High-pressure microwave systems

Table 8.13 Characteristics of high-pressure microwave systems...

Dynamic systems for high-pressure microwave treatment were developed much later than open-vessel systems. Operating under a high pressure reduces the flexibility afforded by working at atmospheric pressure. However, some recently developed devices allow microwave-assisted high-pressure digestion and extraction in a dynamic manner [33,34]. 

The details of the growth of the boron-doped polycrystalline diamond thin films using a high pressure microwave plasma-assisted chemical vapor deposition system (ASTeX) have been... 

Decomposition methods are usually classified as melt decompositions, wet decompositions (with liquid decomposing agents) and dry decompositions by combustion. Sample decomposition methods are varied, and involve open and closed systems (at low and high pressure), UV and thermal activation, low or high temperature, and use of conventional convective or microwave heating. Table 8.4 lists the main sample decomposition methods for trace-element determination. 

Matusiewicz [63,68] has reported the development of a high-pressure, high-temperature, focused-microwave-heated acid (HNO3) digestion system. This microwave technique requires only about 3 % of the time necessary for the thermal high-pressure (HPA) technique. The technique of microwave heating samples in sealed containers to speed up acid digestion has been in widespread use for the past few years [69,70]. 

Although microwave-heated organic reactions can be smoothly conducted in open vessels, it is often of interest to work with closed systems, especially if superheating and high-pressure conditions are desired. When working under pressure it is strongly recommended to use reactors equipped with efficient temperature feedback coupled to the power control and/or to use pressure-relief devices in the reaction vessels to avoid vessel rupture. Another potential hazard is the formation of electric arcs in the cavity [2], Closed vessels can be sealed under an inert gas atmosphere to reduce the risk of explosions. 

Instrument Company systems apply microwave heating, whereas the High Pressure Asher has a conventional heating system. 

In operation containers constructed of microwave-transparent materials, (e.g. quartz or fluoropolymers), are used to hold multiple samples inside the ultraCLAVE . The interior of the stainless steel vessel is protected by a titanium nitride or multi-layer PTFE plasma coating for complete acid and chemical resistance. Sample containers may be open or covered by a lid. After the samples are loaded (manually or robotically) the ultraCLAVE cover is lowered into place by an electric motor controlled from the system s PC. The vessel closure is engaged and secured in place to seal the ultraCLAVE for high pressure operation. 

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