Thermal treatment is one of the most common material synthesis techniques. Thermal treatment (or heat treatment) is collective and general, and using such word in scientific writing could compromise accuracy and lead to ambiguity. Therefore, you might have already seen various terminologies, such as annealing, calcination, and pyrolysis, when reading the literature. This blog aims to differentiate these terms and serve as a reference for writing scientific articles.
- Calcination
International Union of Pure and Applied Chemistry (IUPAC) defines calcination as heating to high temperatures in air or oxygen. It stresses that calcination must be performed involving oxygen gas.
The verb of calcination is "calcinate" which derives from Latin calcinare, meaning "to burn limestone". Burning limestone demands air as a fuel, and therefore, calcination is associated with "in air or oxygen".
- Annealing
Annealing means heating to a high temperature and naturally cooling down. In metallurgy, annealing is an approach to dissipate internal stress in metals, to improve their malleability and ductility.
The cooling process of annealing must be performed naturally. Rapid cooling techniques, such as immersing in cold water or blow air, are collectively termed quenching.
The annealing objects have been extended beyond metals. In principle, any operations involve heating and naturally cooling could be annealing. Additionally, the atmospheres of annealing are not necessarily inert gases.
Annealing usually refers to thermal annealing; however, in polymer sciences, annealing is diverse. For example, placing block copolymers in vapors of volatile solvents (e.g., chloroform) is coined solvent vapor annealing. This process is also called annealing because its net results (microphase-separation and self-assembly of the treated block copolymers) are identical to those of thermal annealing. Therefore, the word "annealing" in polymer-related papers needs to be explicitly specified about the method involved, e.g., thermal annealing and solvent vapor annealing.
- Pyrolysis
IUPAC defines pyrolysis as thermolysis, usually associated with exposure to high temperatures. There are two notes attached to this definition:
Note 1: The term generally refers to reactions in an inert environment.
In contrast to calcination, pyrolysis needs to be performed under atmospheres that are inert to the heating objects (e.g., argon gas).
Note 2: Pyrolysis is the commonly used term for a high–temperature treatment that converts a ceramic precursor to a ceramic.
This note describes the objects of pyrolysis but is often disobeyed by writers. Both organic and inorganic materials can be coupled with pyrolysis.
From the etymology standpoint, we can further distinguish pyrolysis from annealing. Pyrolysis comes from Greek, with pyro- represents fire and -lysis means separation. Thus, pyrolysis literally means "to separate by fire" (one to multiple). Heating processes that fuse various objects (multiple to one) is annealing, or more accurately, sintering, but not pyrolysis.
Carbon generation by heating carbon-containing precursors (e.g., polymers) is a typical example of pyrolysis, and we can name this process with more specific terms—charring and carbonization.
3.1 Charring
Charring refers to heat treatments that result in mixtures of carbonaceous compounds (not just carbon). The compounds are not necessarily black but could have brown or coffee colors.
Charring is around us. For example, caramel is a charring product of sugar, which gives the color of the renowned drink, coke. Charring does not strictly follow Note 1 of pyrolysis; that is, it can occur in the air.
3.2 Carbonization
Further increasing the pyrolysis temperature leads to products of nearly 100% carbon, and this process is coined carbonization. The color of the products is always black due to the presence of carbon.
Carbonization must associate with carbon production. Pyrolysis yielding no or little carbon is not carbonization. For example, heating poly(methyl methacrylate) (PMMA) above 600°C is not carbonization, since PMMA decomposes into gaseous products before its conversion to carbon.
The boundary of temperature separating charring and carbonization, a.k.a. carbonization temperature, lacks a well-defined number, as different carbon precursors have different carbonization temperatures. According to my experience, biomasses typically has carbonization temperatures of at least 600°C.
- Carburization
Carburization is a heat treatment that introduces carbon atoms into guest materials, e.g., iron. The carbon dopants are from carbon-bearing materials, including carbon or carbonizable substances. Because carburization proceeds between solid phases, its rate is substantially slower than those of gas- or liquid-phase diffusions, and demands ultra-high temperatures (e.g., at 1500°C) to facilitate its progress.
Carburization is fundamentally different from carbonization, though the two words look quite similar. Carburization dopes carbon, while carbonization leaves carbon behind by eliminating other non-carbon components. In short, the carbon atoms of carburization are extrinsic, while those of carbonization are intrinsic.
[The author thanks Xiaozhou Yang from Virginia Tech, U.S. for his proofread of this post.]
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