Thermogravimetry and Derivative Thermogravimetry 11 A THERMOGRAVIMETRY 12.D0 10. MASS n 4.00 2.0 4500 500D 5500 TEMPERATURE IN DEGREES CELSIUS DERIVATIVE THERMOGRAVIMETRY 000 -100 dm/dT -200 mg/DEG.-3.00 -40 50 450.0 5000 5800 B00 TEMPERATURE IN DEGREES CELSIUS Figure1 Typical thermogravimetry results,(A)TG curve,(B)DTG curve
Thermogravimetry and Derivative Thermogravimetry 12.00 ~ 1O.DO - 8.00 - 6.00 - 4.00 - 2.00 - 0.00 11 I I I MASS m in mg dm/dT in mg/DEG. 400.0 450.0 500 D 550.0 600 0 TEMPERATURE IN DEGREES CELSIUS B DERIVATIVE THERMOGRAVIMETRY 0.00 -1 w -2.00 -3.00 -4,w -5.00 -603 400 TEMPERATURE IN DEGREES CELSIUS Figure 1 Typical thermogravimetry results, (A) TG curve, (B) DTG curve
Chapter2 reaction starts well before the main reaction temperature.Similarly there is still some residual mass loss well after the mainr An alternative presentation of res original experimental curve to give dm/dt,or rate of mass loss against time,and to plot that against temperature,T or time,t.Alternatively the derivative may be against temperature T giving dm/dT.The production of such curves is called Derivative Thermogravimetry (DTG).Such a curve is shown in Figure 1(B);the spread of the 、 ion over a wide temperat re range appears hereasa relatively broad peak.The DTG curve is of assistance if there are overlapping reactions.Double peaks or a shoulder on a main peak appear in these cases.Slow reactions,with other fast reactions superimposed,then appear as gradient changes in the DTG curve.The area under the DTG peak is proportional to the mass loss,so relative mass losses may be compared.Measurements of relative peak just height s may suffice for some purposes.The position of the peak may not be indicative of any characteristic point in the mechanism of th reaction,only where mass loss is fastest.However,it may be used,if all that is required is to use the peak as a "finger print"of the presence of a substance in a mixture,e.g.a particular mineral in a rock or soil sample. INSTRUMENTATION Balance In the essential form of the apparatus,the substance is placed in a small inert crucible,which is attached to a microbala and has a furnac positioned around the mple. hce The f furnace may be po sitioned in severa places relative to the balance.This is shown in Figure 2 The furnace may be above(C),below (A)or around the side arm of the balance(B).A remote coupling system (D)uses magnetic coupling and ensures that the atmosphere around the sample is completely separated from the balance mechanism.The spring balance (E)is a hi storica version. ot well suited to a recordir ng syste n.The arrangement in the las system(F)has a twin furnace and crucible system to reduce buoyancy effects.The balance is often mounted in a glass envelope,sometimes a metal one.Specialised high-pressure systems use a stainless steel con- struction.Several types of balance have been used in the past,such as mon galvanon er, nd is trolled elec a magnet and moving coil system to restore balance.The control system varies the current passed through the coil to attempt to keep the beam of
12 Chapter 2 reaction starts well before the main reaction temperature. Similarly there is still some residual mass loss well after the main reaction. An alternative presentation of results is to take the derivative of the original experimental curve to give drnldt, or rate of mass loss against time, and to plot that against temperature, T or time, t. Alternatively the derivative may be against temperature T giving dm/dT. The production of such curves is called Derivative Thermogravimetry (DTG). Such a curve is shown in Figure l(B); the spread of the reaction over a wide temperature range appears here as a relatively broad peak. The DTG curve is of assistance if there are overlapping reactions. Double peaks or a shoulder on a main peak appear in these cases. Slow reactions, with other fast reactions superimposed, then appear as gradient changes in the DTG curve. The area under the DTG peak is proportional to the mass loss, so relative mass losses may be compared. Measurements of just relative peak heights may suffice for some purposes. The position of the peak may not be indicative of any characteristic point in the mechanism of the reaction, only where mass loss is fastest. However, it may be used, if all that is required is to use the peak as a “finger print” of the presence of a substance in a mixture, e.g. a particular mineral in a rock or soil sample. INSTRUMENTATION Balance In the essential form of the apparatus, the substance is placed in a small inert crucible, which is attached to a microbalance and has a furnace positioned around the sample. The furnace may be positioned in several places relative to the balance. This is shown in Figure 2. The furnace may be above (C), below (A) or around the side arm of the balance (B). A remote coupling system (D) uses magnetic coupling and ensures that the atmosphere around the sample is completely separated from the balance mechanism. The spring balance (E) is a historical version, not well suited to a recording system. The arrangement in the last system (F) has a twin furnace and crucible system to reduce buoyancy effects. The balance is often mounted in a glass envelope, sometimes a metal one. Specialised high-pressure systems use a stainless steel construction. Several types of balance have been used in the past, such as pivoted beam, cantilever beam, and torsion. The modern microbalance has a rotating pivot, as used in a galvanometer, and is controlled electronically using a zero detection device, usually a light and photocell and a magnet and moving coil system to restore balance. The control system varies the current passed through the coil to attempt to keep the beam of
Thermogravimetry and Derivative Thermogravimetry 13 肉 A 胭 肉 0 Figure2 Layouts for balance and furnace the balance in the zero position.This is known as a null deflection system and has the advantage that it keeps the sampnthesampostion in the furnace throughout the run.Dunn and Sha p2have ved the accu racy and precision of mass measurements.Early apparatus used large samples of one gram or more,but the modern tendency is to use 10-100 mg,and sometimes only 1 mg.The advantage of gram samples is that sufficient residue is left at the end of an experiment for further tests. such as surface chemical,to be carried out on it.The disadvantage is that the sample will not be at a uniform ture at any time it will decompose at different temperatures and differen rates.There is a lower limit in sample size because to read the sample mass to sufficient precision would require the microbalance to read mass to a fraction of a microgram.The electronic control system introduces random fluctuations at this level,and the balance bench on which the balance stands will introduce vibrations,which are also transmitted to the mass record The system of balance plus furnace is called a thermobalance and a typical example is shown in detail in Figure 3. Modern commercial systems will have a built-in computer system. usually to control the furnace programming and to record and process results
Thermoyravimetry and Derivative Thermoyravimetry 13 Figure 2 Layouts for balance and furnace the balance in the zero position. This is known as a null deflection system and has the advantage that it keeps the sample in the same position in the furnace throughout the run. Dunn and Sharp2 have reviewed the accuracy and precision of mass measurements. Early apparatus used large samples of one gram or more, but the modern tendency is to use lO-lOOmg, and sometimes only 1 mg. The advantage of gram samples is that sufficient residue is left at the end of an experiment for further tests, such as surface chemical, to be carried out on it. The disadvantage is that the sample will not be at a uniform temperature at any time. Therefore different parts of it will decompose at different temperatures and different rates. There is a lower limit in sample size because to read the sample mass to sufficient precision would require the microbalance to read mass to a fraction of a microgram. The electronic control system introduces random fluctuations at this level, and the balance bench on which the balance stands will introduce vibrations, which are also transmitted to the mass record. The system of balance plus furnace is called a thermobalance and a typical example is shown in detail in Figure 3. Modern commercial systems will have a built-in computer system, usually to control the furnace programming and to record and process results
Chapter 2 MICROBALANCE MASS SIGNAL m GAS IN OGUE TO VERSIO POWER. FURNACE -SAMPLE THERMOCOUPLE SAS OUT MLINGTCC Figure3 Schematic diagram of a typical thermobalance system Furnace Furnaces,intended to work up to 1100C,use resistive alloy wire or ribbon such as Kanthal or Nichrome,wound on a ceramic or silica tube. For higher temperatures,reaching 1600C,platinum or platinum/rho- dium alloy is used.The winding is coated with furnace cement to hold the wire in place firmly,because it expands greatly in length during heating The tube is th mounted in metal container a nd packed withi tion material.The modern tendency is to use smaller furnaces and insula ting them sufficiently would be diffcult.so they have a cooling water jacket around the outside to keep the outer wall at a low temperature. The temperature of a tube furnace varies along its length,and for ther- mogravimetry,it is good practice to have a long constant temperature zone in the ce re for thi reason the ing is non-uniformly. with turns packed more closely away from the centre but wider nea the centre.A furnace control is used to programme the temperature of the sample.Commercial systems available can cool samples to as low as -160 C,using liquid nitrogen,or heat up to 1600C.The programmed temperature regime is commony linear,with rates from fractionsof a aa nin-1 Moder uip nt is apable cooli g at a heating.I often offer a more complicated heating regime such as holding at a fixed temperature for a programmed time,then resuming heating
14 Chapter 2 -1 I Figure 3 Schematic diagram of a typical thermobalance system Furnace Furnaces, intended to work up to llOO°C, use resistive alloy wire or ribbon such as Kanthal or Nichrome, wound on a ceramic or silica tube. For higher temperatures, reaching 1600 "C, platinum or platinum/rhodium alloy is used. The winding is coated with furnace cement to hold the wire in place firmly, because it expands greatly in length during heating. The tube is then mounted in a metal container and packed with insulation material. The modern tendency is to use smaller furnaces and insulating them sufficiently would be difficult, so they have a cooling water jacket around the outside to keep the outer wall at a low temperature. The temperature of a tube furnace varies along its length, and for thermogravimetry, it is good practice to have a long constant temperature zone in the centre. For this reason the winding is made non-uniformly, with turns packed more closely away from the centre but wider near the centre. A furnace control is used to programme the temperature of the sample. Commercial systems available can cool samples to as low as - 16OoC, using liquid nitrogen, or heat up to 1600°C. The programmed temperature regime is commonly linear, with rates from fractions of a degree to 100°C min? Modern equipment is capable of cooling at a controlled rate as well as heating. It can often offer a more complicated heating regime such as holding at a fixed temperature for a programmed time, then resuming heating
Thermogravimetry and Derivative Thermogravimetry 15 The furnace is capable of being moved away from the balance case to allow access to the sample.A sliding support allows it to move up,down or sideways as required by the particular design.In many cases a rubber "O"ring produces a gas tight seal between the furnace and the balance case. Atmosphere Control The simplest TG experiment would be to heat the sample in static air. However,the sample may react with air in oxidising or burnir an inert gas such as nitrogen o argon I u cd.lnsomecascs,adeliberate ly chosen reactive gas is used.This could be hydrogen used to reduce an oxide to metal or carbon dioxide,which affects the decomposition of a metal carbonate.A flowing purge gas is almost always used.This is fed over the balance mechanism first,then around the sample and then out to waste.As well as mass loss by deco mposition,ther gravimetry may be use ed to follo mass gain by reaction with and uptak f th purge ga Also,physical processes such as evaporation of a liquid,sublimation of a solid and desorption of a gas from the surface of a solid may be followed. Most thermobalances will also operate under vacuum as long as all joints are efficiently sealed. Crucibles Crucibles are made of various materials.The best ones are made of platinum.These are inert with respect to most gases and molten inorganic materials,and only melt at 1769C.If expo osed to hydrogen they do chemisorb hy which might app r as a spurious v ght ga ain.The may also be cleaned in strong acid without any reaction. they are also expensive.They are made of thin platinum to keep the mass low so that they have low heat capacity and follow the furnace tempera- ture without any temperature lag.They must be handled very carefully so as not to squeeze them and distort the shape.Alternative materials are other metals ed alumina, .Other me als such nickel,may be cheaper,but are less inert.They must never be heated to high temperature in an oxidising atmosphere,such as air.Even in inert gases,such as nitrogen from a cylinder,there are traces of oxygen,and reactive metals may not last over repeated use.Ceramic materials can be inert towards oxygen because they are oxides.However,if the material analyse goes thr ough tate it tends to sink into the solid crucible and is very hard to remove by cleaning
Thermogravimetry and Derivative Thermogravimetry 15 The furnace is capable of being moved away from the balance case to allow access to the sample. A sliding support allows it to move up, down or sideways as required by the particular design. In many cases a rubber “0” ring produces a gas tight seal between the furnace and the balance case. Atmosphere Control The simplest TG experiment would be to heat the sample in static air. However, the sample may react with air in oxidising or burning. Usually an inert gas such as nitrogen or argon is used. In some cases, a deliberately chosen reactive gas is used. This could be hydrogen used to reduce an oxide to metal or carbon dioxide, which affects the decomposition of a metal carbonate. A flowing purge gas is almost always used. This is fed over the balance mechanism first, then around the sample and then out to waste. As well as mass loss by decomposition, thermogravimetry may be used to follow mass gain by reaction with, and uptake of, the purge gas. Also, physical processes such as evaporation of a liquid, sublimation of a solid and desorption of a gas from the surface of a solid may be followed. Most thermobalances will also operate under vacuum as long as all joints are efficiently sealed. Crucibles Crucibles are made of various materials. The best ones are made of platinum. These are inert with respect to most gases and molten inorganic materials, and only melt at 1769°C. If exposed to hydrogen they do chemisorb hydrogen, which might appear as a spurious weight gain. They may also be cleaned in strong acid without any reaction. Unfortunately they are also expensive. They are made of thin platinum to keep the mass low so that they have low heat capacity and follow the furnace temperature without any temperature lag. They must be handled very carefully so as not to squeeze them and distort the shape. Alternative materials are other metals, fused alumina, silica or ceramics. Other metals, such as nickel, may be cheaper, but are less inert. They must never be heated to high temperature in an oxidising atmosphere, such as air. Even in inert gases, such as nitrogen from a cylinder, there are traces of oxygen, and reactive metals may not last over repeated use. Ceramic materials can be inert towards oxygen because they are oxides. However, if the material analysed goes through a molten state, it tends to sink into the solid crucible and is very hard to remove by cleaning