Silicon Nanoparticles for Nanophase Ceramics
Nanophase particles—or nanoparticles, as they are commonly called—are particles smaller than 100 nanometers (nm). Their extremely small size gives this class of particles some unique properties. For example, on an approximately 3 nm3 particle, as many as 50 percent of the atoms are exposed on the surface, causing the particle to behave more like a gas than a solid in terms of surface reactivity. Nanoparticles also have a three-dimensional surface, which makes the surface chemistry of this class of particles very exciting.
Producing nanophase ceramic materials from nanoparticles takes advantage of the unique mechanical properties that occur in ceramics with grain sizes of less than 100 nm. For instance, nanophase ceramics can deform plastically and extensively by grain boundary slides. This is in sharp contrast to the brittle nature of conventional ceramics. Additionally, nanophase powders have extensive grain boundaries that can make them useful in cold welding applications. Nanophase ceramics can also have lower thermal conductivity, which can make them useful as thermal barrier coatings.
In general, nanophase ceramics have the potential to provide higher strength, toughness and ductility than their conventional counterparts. These distinctive properties have made nanophase ceramics an area of increasing industrial focus, and a number of major ceramic companies have recently begun research programs in this area. Ceramics based on silicon (Si) nanoparticles, in particular, hold a great deal of promise in electrical, optical, magnetic and mechanical applications. However, the development of Si-based nanophase ceramics is currently limited due to the lack of commercial quantities of Si nanoparticles. In order for Si nanoparticles to be produced on a commercial scale, a greater understanding is needed of the nanoparticles’ synthesis techniques, surface chemistry and reactivity, and handling issues.
Advanced Silicon Materials LLC (ASiMI), one of the world’s largest producers and users of silane, recently established a new research and development/pilot facility at its Butte, Mt. plant to study these issues. By gaining insight on the fundamental reaction kinetics for the large-scale production of Si nanoparticles, researchers at ASiMI hope to further advance nanophase ceramic technology.
Synthesizing Si NanoparticlesThe production of nanoparticles relies on one of three synthesis routes: 1. Mechanical (milling, grinding, alloying, etc.) 2. Thermophysical (electron beam evaporation, laser ablation, sputtering, etc.) 3. Direct chemical (gas or liquid phase) The advantage of both the mechanical and thermophysical synthesis routes is that they can be universally applied to a variety of starting materials. Most substances can be ground to a fine powder or heated until they evaporate. However, mechanical methods have difficulty in producing discrete nanophase particles in the lower size ranges. Furthermore, mechanical grinding will introduce impurities that can have deleterious effects in the final application.
The thermophysical routes usually require high vacuum for evaporation. High vacuum requires an expensive vacuum system, which can be difficult to maintain, especially when dealing with fine particles of solids and reactive gases. In addition, high vacuum systems typically have very low production rates and thus limit the commercial practicality of the method.
The third approach, direct chemical synthesis, requires a fundamental understanding of the reaction kinetics for each desired product. For example, the reaction conditions (temperature, pressure, residence time, concentration, etc.) that are useful to generate nanoparticles of Si will not necessarily apply to the production of nanoparticles of SiC or Si3N4. Therefore, the reaction kinetics are specific for each product, and a significant amount of research effort is required to understand them.
Silane is an electronics-grade gas with silicon purity level on the order of parts per billion. The majority of the silane produced at ASiMI is used to make polysilicon by a chemical vapor deposition process. After nearly 20 years of silane decomposition experience, the company has already developed a strong understanding of the reaction kinetics for the thermal decomposition of silane to silicon. With its new research facility, ASiMI intends to study the development of fundamental reaction kinetics for the production of Si nanoparticles—as well as a variety of Si-based nanoparticles, including SiC, Si3N4 and MoSi—through direct chemical synthesis.
Overcoming Other ObstaclesOnce the reaction kinetics of Si and Si-based nanoparticles are understood, a model can be developed that will allow the process to be fine-tuned. Additionally, an effective model will allow the scale-up functions to be developed to produce a wide range of particle sizes. The kinetic models can also be used as a guide to influence and control the surface chemistry. For example, by adjusting the reaction kinetics to emphasize nucleation over particle growth, one can form ultra-fine particles.
The surface chemistry of nanoparticles will be an important parameter associated with converting nanoparticles into nanophase ceramics. Since nanoparticles have a very high surface area, on the order of 25-250 m2/gm, they tend to be very reactive. By tailoring the reaction kinetics, it may be possible to adjust the surface chemical composition to a discreet surface functionality. Understanding the surface chemistry and surface reactivity can be major factors in overcoming current difficulties associated with both agglomeration of nanoparticles and forming high density green bodies prior to sintering.
In addition to synthesis issues, there can be difficulties with transportation and storage of nanoparticles due to their low bulk density and high surface reactivity. One viable supply approach is to produce and convert nanoparticles into nanophase ceramics in a continuous or semi-continuous process. The manufacturing site for nanoparticles may be advantageously located physically close to where the nanophase ceramics are produced to avoid the difficulties of isolation, packaging, transportation, storage and unpacking of the nanoparticle material.
Another limitation to producing Si nanoparticles from silane is that silane is pyrophoric; however, researchers at ASiMI have discovered ways to ensure safe handling and transportation, thereby enabling silane to successfully be used as a reactant in higher temperature and concentration ranges. This has tremendous potential to generate targeted particle size and surface functionality for Si based nanoparticles that have not been available to date.