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KW - Self-propagating high-temperature synthesis (SHS)

High Temperature Synthesis

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Self-propagating high-temperature synthesis of …

The selection of materials from which certain parts of the cells were made, is also very important. Thereby, maybe the most important parts are the anodes. They need to be resistant to chlorine which is generated on the surface of the anodes. The almost ideal material for the anodes is platinum, which corrodes at a very low rate (compared to the graphite, about which I will say more later). Thanks to the negligible corrosion of platinum, at the end of the reaction one gets a solution with very little impurities, and because of that, the further refinement of the electrolyte is greatly simplified. The only drawback of platinum is its very high price, which is the only reason why I didn't use this material. There are various alternatives to this material. The most well known are lead(IV) oxide and graphite. The first of the mentioned is also used in lead batteries, and in this case, it is useful because it is relatively ressistant to corrosion, even when the eleytrolysis is done at higher temperatures (which increases the yield of the electrolysis). Because of the unavailability of anodes made of this material, I used graphite which was the most simple solution at the time. Graphite is cheap and can easily be found. Unfortunately, it has a few drawbacks - it is not completely resistant to the conditions in the cells during electrolysis, so it corrodes at a relatively high rate. That creates an additional problem, it pollutes the electrolyte, and that can create even more complications later, so it is necessary to filter the electrolyte after the process is complete. In spite of the mentioned drawbacks, by keeping the conditions as close to ideal, the corrosion of the graphite anodes can be reduced to a minimum.

Self-Propagating High Temperature Synthesis

After 160 hours have passed since the start of the electrolysis, the process is finished. The electrolyte is filtered a few times with the help of a medicinal gauze, in order to filter out larger unwanted particles. After that, the electrolyte was further filtered through cotton wool placed in a bottleneck (of a larger two-liter bottle) that was cut off. Gradually, by repetitive filtration, a yellow colored clear solution was obtained. Since the filtration was progressing at a very slow rate, I took a smaller amount of the already filtered solution, and the rest of the solution was slowly filtered for a few more hours. In the filtered solution, along with sodium chlorate, there was also some sodium hypochlorite. Because of that, the solution was heated until the boiling point was reached, and was kept at that temperature for about 15 minutes. Thanks to this step, all the sodium hypochlorite converted to more sodium chlorate (which is also the basis of the hypochlorite method of chlorate synthesis). After heating for 15 minutes, I checked the pH of the solution, and added a bit of sodium hydroxide solution so that the pH would get close to 8. If one assumes that all the NaCl passed into NaClO3, that would mean that from the starting 350 grams of sodium chloride, one could get around 627 grams of sodium chlorate, which is only possible in theory (the yield of this type of homemade cells is mostly around 50%). Although the yield of the process was surely much less than 100%, I calculated the amount of needed potassium chloride for the reaction of the ion exchange by taking into account the theoretical yield of 100%. That way, I was sure that all of the sodium chlorate passed into potassium chlorate. However, some of the potassium chloride remained unused (which is not a problem because thanks to its high solubility, it remains in the solution and doesn't cause problems when extracting potassium chlorate).

Wet Chemical and High Temperature Synthesis - Materion

synthesis temperature self high R

Applications of SVS products noted by the Soviets include abrasives, high temperature heating elements and electrodes, solid lubricants, semiconductor materials, and polishing pastes. The SVS technology has been used to apply various protective coatings to metal substrates and in such diverse applications as the production of N and P fertilizers. Some Soviet researchers envision SVS as a possible means for actually casting high temperature refractory materials. Other potential uses that could have significant implications are the production of nonferrous metal powders, the direct reduction of Fe from ferrous ores, and the smelting of high alloy, high temperature metals.

T1 - Study of self-propagating high-temperature synthesis in the Ti-TiO 2-Al-Air system and preparation of ceramic materials based on titanium nitride and corundum


Self-propagating high-temperature synthesis (SHS) of crystalline nanomaterials

The physical structure of the final product of a SVS reaction depends primarily on the ratio of the synthesis temperature to the melting temperature of the final product. Sample dimensions are of secondary importance. Powders, sintered samples, and solidified materials from a melt have all been formed in various systems. Nonpulverized-powder products arc generally similar in geometry to the initial metal powders. Particle sizes range between 10 and 500 m. Sintered products are in the form of cakes that can be easily machined. Products solidified from a melt enable finished articles of materials with high melting points to be produced during the course of the reaction. The Soviets believe that it may be possible to actually cast some refractory materials.

The high temperature developed during combustion results in almost complete transformation of the initial substances into the final product. Unreacted elements represent only 0.01 to 0.2 wt% of the product. High purity is therefore another advantage inherent in the SVS technique. The purity of the final product is essentially a function of the purity of the initial elements. Contamination normally does not occur. In fact, the high temperature of combustion provides a "self-purification" feature by evaporating those impurities that arc volatile and by removing oxide films on the metal particles by a reduction process. Titanium carbide formed by SVS typically will have an O content ranging between 0.02 and 0.2 wt%, and in most nitrides, the O content seldom rises beyond 0.5 wt%.

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Self-Propagating High-Temperature Synthesis

Our expert chemists and engineers use wet chemical and high temperature synthesis to develop new materials and produce established ones to meet our customers’ needs. Our capabilities include:

Kinetics: Self-propagating high-temperature synthesis …

Ceramic materials offer a unique combination of properties that makes them indispensable for meeting the materials requirements for many high technology applications. One of the chief military uses of ceramics is in armor systems. The combination of high hardness, high elastic modulus, and low density that some ceramics offer makes them very effective in defeating small-arms projectiles. Because their properties are retained at high temperatures, ceramics are excellent candidates for heat-engine applications.

SHS (Self-propagating High temperature Synthesis)

Large-scale production of ceramic bodies by conventional powder-consolidation methods is somewhat cumbersome because high temperature furnaces (1200° to 1800°C) and relatively long processing times (several hours) are required. As a result, it is difficult to achieve and maintain high productivity. Moreover, solid-state reactions of powder mixtures in high temperature furnaces are often incomplete, allowing unreacted substances to act as impurities and leading to poor-quality products. The Soviets recognized the shortcomings of conventional ceramic-production techniques and, in the early 1970s, initiated several programs to develop better, more cost-effective production methods. These techniques included gas-phase plating, synthesis in a low temperature plasma and shock-wave compression, as well as a new method called self-propagating high temperature synthesis, or "SVS."* The SVS method is a simple, economical process discovered by the Soviets for producing high quality ceramics and other refractory compounds through the exothermic reaction caused by the spontaneous propagation of a combustion wave through the initial reactants. This process - also called gasless combustion, high temperature chain fusion, or spin combustion - is very similar to common thermite reactions and provides the following advantages:

High temperature synthesis | Asynt

The great advantage of producing ceramic materials by the SVS method is that long processing times in high temperature furnaces arc not necessary. To make TiC, for example, powders of Ti and are mixed and press-formed into a pellet and placed in a simple, cylindrical reaction vessel, or reactor, made of stainless steel. The pellet is ignited on one end by an electrically heated coil of wire, which provides the heat impulse to initiate the chemical reaction between the Ti and in the heated surface layer. The reaction forms a combustion wave, or synthesis wave, that rapidly spreads along the axis of the cylindrical sample forming TiC (Fig. 1). The reaction continues spontaneously and is caused by the high heat released by the combustion process. Various reactor modifications can be used to achieve high or low pressure, constant pressure, and/or cryogenic temperature operation. High pressures, for example, can be obtained in cryogenic reactors from the evaporation of liquid N2. This type of reactor is depicted in Fig. 2 and is used for making ceramics such as TiN. Theoretically, the quantity of material that can be produced depends only on the size of the reactor. In early 1972, reactors of sufficient size were available to synthesize up to 10 kg of material per reaction.

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