Thermogravimetry
Encyclopedia
Thermogravimetry is a branch of physical chemistry
, materials research, and thermal analysis
. It is based on continuous recording of mass changes of a sample of material, as a function of a combination of temperature with time, and additionally of pressure and gas composition.
, also called thermobalance where that arm and the sample are placed in a furnace
. The furnace temperature is controlled in a pre-programmed temperature/time profile (most commonly), or in the rate-controlled mode, where the pre-programmed value of the weight changes imposes the temperature change in the way necessary to achieve and maintain the desired weight-change rate. The most common temperature profiles are: jumping to isotherm and holding there for a specified time ("soak"); temperature ramping at constant rate (linear heating or cooling); and combination of ramp and soak segments. The profile "ORTA" ("oscillation-rate thermal analysis") is used in other methods of thermal analysis
, but not in TG, due to unavoidable disturbance forces. The rate-controlled method is very time-efficient, but for some types of materials it produces incorrect results or "ghost effects"; since this method does not reveal the automatically imposed temperature profile, the users may be misled by their trust for the "sophisticated, computerized program, which saves the analysis time tremendously".
The gaseous environment of the sample can be: ambient air, vacuum, inert gas, oxidizing/reducing gases, corrosive gases, carburizing gases, vapors of liquids or "self-generating atmosphere". The pressure can range from high vacuum or controlled vacuum, through ambient, to elevated and high pressure; the latter is hardly practical due to strong disturbances.
The commonly investigated processes are: thermal stability and decomposition, dehydration, oxidation, determination of volatile content and other compositional analysis, binder-burnout, high-temperature gas corrosion etc. The kinetic data obtained by TG are reliable only for irreversible processes, whereas reversible ones are grossly affected by diffusion, and only special procedures can handle them. Although many industrial processes could benefit from thermogravimetric investigations, the industry is often discouraged by the natural discrepancies between the data produced by milligram-size samples, and those of the bulk processes. In this respect gram-size and larger TG samples are more suitable for optimization research of industrial processes.
The conventional TG focuses on various aspects of ANALYSIS of materials; the other facet of thermogravimetry is studying SYNTHESIS, e.g. using a thermobalance to monitor the making of materials. The industrial processes of chemical vapor deposition
(CVD), chemical vapor infiltration
(CVI), metallurgical carburization
, synthesis of carbon-carbon composites can greatly benefit from modeling them with large-sample TG instruments.
Physical chemistry
Physical chemistry is the study of macroscopic, atomic, subatomic, and particulate phenomena in chemical systems in terms of physical laws and concepts...
, materials research, and thermal analysis
Thermal analysis
Thermal analysis is a branch of materials science where the properties of materials are studied as they change with temperature. Several methods are commonly used - these are distinguished from one another by the property which is measured:...
. It is based on continuous recording of mass changes of a sample of material, as a function of a combination of temperature with time, and additionally of pressure and gas composition.
Process
A sample of material (ranging from 1 mg to 100 mg, but sometimes as large as 100 g) is placed on an arm of a recording microbalanceMicrobalance
A microbalance is an instrument capable of making precise measurements of weight of objects of relatively small mass: of the order of a million parts of a gram. In comparison, a standard analytical balance is 100 times less sensitive; i.e. it is limited in precision to 0.1 milligrams...
, also called thermobalance where that arm and the sample are placed in a furnace
Furnace
A furnace is a device used for heating. The name derives from Latin fornax, oven.In American English and Canadian English, the term furnace on its own is generally used to describe household heating systems based on a central furnace , and sometimes as a synonym for kiln, a device used in the...
. The furnace temperature is controlled in a pre-programmed temperature/time profile (most commonly), or in the rate-controlled mode, where the pre-programmed value of the weight changes imposes the temperature change in the way necessary to achieve and maintain the desired weight-change rate. The most common temperature profiles are: jumping to isotherm and holding there for a specified time ("soak"); temperature ramping at constant rate (linear heating or cooling); and combination of ramp and soak segments. The profile "ORTA" ("oscillation-rate thermal analysis") is used in other methods of thermal analysis
Thermal analysis
Thermal analysis is a branch of materials science where the properties of materials are studied as they change with temperature. Several methods are commonly used - these are distinguished from one another by the property which is measured:...
, but not in TG, due to unavoidable disturbance forces. The rate-controlled method is very time-efficient, but for some types of materials it produces incorrect results or "ghost effects"; since this method does not reveal the automatically imposed temperature profile, the users may be misled by their trust for the "sophisticated, computerized program, which saves the analysis time tremendously".
The gaseous environment of the sample can be: ambient air, vacuum, inert gas, oxidizing/reducing gases, corrosive gases, carburizing gases, vapors of liquids or "self-generating atmosphere". The pressure can range from high vacuum or controlled vacuum, through ambient, to elevated and high pressure; the latter is hardly practical due to strong disturbances.
The commonly investigated processes are: thermal stability and decomposition, dehydration, oxidation, determination of volatile content and other compositional analysis, binder-burnout, high-temperature gas corrosion etc. The kinetic data obtained by TG are reliable only for irreversible processes, whereas reversible ones are grossly affected by diffusion, and only special procedures can handle them. Although many industrial processes could benefit from thermogravimetric investigations, the industry is often discouraged by the natural discrepancies between the data produced by milligram-size samples, and those of the bulk processes. In this respect gram-size and larger TG samples are more suitable for optimization research of industrial processes.
The conventional TG focuses on various aspects of ANALYSIS of materials; the other facet of thermogravimetry is studying SYNTHESIS, e.g. using a thermobalance to monitor the making of materials. The industrial processes of chemical vapor deposition
Chemical vapor deposition
Chemical vapor deposition is a chemical process used to produce high-purity, high-performance solid materials. The process is often used in the semiconductor industry to produce thin films. In a typical CVD process, the wafer is exposed to one or more volatile precursors, which react and/or...
(CVD), chemical vapor infiltration
Chemical vapor infiltration
Chemical vapor infiltration is a variant on Chemical Vapor Deposition . CVD implies deposition onto a surface, whereas CVI implies deposition within a body...
(CVI), metallurgical carburization
Carburization
Carburizing, spelled carburising in the UK, is a heat treatment process in which iron or steel is heated in the presence of another material which liberates carbon as it decomposes. Depending on the amount of time and temperature, the affected area can vary in carbon content...
, synthesis of carbon-carbon composites can greatly benefit from modeling them with large-sample TG instruments.