Biomedical Engineering Reference
In-Depth Information
4.1
Synthesis of Nanopowders
4.1.1 Top-Down Approach
Many different methods are available for producing small grained
nanostructures. Mainly they are based on the production of ine
grained powders (down the nanometer scale) and a subsequent
Hot Isostatic Pressing (HIP) for consolidation. Mechanical processes
include mechanical grinding, high-energy ball milling, mechanical
alloying, and reactive milling [2, 45]. The advantages of these
techniques are that they are simple, require low-cost equipment,
and provided that a coarse feedstock powder can be made, the
powder can be processed. However, there can be dificulties such as
agglomeration of the powders and contamination from the process
equipment. Mechanical processes are commonly used only for in
organics and metals/alloys.
In high intensity ball milling, high impact collisions are used
to reduce microcrystalline materials down to nanocrystalline
structures without chemical change. A relatively new technique
termed Mechanochemical Processing (MCP) technology is a
novel solid-state process for the manufacture of a wide range of
nanopowders [31]. Dry milling is used to induce chemical reactions
through ball/powder collisions that result in nanoparticles formed
within a salt matrix. Particle size is deined by the chemistry of
the reactant mix, milling, and heat treatment conditions. Particle
agglomeration is minimized by the salt matrix, which is then
removed by a simple washing procedure.
4.1.1.1 Milling processes
In this topic, we concentrate generally on nanoprocessing
conducted entirely in a solid state. This is a class of methods based
on processes conducted in high-energy ball mills. Such methods
are based on high-energy grinding and milling of materials. They
provide top-down approach toward manufacturing nanomaterials,
bionanomaterials, and/or bionanocomposites [22, 25, 34, 35, 49].
Use of mechanical milling for synthesis of new alloys, compounds,
and nanomaterials was not envisioned until recent decades. The
primary objectives of milling have always been mixing or blending,
change in particle shape, and size reduction. The milling in ball mills
combines both crushing/shearing and impact forces combined in
various proportions, depending on the equipment used. A variety of
