurs on convex surfaces. The growth direction of all TiO(2) nanotubes is perpendicular to the local surface and thus is different on different facets of the same Ti foil. At the edge of the Ti foil where two facets meet, the nanotube growth direction is bent, resulting in a large stress release that causes the formation of cracks.

Novel patterns by focused ion beam guided anodization.

Focused ion beam patterning is a powerful technique for guiding the growth of ordered hexagonal porous anodic alumina. This study shows that, with the guidance of the focused ion beam patterning, hexagonal porous anodic alumina with interpore distances from 200 to 425 nm can be fabricated at 140 V in 0.3 M phosphoric acid. When the interpore distance is increased to 500 nm, alternating diameter nanopore arrays are synthesized with the creation and growth of new small pores at the junctions of three large neighboring pores. Moreover, alternating diameter nanopore arrays in hexagonal arrangement are fabricated by focused ion beam patterning guided anodization. Interpore distance is an important parameter affecting the arrangement of alternating diameter nanopore arrays. Different types of novel patterns are obtained by designing different focused ion beam concave arrays. The fundamental understanding of the process is discussed.

Unique nanopore pattern formation by focused ion beam guided anodization

The fabrication of ordered porous templates in large areas with sophisticated patterns beyond mono-sized pores in hexagonal packing remains a great technical challenge. Conventional anodization cannot overcome this limitation and a new approach needs to be sought. This study is focused on designing pore patterns in square and hexagonal arrangements via focused ion beam lithography with varying interpore distances in order to form organized pore arrays with more sophisticated patterns. The results demonstrate that the small pores from the anodization can be meshed with the larger pores from the focused ion beam lithography and unique, complicated nanopore patterns can be created. Large pore patterns that cross the grain boundaries are also made possible with the guidance of the pores from focused ion beam lithography. A fundamental pore pattern formation mechanism is proposed.

Thermal stability and electrical conductivity of carbon-enriched silicon oxycarbide

Silicon oxycarbide (SiOC) is an interesting polymer-derived system that can be tailored to embody many different properties such as lightweight, electrochemical activity, and high temperature stability. One intriguing property that has not been fully explored is the electrical conductivity for the carbon-rich SiOC compositions. In this study, a carbon-rich SiOC system is created based on the crosslinking and pyrolysis of polyhydromethylsiloxane (PHMS) and divinylbenzene (DVB) mixed precursors. The carbon-rich nature can effectively delay SiOC phase separation and crystallization into SiO2 and SiC during pyrolysis. In an oxidizing air atmosphere, the SiOC materials are stable up to 1000 °C with <0.5 wt% weight loss. Before the onset of electrical conductivity drop at ∼400 °C, the material has electrical conductivity as high as 4.28 S cm−1. In an inert argon atmosphere, the conductivity is as high as 4.64 S cm−1. This new semi-conducting behavior with high thermal stability presents promising application potential for high temperature MEMS devices, protective coatings, and bulk semi-conducting components that must endure high temperature conditions.

Effects of titania nanotube distance and arrangement during focused ion beam guided anodization

Anodic TiO2 nanotube arrays possess exciting application potentials in solar cells and photocatalysis. However, self-organized anodization alone can only produce randomly arranged nanotubes. In this study, focused ion beam (FIB) guided anodization is used to create highly ordered TiO2 nanotube arrays with different intertube distances and tube arrangements. On the one hand, FIB patterning greatly enhances the organization of the nanotubes for the classic hexagonal close packing. On the other hand, fundamentally different nanotube arrangements, such as square, oblique hexagonal, and alternating-sized patterns, are also created. The self-compensating effect enables the development of alternating-sized TiO2 nanotube hexagonal arrays from graphite lattice FIB guiding patterns. The mechanism of the FIB guided anodization is proposed. This approach presents great opportunities in producing ordered TiO2 nanotubes and new nanotube arrangements for the desired applications.