Field emission (FE) is based on the physical phenomenon of quantum tunneling, in which electrons are injected from the surface of materials into vacuum under the influence of an applied electric field. A variety of field emission cold cathode materials have been developed to date. In this review, we shall focus on several kinds of novel cold cathode materials that have been developed in the past decade. These include materials for microfabricated field-emitter arrays, diamond and related films, carbon nanotubes, other quasi one-dimensional nanomaterials and printable composite materials. In addition, cold cathode materials have a wide range of applications such as in flat panel displays, high-power vacuum electronic devices, microwave-generation devices, vacuum microelectronic devices and vacuum nanoelectronic devices. Applications are in consumer goods, military industries and also space technology. A comprehensive overview of the various applications is presented. Recently, recognizing the strong possibility that vacuum nanoelectronic devices using quasi one-dimensional nanomaterials, such as carbon nanotubes may emit electrons with driving voltages comparable to that of a solid-state device, there is a growing interest in novel applications of such devices. With such exciting opportunities, there is now a flurry of activities to explore applications far beyond those considered for the conventional hot cathodes that operate on thermionic emission. We shall discuss the details of a number of fascinating potential applications.

Novel cold cathode materials and applications

Vibrations of carbon nanotubes and their composites: a review

Carbon nanotubes – Production and industrial applications

Carbon nanotubes (CNTs) are allotropes of carbon with a nanostructure that can have a length-to-diameter ratio greater than 1,000,000. Techniques have been developed to produce nanotubes in sizeable quantities, including arc discharge, laser ablation, and chemical vapor deposition. Developments in the past few years have illustrated the potentially revolutionizing impact of nanomaterials, especially in biomedical imaging, drug delivery, biosensing, and the design of functional nanocomposites. Methods to effectively interface proteins with nanomaterials for realizing these applications continue to evolve. The high surface-to-volume ratio offered by nanoparticles resulted in the concentration of the immobilized entity being considerably higher than that afforded by other materials. There has also been an increasing interest in understanding the influence of nanomaterials on the structure and function of proteins. Various immobilization methods have been developed, and in particular, specific attachment of enzymes on carbon nanotubes has been an important focus of attention. With the growing attention paid to cascade enzymatic reaction, it is possible that multienzyme coimmobilization would be one of the next goals in the future. In this paper, we focus on advances in methodology for enzyme immobilization on carbon nanotubes.

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Carbon Nanotubes: A Review on Structure and Their Interaction with Proteins

Microwave processing has been emerging as an innovative sintering method for many traditional ceramics, advanced ceramics, specialty ceramics and ceramic composites as well as polymer and polymer composites. Development of functionally gradient materials: joining; melting; fibre drawing; reaction synthesis of ceramics; synthesis of ceramic powder, phosphor materials, whiskers, microtubes and nanotubes; sintering of zinc oxide varistors; glazing of coating surface and coating development have been performed using microwave heating. In addition, microwave energy is being explored for the sintering of metal powders also. Ceramic and metal nanopowders have been sintered in microwave. Furthermore, initiatives have been taken to process the amorphous materials (e.g. glass) by microwave heating. Besides this, attempt has been made to study the heating behaviour of materials in the electric and magnetic fields at microwave frequencies. The research is now focused on the use of microwave processing for industrial applications.

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Prospects of microwave processing: An overview

The relationship between the microwave dielectric properties and the IR active phonons of xBa(Mg1/3Ta2/3)O3–(1−x)Ba(Mg1/3Nb2/3)O3 ceramics was investigated. The IR modes were assigned, and the origin of dielectric response was determined. Among the 15 prominent IR modes, we found that the normal vibrations of the O layers and that of the Ta/Nb layers are strongly correlated to the measured dispersion parameters, such as the resonant strength (4πρ) and the damping coefficient (γ). The frequency shifts of the normal modes of the O layers and that of the Ta/Nb layers explain the linear decrease of microwave dielectric constant (K) as x increases, while the width of these modes correlate with the Q×f value.

Correlation of microwave dielectric properties and normal vibration modes of xBa(Mg1/3Ta2/3)O3-(1-x)Ba(Mg1/3Nb2/3)O3 ceramics: II. Infrared spectroscopy

The effects of liquid-phase sintering aids on the microstructures and PTCR characteristics of (Sr0.2Ba0.8)TiO3 materials have been studied. The grain size of sintered materials monotonically decreases with increasing content of Al2O3–SiO2–TiO2 (AST). The ultimate PTCR properties with ρht/ρrt as great as 105.61 are obtained for fine-grain (10-μm) samples, which contain 12.5 mol% AST and were sintered at 1350°C for 1.5 h. The quantity of liquid phase formed due to eutectic reaction between AST and (Sr,Ba)TiO3 is presumably the prime factor in determining the grain size of samples. The grains grow rapidly at the sintering temperature in the first stage until the liquid phase residing at the grain boundaries reaches certain critical thickness such that the liquid–solid interfacial energy dominates the mechanism of grain growth.

Effect of Sintering Aids on Microstructures and PTCR Characteristics of (Sr0.2Ba0.8)TiO3 Ceramics

The electrical properties of semiconducting ferrite materials are investigated. A simplified model, consisting of bulk material separated by highly resistive grain boundary layers, is proposed. The nature of grain boundary on its electrical behavior is also discussed. Complex impedance technique is used to analyze the electrical response of polycrystalline specimens. The characteristics of the complex impedance dispersion curves for MnZn‐ferrite specimens are affected by the sintering and annealing conditions. All the complex impedance plots differ from idealized semicircles, but can be fitted by circular arcs very well. An experimental equivalent circuit is proposed to explain the observed R‐X dispersion. The tremendous increase in the grain boundary resistivity due to controlled‐atmosphere sintering is ascribed to the preferential oxidation. The decrease in the apparent resistance of the specimens in the high‐frequency region arises from the polarization effect. This is attributed to the special microstructure of such material, namely, semiconducting bulk material with insulating grain boundaries.

Modeling of electrical response for semiconducting ferrite

Laser deposition technique has been applied to synthesize (Sr0.2Ba0.8)TiO3 thin films. The crystal structure of the films and the effect of deposition parameters on them have been systematically examined. Among the important deposition parameters, the substrate temperature and the chamber oxygen pressure affect the characteristics of the films most prominently. The crystalline phase of (Sr0.2Ba0.8)TiO3 films can only be obtained when the substrate temperature is higher than 550 °C. Furthermore, both the low deposition oxygen pressure and the high substrate temperature during deposition facilitate the formation of (100) grains. The metal‐insulator‐metal structure of the films deposited on Pt‐coated Si substrates, possesses good ferroelectric properties, which are indicated by butterfly‐shaped capacitance‐voltage characteristics and demonstrated by hysteresis loop of remanence Pr=1.56 μC/cm2 and coercivity Ec=13.5 kV/cm. The charge‐discharge test reveals that the effective dielectric constant can reach a va...

Structural and optical properties of laser deposited ferroelectric (Sr0.2Ba0.8)TiO3 thin films

The effect of sintering aids SiO2, Al2O3, and TiO2 on the microstructure and electrical properties of Sb3+‐doped BaTiO3 materials is examined. The SiO2 acts as a liquid‐phase former, which not only enhances the atomic diffusion, but also purifies the grain interior and increases the grain conductivity. The room‐temperature resistivity of the materials is lowered. An abundance of the liquid phase due to excess TiO2 will, however, result in a clustering of particles and the formation of a nonuniform microstructure. The Al3+ ions inhibit the grain growth and lead to a small and uniform grain size distribution. These ions also act as electron traps which increase the Schottky barrier height and in turn, raise the high‐temperature resistivity of the samples.

Hsiu-Fung Cheng

Papers

Correlation of microwave dielectric properties and normal vibration modes of xBa(Mg1/3Ta2/3)O3-(1-x)Ba(Mg1/3Nb2/3)O3 ceramics: II. Infrared spectroscopy

Effect of Sintering Aids on Microstructures and PTCR Characteristics of (Sr0.2Ba0.8)TiO3 Ceramics

Modeling of electrical response for semiconducting ferrite

Structural and optical properties of laser deposited ferroelectric (Sr0.2Ba0.8)TiO3 thin films

Effect of sintering aids on the electrical properties of positive temperature coefficient of resistivity BaTiO3 ceramics