Polysilazane
In organosilicon chemistry, polysilazanes are polymers in which silicon and nitrogen atoms alternate to form the basic backbone (···−Si−N−Si−N−···).
The industrial manufacture of chlorosilanes using the Müller-Rochow process, first reported in the 1940s, served as the cornerstone for the development of silazane chemistry.
[2] At this time, suitable (“pre-ceramic”) polymers heated to 1000 °C or higher were shown to split off organic groups and hydrogen and, in the process, the molecular network is rearranged to form amorphous inorganic materials that show both unique chemical and physical properties.
Linking together of these basic units can result in polymeric chains, rings or three-dimensionally crosslinked macromolecules with a wide range of molecular mass.
While the monomer unit describes the chemical composition and the connectivity of the atoms (coordination sphere) it does not illustrate the macro-molecular structure.
In this chemical reaction, different chlorosilanes react at similar rates so that the monomers are statistically distributed in the copolymer.
The patented procedure is used today by Merck KGaA, formerly AZ Electronic Materials in the manufacture of polysilazanes.
Synthesis took place in two steps: first a trichlorosilane was reacted with dimethylamine and the resulting monomeric aminosilane was separated from dimethylammoniumchloride.
The chlorine atoms liberated from the chlorosilane are tied to the trimethylsilyl groups of HMDS so that no chlorine-containing solid salts are formed.
Numerous additional procedures for the synthesis of Si–N based polymers have been described in the literature (for example a dehydrogenation coupling between Si–H and N–H or ring-opening polymerizations) but none are currently employed commercially.
Although this results in a higher price, the material is routinely used as a coating in the electronics industry due to its special properties (insulating effect in thin layers).
Silicon-nitrogen compounds with alternating silicon- ("sila") and nitrogen atoms ("aza") are designated as silazanes.
Small ring-shaped molecules with a basic network of Si-N are named cyclosilazanes (for example cyclotrisilazane [H2Si−NH]3).
In contrast to this, polysilazanes are silazane polymers consisting of both large chains and rings showing a range of molecular masses.
Solid polysilazanes are produced by chemical conversion of the liquid materials (crosslinking of smaller molecules).
After the synthesis, an aging process frequently takes place in which dissolved ammonia plays an important role.
The rate of the reaction with water (or other OH containing materials like alcohols) depends on the molecular structure of the polysilazanes and the substituents.
Perhydropolysilazane [H2Si−NH]n will decompose very quickly and exothermically with contact to water while polysilazanes with large substituents react very slowly.
Due to the organic groups that are often used to give good polymer processability, ceramic yield is normally in the range of 60-80%.
The development effort for these rather expensive chemicals is relatively high because of changing commercial availability among other things.
The “free” surface of the coating can react with humidity thereby creating a siloxane-like structure with excellent “easy to clean” properties.
They show excellent barrier properties (against water vapor or oxygen) and a low electrical conductivity.
[6][8] Since most ceramic materials are produced by powder processing and sintering, near net shape forming is very difficult for complex components.
After compounding, casting and curing, the thermoset material can be pyrolyzed in one step to give a ceramic component in high yield.
Dow Corning modified the HPZ polymer as a precursor for SiCN fibers, and Hoechst AG did successful experiments with VT50.
More recently, G. Singh at Kansas State University demonstrated synthesis of boron-modified polysilazane for synthesis of Si(B)CN functionalized carbon nanotubes, which were stable in air up to 1000 C.[9][10] The PDC-CNT composites are being explored for applications such as damage resistant coatings for high power laser thermal detectors [11][12][13] as well as Li-ion battery anodes.
