THERMOGRAVIMETRIC ANALYSIS OF MgAI204, SrAI2O4 AND NiAI2O4

Transcription

1 CHAPTER 5 THERMOGRAVIMETRIC ANALYSIS OF MgAI204, SrAI2O4 AND NiAI2O4 5.1 Introduction To track the formation temperature of a material during heat treatment and to determine the optimum condition for preparation, thermogravimetric methods are very much effective. Thermogravimetric (TG) methods are also usehl in the study of decomposition, oxidation, vaporisation, sublimation and desorption processes.' One of the major problems in all scientific studies is the presence of occluded water in the sample under study. Thermogravimetric method can effectively be employed to determine the calcination temperatures required for the step-wise removal of physisorbed water, inter layer water, structural water etc. The evolution of C02 and NH3 from the interlayer due to the destruction of layered structure can be identified using this technique.' The shape of the TG curve indicates the endothermic or exothermic effects involved in the high temperature region. Also the departing temperature of certain elements, which cannot be removed by washing process, can be estimated with thermograms? In the case nanoparticles, the calcination temperature determines the size of the particle and crystallinity. Crystallisation temperature is different for different materials. In the thermogravimetric analysis, the temperature region which does not contain any peaks represents the crystallisation region. So this is an efficient tool for the determination of temperatures at which crystallisation or recrystallisation occur^.^ Study of phase transition in nanoparticles in comparison with that in the bulk materials has wide spread appreciation.5 For example, in the case of nanostructured Ni film a thermal study revealed an additional exothermic peak at 600K and was explained on the basis of grain growth process.6 The free energy of formation of nanoparticles is higher compared to bulk materials. So when these materials are exposed to high

2 temperature, the structure and atomic configuration will change. As a result of these changes, some exothermic or endothermic peaks appear in the thennogram.' Thus, it seems that the scope of these techniques is extremely broad and includes studies of thermal stability and decomposition, solid - state transition, melting, purity, crystallanity and ageing behaviour. 5.2 Theory Thermal analysis technique is widely used to study the physical changes occurring in a material when it is subjected to a varying temperature programme.8 This method has application in quality control and research uses in industrial products such as polymers, pharmaceuticals, metals, dielectric materials etc. Thermogravimetry (TG), Differential Thermal Analysis (DTA), and Differential Scanning calorimetryg are the three major thermal analysis techniques employed nowadays. 5.3 Thermo Gravimetric Methods (TG) In thermogravimetric analysis the mass of a given quantity of a sample in a. controlled atmosphere is recorded continuously as a function of time or temperature as the temperature of the sample is linearly increased with time. The graph, which shows the variation of mass with time is called a thermogram or a thermal decomposition curve." Instrumentation A typical modem commercial thermo gravimetric instrument consists of (1) a sensitive balance (2) a furnace (3) a purge gas system to provide inert or reactive atmosphere and (4) a computer system to control the temperature of the furnace, data collection and to display the result in the form of a thermogram. Fig.S.l shows the schematic diagram of a thermal balance.

3 Fig.S.1 Schematic diagram of thernial balance A. Beam balance B. Sample cup and holder C. Counter weight D. Lamp and Photodiodes E. Coil I?. Magnet G. Control amplifier H. Tare calculator I. Amplifier J. Recorder Different' thermo balance designs are availabie commercially, which are capable to give qualitative information about samples ranging in mass fiom a few milligrams to several grams. The sample holder is always inside the furnace and the rest of the balance is thermally insulated. A change in the sample mass causes a deflection of the beam, which interposes a light shutter between a lamp and one of the two photodiodes. The resulting imbalance in the photodiode current is amplified and fed into coil E which is situated between the pole pieces of a magnet 'F'. The magnetic field generated sets the beam to its original position. The amplified photodiode current is transformed into mass loss information by data acquisition system. The plot, which represents mass loss with temperahre in real time, can be obtained with the help of a computer. The temperature range, which most furnaces can handle, is from ambient to IS00 "C. Rate of cooling and heating can be set at any desired value. Proper insulation and cooling of the interior of the furnace are required to protect the ' balance from heating. The temperature recorded in a thennogram is ideaily the actual temperature of the sample, which can be measured by immersing a thermocouple directly into the sample. Usually the recorded temperatures are measured with a thermocouple located as close as possible to the sample to avoid certain problems. So the recorded temperature generally differs from the actual

4 sample temperature. Most modem thermo balances use a computerised temperature control method that automatically compare the voltage output of the thermocouple with a voltage versus temperature table that is stored in the ROM. The microcomputer uses the difference between the temperature of the thermocouple and the temperature specified in ROM to adjust the voltage to the heater. This is the best method to achieve excellent correlation between the specified temperature programme and the temperature of the sample. 5.4 Differential Thermal Analysis (DTA) In this technique the difference in temperature between a substance and a reference material is measured as a function of temperature while the substance and an inert reference material are subjected to a controlled temperature programme. If T, is the temperature of the sample and T, the temperature of the reference sample, then the plot between the difference in temperature (AT = T, - T,) and sample temperature is called the differential thermogram. This themrogram can he used to study the mass loss at different temperatures. Fig. 5.2 Schematic diagram of a differential thermal analyser

5 5.4.1 Instrumentation Fig.5.2 shows the schematic representation of a DTA system. The sample 'S' and reference material 'R' are placed in separate aluminium dishes. Separate thermocouples are attached to the dishes to measure the temperatures separately in an electrically heated furnace.. The output voltage E, from the sample passes into a microcomputer, where it is used to control the input current of the fumace, so that the furnace temperature increases gradually with time at a predetermined rate. The difference in the thermo emf AE between sample and reference material can be converted in to AT. Also from E, the sample temperature T, can be obtained. The differential thermogram may be plotted taking T, as the abscissa and AT as the ordinate. Peak areas (A) in differential thermogram depend upon the mass of the sample m, the enthalpy AH, of the physical or chemical process, and certain geometric and heat conductivity factors. The relevant factors are related by the equation where G is a calibration factor that depend upon the sample geometry. K' is related to the thermal conductivity of the material. For a given species, K' remains a constant. 5.5 TG - DTA Measurements of MgA1204 The decomposition of the prepared powder and its structural evolutions has been studied by thermogravimetric and differential thermal analyses. TG and DTA curves of the thermal degradation of the magnesium aluminate precursor powder are shown Fig.5.3. The major steps of the decomposition take place between100 "C and 517 O C. TG pattern shows weight loss steps at 100, 234, 295 and 517 "C, with an overall weight loss of 19%. These results have been compared with that of MgA1204 obtained from A1 (NO3)3 and Mg (NO& as starting materials. The weight loss of MgA1204 precursors has been reported to be at temperatures 100 "C, 176 OC, 262 OC, 332 "C and 478 OC.''.'~ The reaction pathways in the formation of MgA1204 nanoparticles are determined with the thermal analysis in conjunction with X-ray

6 diffraction studies. The as prepared sample is magnesium-aluminium mixed hydroxide. The peak at 281 C may be associated with the loss of hydroxides from aluminium in the mixed magnesium-aluminium hydroxide sample. During the third mass loss at 318 OC, a large exothermic peak begins, which is attributed to the loss of hydroxide fiom magnesium in the mixed hydroxide. The weight losses associated with these endotherms are expected of the form The large exothermic peak is likely to be the result of a phase transition to the oxide phase. Thus, the first three weight loss steps can be attributed to the evaporation of physisorbed water, interlayer water, structural water, C02 and NH3 from the interlayer. Above 517 "C, no more detective weight loss was observed. According to the reported value for the bulk material, weight losses are absent after 600 OC. The DTA curve contains several exothermic and endothermic peaks. In the DTA curve the exothermic peak temperature is related to the weight loss stages in the TG profile. So, they are characteristic of energy changes corresponding to different weight losses. Above 517 OC, the gradual shift of the base line indicates that the solid-state reaction to form nano MgA1204 began because there after no distinctive weight loss is observed. No other features were observed with the rise in temperature which indicates that once formed, this nanostructure of spinel is very stable. The DTA result matches with the result obtained from TG. In the case of bulk material the crystallisation starts only after 600 OC. Thus the calcination temperature required for nanostructured material is lower compared to that of bulk material. According to the XRD result the crystallisation occurs after this calcination temperature 517 "C. The weak exothermic peak at 517 OC with no corresponding mass loss is expected to be the crystallization transition in which the amorphous oxide powder becomes nanocrystallites with the release of lattice energy. This is also in very good agreement with the results from X-ray diffraction studies. At 750 "C very weak Bragg reflections are finally observed. Both TG and DTA results confirm that nanocrystalline MgA1204 spinel can be obtained by calcination of the sample at about 517 OC. This temperature is significantly lower than that required by the

7 conventional solid-state reaction process where calcination temperature is usually above 1300 "c.'~ Considering the least acceptable calcination temperature, 750 "C, 900 "C, 100 "C and 1100 OC are selected' as calcination temperatures for the preparation of samples having participle sizes 7 nm, 9 nm, 19 nm and 33 nm respectively. Fig. 5.3 TG / DTA curve of MgA1204 precursor powder 5.6 TG - DTA Measurements in SrAl~04 To study the formation of crystalline SrA1204 phases during heat treatment and to determine the effective processing conditions for synthesis, the product derived from the co-precipitation method was characterised by differential thermal analysis (DTA) and thermo gravimetric analysis (TGA). The surnmerised DTA and TGA profiles are shown in Fig.5.4. Evaporation of absorbed water and other interlayer water occurs at 255 "C. In the case of SrA1204 prepared by sol-gel method the evaporations of volatile impurities and absorbed water occurs in the temperature range 80 "C to 250 OC'~. A total weight loss of 21% occurs between the temperatures 255 "C and 596 OC.'~ Some prominent weight losses occur at 480 "C and 562 "C, which may be due to decomposition of the precursor material. After 596 "C there is no notable change in the phase, which indicates the stable phase formation after 596 "C. The DTA analysis shows that crystallisation occurs after 620 "C because there is no apparent change in the weight loss after that temperature.

8 According to earlier report, in SrA1204 prepared by sol-gel method, broad exothermic region occurring in the temperature range 800 OC to 1200 OC in the TGA profile was interpreted as an indication for the formation of the SrA1204 phase. In the case of nano, SrA1204 some lower calcination is required for the formation of crystalline phase when compared to the bulk form. This method is appropriate to find out the calcination temperature required for the synthesis of nanostructured materials. This experimental fact can be confirmed with the help of X-ray diffraction patterns. Based on these studies, calcination temperatures were selected at 750, 900, 1000 and 1100 "C to prepare SrA1~04 nano crystallites of particle sizes 6 nm, 9 nm, 22 nm and 38 nm. Fig.5.4.TGlDTA curve of SrA1204 precursor power 5.7 TG - DTA Measurements in NiA1204 The thermo gravimetric analysis was done using mass spectrometer in the temperature range OC at the rate of 100 "Clminute. The thermal decomposition of the material at different temperatures was studied using DTA. The TG and DTA curves of the precursor NiA1204 material are shown in Fig.5.5. An initial weight loss of 45% takes place between room temperature and 450 C. The shape of the TG curve clearly indicates that several effects are involved in this process. The weight loss occurs at temperatures 140, 244 and 444 OC. According to earlier reports, the weight loss steps occur at 96, 163 and 330 "C with an overall weight loss of 42.5%, for samples prepared by mixing the respective nitrates with

9 the help of ultrasound radiation..reevanandem2 et a1 attributed the weight losses at 96 "C and 163 O C to the loss of physisorbed and inter layer water. The weight loss at 330 OC was interpreted as the removal of structural water, C02 and NH3 from the interlayer because of the destruction of layered ~tructure.~ The weight losses in the present case also may be due to these changes at the respective temperatures. The disagreement between the current and reported values may be due to the change in the preparation method. Also in the TG curve no further loss was noted after 445 "C. Absence of loss peaks in a TG curve is an indication of initiation of crystalline phase, which can also be verified by XRD study? The continuous loss of water molecules leads to the formation of amorphous nickel aluminate and its subsequent crystallisation, at high temperature. 5.8 Conclusion Ibe.bO do 30 JI * dr dr $4 0 % ~ Fig 5.5 TG and DTA curve of the NiA1204 precursor TG and DTA methods are the best tools to study the phase changes with temperature. Theoretical studies of different thermal methods have been discussed in this chapter. The temperatures at which different reactions were taking place and departing of different occluded components were studied. The minimum calcination temperature required for the formation of crystalline nano aluminates was calculated from the thermal studies. The degree of crystallanity at different calcination temperatures was also studied in this chapter. X-ray diffraction together

10 with TGA/DTA studies in MgA1204 suggests that the initial dried white precursor is likely a mixed magnesium-aluminium hydroxide phase. This hydroxide phase loses different types of absorbed water to form an amorphous oxide at about 318 "C. The sample crystallizes at 520 OC and forms nanocrystalline MgA1204 spinel. The crystallisazation temperature for SrA1204 is determined as 620 "C, which is in confirmation with the X-ray diffraction study. Also, the temperatures at which prominent changes in phases occur in SrA1204 were studied. TGIDTA study of NiA1204, revealed that nanocrystalline NiA1204 begins to develop at 445 OC. 5.9 References 1. R.V.S.S.N. Ravikumar; A.V. Chandrasekhar, Cryst. Res. Technol. 37(10) (2002) P. Jeevnandan, Y. Koltypin, A Gedanken, Mater. Sci. Eng. B 90 (2002) F.A. Lopez, A. L. Delgado, J.L. M Devidales, E. Vila, J. All. Comp.265 (1998) F. Meyer, R. Hempelmann, S. Mathur, M. Veith, J. Mater. Chem. 9 (1999) W. Chang, F. Cosandey, H. Hahn, Nanostruct. Mater.2 (1993) A.Cziraki, 2. Tonkovies, I. Geroes, Mat. Sci. Engg. A179 / A180, (1994) X.Y. Qin, X.J. Wu, L.F. Cheng, Nanostruct. Mater. 2 (1993) R.C. Mackenzie, Thermochim. Acta. 1 (1979) B. Wunderlich, Thermal analysis, San Diego: Academic Press, C.M. Earnest, Anal. Chem. A 56 (1984) J. Wang, C. Liehi, Appl. Surf. Sci. 161 (2000) R. Christy, Vestal, Z. J. Zhang, 3. Soli.Sta. Chem. 175 (2003) L.B. Kong, J. Ma, H. Huang, Mater. Lett. 56 (2002) I. C. Chen, T. M. Chen, J. Mater. Res. Soci. 16 (5) (2001).