chemistry

Self-propagating high-temperature synthesis

Self-propagating high-temperature synthesis is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Self-propagating high-temperature synthesis rather than just read about it. In short: Self-propagating high-temperature synthesis (SHS) is a method for producing both inorganic and organic compounds by exothermic combustion reactions in solids of different nature. Reactions can occur between a solid reactant coupled with either a gas, liquid, or other solid.

Key takeaways

Self-propagating high-temperature synthesis belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Self-propagating high-temperature synthesis to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Self-propagating high-temperature synthesis from memory before moving on to harder problems.

Reference excerpt

Self-propagating high-temperature synthesis (SHS) is a method for producing both inorganic and organic compounds by exothermic combustion reactions in solids of different nature. Reactions can occur between a solid reactant coupled with either a gas, liquid, or other solid. If the reactants, intermediates, and products are all solids, it is known as a solid flame. If the reaction occurs between a solid reactant and a gas phase reactant, it is called infiltration combustion. Since the process occurs at high temperatures, the method is ideally suited for the production of refractory materials including powders, metallic alloys, or ceramics. The modern SHS process was reported and patented in 1971, although some SHS-like processes were known previously.

Advantages and Disadvantages Self-propagating high-temperature synthesis is a green synthesis technique that is highly energy efficient, using little if any toxic solvents. There have been environmental analysis conducted to show that SHS has a lesser environmental impact than traditional solution-phase processing techniques. The technique uses less energy for production of materials, and the energy cost savings increase as synthesis batch sizes increase. SHS is not a suitable technique for production of nanoparticles. Typically, the high-temperature nature of the process leads to particle sintering during and after the reaction. The high-temperatures generated during synthesis also lead to problems with energy dissipation and suitable reaction vessels, however, some systems use this excess heat to drive other plant-processes.

Methodology In its usual format, SHS is conducted starting from finely powdered reactants that are intimately mixed. In some cases, the reagents are finely powdered whereas in other cases, they are sintered to minimize their surface area and prevent uninitiated exothermic reactions, which can be dangerous. In other cases, the particles are mechanically activated through techniques such as high energy ball milling (e.g. in a planetary mill), which results in nanocomposite particles that contain both reactants within individual chemical cells. After reactant preparation, synthesis is initiated by point-heating of a small part (usually the top) of the sample. Once started, a wave of exothermic reaction sweeps through the remaining material. SHS has also been conducted with thin films, liquids, gases, powder–liquid systems, gas suspensions, layered systems, gas-gas systems, and others. Reactions have been conducted in a vacuum and under both inert or reactive gases. The temperature of the reaction can be moderated by the addition of inert salt that absorbs heat in the process of melting or evaporation, such as sodium chloride, or by adding "chemical oven"—a highly exothermic mixture—to decrease the ratio of cooling.

Examples The reaction of alkali metal chalcogenides (S, Se, Te) and pnictides (N, P, As) with other metal halides produce the corresponding metal chalcogenides and pnictides. The synthesis of gallium nitride from gallium triiodide and lithium nitride is illustrative:

GaI3 + Li3N → GaN + 3 LiI The process is so exothermic (ΔH = -515 kJ/mol) that the LiI evaporates, leaving a residue of GaN. With GaCl3 in place of GaI3, the reaction is so exothermic that the product GaN decomposes. Thus, the selection of the metal halide affects the success of the method. Other compounds prepared by this method include metal dichalcogenides such as MoS2. The reaction is conducted in a stainless steel reactor with excess Na2S. Self-propagating high-temperature synthesis can also be conducted in an artificial high gravity environment to control the phase composition of products. SHS has been used to vitrify various nuclear waste streams including ashes from incineration, spent inorganic ion exchangers such as clinoptilolite and contaminated soils.

Reaction Kinetics Due to the solid-state nature of SHS processes, it is possible to measure reaction kinetics in-situ using a variety of experimental techniques, including electrothermal explosion, differential thermal analysis, combustion velocity approaches, among others. There have been a variety of systems studied, including intermetallic, thermite, carbides, and others. Using SHS, it was shown that the particle size has a significant effect on the reaction kinetics. It was further shown that these effects are related to the relationship between the surface area/volume ratio of the particles, and that the kinetics can be controlled via high-energy ball-milling. Depending on the morphology of the reactants, it is possible to initiate a SHS reaction where a liquid phase occurs prior to phase formation or to directly result in solid-phase products without any melt.

References

External links ISMAN About SHS Combustion Synthesis Info by ESA Combustion Synthesis Archived 2009-12-26 at the Wayback Machine Bentham Ebook by Maximilian Lackner [2] Materials formed by SHS for needs of moon colonies.

Illustrations

Worked examples

Example 1 — a first encounter with Self-propagating high-temperature synthesis

Start with the simplest possible case. Write down what Self-propagating high-temperature synthesis claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Self-propagating high-temperature synthesis before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Self-propagating high-temperature synthesis ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Self-propagating high-temperature synthesis

In research: Self-propagating high-temperature synthesis appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Self-propagating high-temperature synthesis in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Self-propagating high-temperature synthesis is common in secondary-school and first-year university syllabi. It links to neighbouring topics Solid-state chemistry, so understanding it makes those chapters shorter.
In everyday life: Look for Self-propagating high-temperature synthesis outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Self-propagating high-temperature synthesis in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Self-propagating high-temperature synthesis means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Self-propagating high-temperature synthesis out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Self-propagating high-temperature synthesis in simple terms?

Why does Self-propagating high-temperature synthesis matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Self-propagating high-temperature synthesis?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Self-propagating high-temperature synthesis.