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Factors affecting the TG curve

n Many factors have an effect on TG curves

They can be caused from the 1) equipment(measurement conditions) or 2) sample

Most important factors caused by the equipment

n Heating rate of the furnacen Gas atmosphere in the furnacen Geometry of the furnace and the sample holdern Crucible materialn Registration interval of the data points

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Heating rate1. Isothermal TG

n Sample mass is registered as function of time at constanttemperature

n Isothermal measurements are often used in kineticmeasurements. The equipment can be simple balance andtube furnace.

2. Dynamic TG

n Most common methodn Sample is heated (cooled) with a certain rate usually linearly

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n High heating rate shifts the beginning and end temperatures ofdecomposition reactions even 100 C up (graphite, siderite FeCO3)

n With low heating rate the tempearatures are lower and thedecomposition reaction occurs at narrower T range.

n Resolution of different steps in TG curve usually is better with lowheating rates and more intermediate products are observed(CuSO4 · 5 H2O, NiSO4 · 7 H2O)

n However, some times resolution is worse with low heating rates

n F.ex. - very exothermic reaction- low heating rate

à evolved heat can make earlier the next reaction andsuccessive reactions can not be resolved.

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Wendlandt, W.W., Thermal Analysis, 3rd Ed. Wiley & Sons, New York 1986

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20 °C/min2 °C/min

CuSO4 · 5H2O

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3. Quasi-isothermal TG (high resolution TG, controlled heating rate TG

n Linear heating close to reaction temperature

n When mass change vs time (from DTG signal) exceeds a threshold,

heating rate decreases close ~ 0 (cooling if needed)

n Advantages:

n Better resolution (close to each other occuring reactions can be

separated better)

- In highly exothermic reaction strong temperarture increase may

disturb

n Can be better used in measuring kinetic parameters (overlapping

reactions do not disturb)

n Data is obtained in one measurement (cf. isothermal measurements)

n Disadvantages:

n Measuring time increases

- Can be partly compensated by increasing the heating rate when

no reactions occur

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Quasi-isothermal TG: Derivatograph Q-TG

Paulik, Paulik and Erdey solved the problem by feedbackfrom the measured DTG signal

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Gas atmosphere in the furnacen Atmosphere has a great effect on the TG curven Many options:

- dynamic- inert ( N2, Ar, He) - oxidative (air, O2)- reductive (H2 (H2/Ar), CO) - vacuum- reactive (Cl2, SO2) - water vapor

- static (sample itself creates the atmosphere)

Dynamic gas atmosphere

Reaction possibilities:1. Asolid ↔ Bsolid + Cgas

2. Asolid → Bsolid + Cgas

3. Asolid + Bgas → Csolid + Dgas

n Inert gas removes gaseous products from reactions 1 and2

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n If the gas atmosphere is the same as the gas released fromthe reactions, reversible reaction (1) affects according to LeChatelier principle temperature, for example in case ofCaCO3

n In reaction type 3 gas atmosphere (B) can affect thedecomposition mechanism and significantly the end product(oxidative/reductive)

Advantages of dynamic gas atmosphere:- Decreases condensation of the reaction products to the

cool parts of the balance mechanism- Removes corroding products- Diminishes secondary reactions- Cools the mechanical parts of the balance

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Paulik, J., Paulik, F., Erdey, L., Anal. Chim. Acta 34 (1966) 419

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n Different gas atmospheres have been utilized for example instudied of high-temperature superconductors

Especially YBa2Cu3O7-x (x varies between 0-1)

n Oxygen content is critical for superconductivityn In superconducting phase x is small ~ 0,1 (orthorhombic)n If x ~ 1à non-conducting YBa2Cu3O6 (tetragonal)n Oxygen content can be controlled by heating or cooling in

oxygen atmosphere (or in air)

n By heating in reducing atmosphere (H2/Ar) the oxygen contentof the original sample can be measured from the mass decrease(copper reduces to metallic) and also the oxidation state of Cucan be determined. Heating and cooling in inert and oxygen canalso tell the oxygen content (release/in-take)

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Oxygen Desorption (in N2)

CuBa2YCu2O7-d :Gradually ® mixed-valent Cu

Ag2O2 :in two discrete steps

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in O2

in N2

YBa2Cu3O7-x heatingin oxygen and nitrogen

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Heating of YBa2Cu3O6 in oxygen

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OXYGEN-DEFICIENT SAMPLESTemperature-Controlled Oxygen-Depletion

(TCOD)

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Reactive gas atmosphere

For example Cl2, SO2, H2O

n It must be secured that the reactive gases go not go tobalance mechanism

n Inlet of the reactive gases occurs usually directly to thefurnace (not through the balance)

Vacuum

n Reactions take place at clearly lower temperatures in vacuumn Effect on temperature (mechanism) can be large as can be

seen for example with volatile metal complexesn Vacuum in the equipment varies depending on the

manufacturer, at best ~ 10-4 mbar

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Static gas atmosphere (created by the sample)

n The gases relase slowly from the samplen The releasing gas form above the crucible its own atmosphere

which different to that orginally in the furnacen If that atmosphere is desired labyrint crucibles are recommended

Problems:n Gas atmosphere is not easy to controln The gases can react back with the solid sample in the cruciblen The gases may cause corrosion problemsn Temperature of the sample more difficult to measure (often larger

masses and crucibles with large thermal mass is used)Advantages:n Better resolution in the TG curve (same effect as the slow heating

rate has)

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Sample holder and crucibles

n Sample holder should transfer heat as evenly and effectively aspossible

Affecting factors:- shape- thermal conductivity (metal-ceramic)- thermal mass

n Thermal expansion of sample holder- in horizontal furnaces the holder acts also as balance’s rodà change in length causes change in torque and error in

mass measurementn The sample type affects the selection of crucible (metal-

ceramic)n The shape of the crucible affect the reaction temperatures

(CaCO3)

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Effect of crucible geometry on the decomposition of CaCO3

Niinistö, L., Thermal Analysis. Teoksessa Analytical Chemistry, toim. Kellner, R., Mermet, J.-M,Otto, M., Valcarcel, M., Widmer, H.M., Wiley-VCH, Weinheim 2004.

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Most important factors caused by the sample

n Amount of samplen Particle sizen Packing of the samplen Thermal conductivityn Reaction heat (endo ↔ exo)n Solubility of the gases to the sample

Sample size

n Increasing sample mass requires more time to decompose(react) completelyà Tf increaes in general

- Exception: exothermic decomposition reactionsà rapid increase in the temperature of sample can

compensate the increase of sample mass

n Resolution is generally better with samll sample sizes

n Thermal conductivity affects simultaneously with thesample mass

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Wendlandt, W.W., Thermal Analysis, 3rd Ed., Wiley & Sons, New York 1986.

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Particle size

n Not studied in very detailedn In general, decreasing the particle size decreases the

reaction temperatures (more surface area)

Example. Oxidation of CaS- in samples with bigger particle sizes the steep mass

decrease in TG curve > 1000 °C:ssaReason: Oxygen diffuusio in larger particles is slower

and CaSO4 which is formed at the surface starts todecompose before the inner parts of the particles haveoxidized.

Packing of the sample powder

n Has been studied very littlen Dense packing increases generally slightly the reaction

temperatures.

Thermal conductivity of the sample

n If the thermal conductivity is very poor (polymers), it issuggested to use small sample mass.

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Resolution (distinguishing of close/overlapping processesfrom each other)

• Importan criterionm in selecting measurementsparameters and conditions

• In general:- small sample size- low heating rate- good thermal conductivity of the flowing gas

à improve resolution

■ As conclusion, many factors affect the TG curves

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Sources of errors in TG

n Temperature calibrationn Mass calibrationn Position of thermoelement in the furnacen Thermal conductivity of sample holder and cruciblen Thermal conductivity of the gas environmentn Reaction enthalpy (causes a difference between the

temperature of furnace and sample)

- In exothermic reactions the sample temperature ishigher than that measured from furnace

Esample CaC2O4(s) → CaCO3(s) + CO(g)

- If heated in oxygen CO oxidizes to CO2- This reaction is very exsothermic and the temperatureof solid calsium oxalate increases and the decompositionis accelerated- Decomposition occurs at lower temperature in oxygenthan in nitrogen atmosphere.

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n Buoyoance- Density of the gases decreases when temperatureincreases à buoyoance decreases and sample massseemsto increase (TG curve goes up)- In very accurate measurements the buoyoancecorrection is needed

n Aerodynamiset disturbances- Appear in dynamic gas atmospheres but dimishes whendecresing the gas flow rate or in vacuum

n Electrostatic disturbances- Sample holder hits the wall of furnace- Aluminum crucible may show electrostatic disturbancesin very dry conditions

n Although all aspects are tied to take in accountinterpretation of TG curves can some times be difficult

q It may be useful to test with general test material:Ca(COO)2 · H2O

q Care must be taken when comparing results obtained withdifferent instruments

q Results obtained with same instrument are comparableonly if conditions and parameters are the same in differentmeasurements.

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Van der Plaats, G., The Practice of Thermal Analysis, Mettler, Switzerland 1991.

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Applications of TG

n Moisture contentn Dry material contentn Amount of volatile compoundsn Sublimationn Mechanisms of decomposition and oxidation

reactionsn Kinetic studies

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Onset 138 oC

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Van der Plaats, G., The Practice of Thermal Analysis, Mettler, Switzerland 1991.

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Van der Plaats, G., The Practice of Thermal Analysis, Mettler, Switzerland 1991.

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