faster reaction on solid buyproduct comps

[skatastamoutra]

Is it true that metal powders (or any fuel that gives solid buyproducts upon oxitation) with oxidisers burn faster when compresed? It sounds reasonable because there is not needed any surface area between particles for the gasses to travel, because there is no gas. So tight contact results on faster reaction (i hope :p ).

[Bert]

Is it true that metal powders (or any fuel that gives solid buyproducts upon oxitation) with oxidisers burn faster when compresed?
Your question is a little too broad to answer Yes or No- An important piece of information to answer it would be if ALL the reaction products are solid, or if some are gaseous.

If some of the reaction products are gaseous, reaction will be faster if some open space between particles is present so hot gasses may transfer heat to the unreacted zone and propagate the reaction. A loose pile of black powder grains for example will burn very rapidly, while the same material compacted heavily into a single large grain will burn much more slowly as the gas cannot permeate the interior and light the whole mass at once. You also must consider that the reaction products that you think of as solids may not be solids at the reaction temperature- A mix of Aluminum and Manganese dioxide for instance has Aluminum oxide and Manganese GAS as the products. It burns like BP unconfined, but will burn in an orderly progresion compressed into a single grain.

Certain military delay mixes that consist of solids only, and have only solids as reaction products may not propagate the reaction from one end of a mass to the other unless they are well consolidated, as you proposed. The degree of compaction will affect the speed of the reactions propagation in such a case- These mixes are rarely encountered in civil pyrotechnics.

What do you want to make? If it's a metal fueled report mix, keep it uncompressed. If it's a metal fueled star, consolidate it. If it's a gasless artillery shell delay fuse mix, you allready knew the answer, eh?

[skatastamoutra]

I was working on metal powders and i experimented on a potassium nitrate - wax coated 600mesh Al mix and i noticed in some ocasions compresion led to faster deflagration.