8.6 Microspheres

• model substances for calibration purposes

Later it was appreciated that monodisperse spheres could be useful in sintering and densification as a means of controlling grain size. Glass spheres produced by sol-gel are now considered to have a rather more furturistic use in inertial fuel confinemnet in nuclear fusion. I do not want to digress here into the theory of nuclear fusion, but I will differentiate between nuclear fission and nuclear fusion via a very simple explanation:

fission	the release of nuclear energy when the isotopes, generally the elements from uranium onwards (i.e., getting heavier), split to form nucleai of lighter elemts. On doing this, a quantity of energy in released.
fusion	the process whereby two nuclei of lighter elements such as hydrogen and its isotopes are brought close enough together to form a new nucleus of a heavier atom which would be helium in the case if of two hydrogen atom nuclei combining and again with teh release of energy.

The first process has been used for many years as a source of heat for conversion to electrical power, but it has many drawbacks including:

• The starting materials are isotopes that are scarce and expensive to isolate.
• The by-products of fusion are usually highly radioactive aand often toxic.
• The quantities of energy released are small compared to fusion

However, fusion is not easy to bring about in a controlled way. We have all seen the uncontrolled release of fusion energy in the hydrogen bomb. This bomb in turn requires an atomic bomb (thermonuclear device) as its detonator.

Physicists have taken two main approaches to fusion. In Britain and Europe, the projects centre very much on the generation of ultra-high temperature plasma (plasma is the collection of atomic nuclei with the electrons stripped off) and confinement of this plasma in the magnetic field in a toroid for long enough to cause a fusion reaction. The work began with the Zeta and has now advanced to the JET , (the Joint European Taurus), which it is hoped will start to provide some good results towards the turn of the century. In the USA, there has been a considerable focus on laser induced fusion. High energy pulsed lasers have the property of being able to compress matter at very highg densitites, e.g. 1000 times its normal solid density. Compression is what is needed if two nuclei are brought close enough together to fuse. The reaction is basically initiated by compression and highg temperature. During this stage, a high level of radiation is produced which increases the reaction rate and enhances the speed of the reaction. The most likely fuel to date is a mixture of the two isotopes of hydrogen; deuterium and tritium. The energy from this reaction is a form of hihg energy neutrons which can then be used as energy in a heat transfer medium such as lithium metal or in radionuclear chemical reactions to produce gaseous fuels such as hydrogen or methane.

The basic compression mechanism is one of implosion (collapsing inwards, an effect that has occasionally been seen in evacuated television tubes). The problem then is how to confine small controlled quantities of fuel in the laser beam long enough to undergo the complete reaction. For many years, glass shells or spheres have been consdiered as one of the best forms of confinement of the fuel. The principle is that the laser beam hits the outer surface of the sphere (which is a shell) for a very short period. Inside the shell is the fuel. Thermaleffects cause the ablation of the surface of the shell and the contration of the inner surface causing compression. The shell, thus, momentarily beomes a pressure vessel and so it needs to be almost perfectly spherical. The requirements are stringent:

• the diameter must lie between 100 and 500 micrometers
• wall thickness of .5 to 10 micrometers
• concentricity of inner and outer surface better than 5%
• asphericity less than 1 percent
• surface irregularies less than .5 micrometer

  There are traditional processes such as glass spheres but one of the most promising approaches has been via sol-gel. This involves the incorporation of a blowing agent in the sol prior to gelation. The solution is then gelled, dried and finally crushed to a powder of specified particle size. This powder is then introduced into the top of a heated vertical tube furnace and as the particles hit the hot zone the blowing agent expands to from microspheres. The spheres can then be filled with either a gas or a liquid by permeating the vapor into the sphere at elevated temperature which then condenses on cooling. The glass must, therefore, be permeable at elevated temperature and impermeable at room temperature.

  Vocabulary

  sintering,
  process of forming objects from a metal powder by heating the powder at a temperature below its melting point. In the production of small metal objects it is often not practical to cast them. Through chemical or mechanical procedures a fine powder of the metal can be produced. When the powder is compacted into the desired shape and heated, i.e., sintered, for up to three hours, the particles composing the powder join together to form a single solid object.