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 Total Downloads : 1252
 Authors : Babita A. Saiyed
 Paper ID : IJERTV1IS8004
 Volume & Issue : Volume 01, Issue 08 (October 2012)
 Published (First Online): 29102012
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
The Study Of Thermal Stability And Decomposition In Cadmium Oxalate Single Crystals
Babita A. Saiyed
Shree P.M.Patel College of Electronics & Communication, Anand Peoples Medicare Society, Nr. Sardar Baug, Anand388001 ABSTRACT
Cadmium Oxalate Single crystal grown using well known technique of crystal growth i.e gel technique. The grown crystal characterized using thermal technique. Their thermal behaviour is investigated using thermo analytical techniques (TGA,DTA, DSC). The thermogram reveals that the anhydrous oxalate is formed by liberating three molecules of water in the first step of transition gives the same reaction in both air and nitrogen atmosphere .The second phase transition results in CdO in air atmosphere while Cd is formed in nitrogen atmosphere at 385C. Based on the data obtained from thermograms, different mechanic and nonmechanic equations are used to calculate kinetic parameters such as activation energy, order of reaction, frequency factor and entropy of the grown crystal.
Key words: Gel method, crystal growth,cadmium oxalate, characterisation
INTRODUCTION:
The thermal methods of investigations are generally referred to as thermo analytical techniques. This is an important experimental method for characterizing a system by measuring the changes in physico chemical properties as functions of increasing temperature with time. Cadmium oxalate crystals are obtained by diffusion of cadmium ions through silica hydrogel impregnated with oxalic acid at room temperature. The study of thermal analysis is significant for knowing the different phases and stages of stability and hence the grown crystals have been subjected to thermal treatments in air and nitrogen atmosphere using gravimetric thermal techniques. The methods used in the present analysis are thermogravimetry (TG) and Differential Thermal Analysis (DTA) and DSC. Our aim to do thermal analysis is to measure the temperature of transition, reliability of crystal for particular application, compositional analysis, stability of substance and other dynamic properties.
EXPERIMENTAL
The thermogravimetric analysis (TGA) is carried out using Perkin Elmer analyzer at ambient temperature respectively. The sample is heated at 10 Â°C/min. in the temperature range 50Â°C900Â°C. The thermograms have been used for evaluation of some important kinetic parameters in respect of the decomposition phases of cadmium oxalate crystals. The DTA analysis is performed on Mettler 2000 C system. During the analysis, thermal energy is added or subtracted from the sample and the reference material at the same temperature. A difference between temperatures of reference material and sample yields a direct calorimetric measurements of the transition energy. The DSC analysis is performed on Perkin Elmer DSC I Instrument . DSC is carried out only in nitrogen .
RESULTS AND DISCUSSIONS
From the curves obtained from TGA, DTA and DSC one can say that the decomposition in air atmosphere consists of two steps only while in the nitrogen atmosphere that has three steps, which is because in air atmosphere the final product is the metal oxide and do not reach to the metal.
TGA DTA Curve at heating rate of 10 0C/min in N2 atm. TGA DTA Curve at heating rate of 10 0C/min in N2 atm..
Atm. 
Step 
Temp. Range(C) 
Mean Temp.(C) 
Mass Loss % 
Reaction 

Obs. 
Cal. 

N2 
I 
55 155 
105 
21.25 
21.23 
CdC2O4 . 3H2O = CdC2O4 + 3H2O 
II 
265 311 
339.5 
49.58 
49.53 
CdC2O4 = CdO +CO + CO2 

III 
400596 
498 
55.417 
55.8 
CdO = Cd + 1/2O2 

Air 
I 
56 — 145 
98 
21.25 
21.23 
CdC2O4 . 3H2O = CdC2O4 + 3H2O 
II 
264 303 
283.5 
49.58 
49.53 
CdC2O4 = CdO + CO + CO2 
Thermal kinetics:
Horowitz – Metzger relation
1 1 1 n
E
Where TTm = q, n 1: n = 1/2, 1/4, 2/3, etc. From the
log
1 n
2.303RTm2
plot the activation energy (E) can be calculated from the slope of the graph.
= Weight loss up to particular temperature / Total weight loss in the step
R = Gas constant = 8.31432 x 103 J K 1mol 1 , = Heating rate (K/min1) ,T = Temperature (K) Z = Frequency factor (min1) ,Tm = Temperature of maximum reaction rate.
Piloyan – Novikova relation
ZR
E From the plot log (/T2) 1/T, E can be calculated from the slope and Z
log log
2 E 2.303RT
T
1/2
from the intercept of the graph obtained.
CoatsRedferm relation
1/2
From the plot
log 21
1/T, E can be
log 21 log ZR E
T2
T
2
E

RT
calculated from the slope and Z from the intercept of the graph obtained. Hence, the values of entropy S* are obtained using the following equation:
Z kTm exp S *
h R
Kinetic parameters evaluated from nonmechanistic equations for thermal analysis
Relation used
Atm.
Step
Order of Reaction (n)
Frequency Factor
Z
Activation Energy
Entropy S*
1 1
(J. K mole )
(eV)
(J/mole)
H – M
Air
I
Â½
0.599
57724.432
P – N
6
1.09 x 10
0.609
58688.112
131.18
C – R
1
6
1.27 x 10
0.651
62735.568
129.80
Broido
1
0.716
68999.488
H – M
II
Â½
1.95
187917.60
P – N
10
4.07 x 10
2.04
196590.72
47.03
C – R
1
10
9.09 x 10
2.164
208540.35
40.34
Broido
1
2.96
285249.28
H – M
N
2
I
Â½
0.6
57820.8
P –
5
4.83 x 10
0.582
56086.18
138.46
C – R
1
5
8.42 x 10
0.635
61193.68
136.78
Broido
1
0.691
66590.29
H M
II
Â½
2.334
224922.91
P – N
10
2.17 x 10
2.0
192736.0
52.34
C R
1
10
6.5 x 10
2.12
204300.16
43.22
Broido
1
2.52
242847.36
H M
III
Â½
0.945
91054.77
P – N
5
2.03 x 10
0.715
68903.12
151.23
C R
1
5
6.82 x 10
0.899
86634.83
141.16
Broido
1
1.177
113425.14
Broido relation: – lnln(1/y) = (E/(RT)) + constant. Here y = fraction of the number of initial molecules not yet decomposed.Where WT = weight of active material at temperature T,W0 = weight of the material taken initially,W = weight of the material at the end of reaction, Plotting lnln(1/y) 1/T gives the value of the activation energy by the slope of this plot.Kinetic parameters evaluated from nonmechanistic equations for thermal analysis of CdC2O4 . 3H2O
k T
Borchardt and Daniels
A a
k = Specific reaction rate constant ( min1 ),A =
Total area of a peak (min 0C ) , T = Peak height at any temperature T ( 0C ), a = Area of the peak at the temperature T (min 0C )
Piloyan, Ryabchikov and Novikova
lnT C' E RT
The values of T are taken
directly from the DTA curve in units of length.Thus by ploting the graph of lnT 1/T we have calculated the required activation energy and frequency factor
Some kinetic parameters calculated from DTA using different models
Relation used
Atm.
Stage
Activation energy E (eV)
Frequency Factor
1
Z (min )
Order of reaction (n)
Entropy S
1 1
(J. K .mol )
P R N
N
2
I
0.686
9
1.075 x 10
1
74.556
II
2.04
16
2.24 x 10
62.521
Air
I
0.683
10
1.67 x 10
1
51.75
II
1.83
14
6.30 x 10
32.836
B D
N
2
I
0.779
8
3.268 x 10
1
84.469
II
2.33
16
3.49 x 10
66.207
Air
I
0.672
8
2.264 x 10
1
87.508
II
2.064
15
1.425 x 10
39.617
Matusita and Sakka (MS) relation log[ln(1x)] = – n log mE + constant
2.303 RT
Where E is the activation energy, is the healing rate, n and m are numerical constant, depend upon the mechanism of crytallization
Some kinetic parameters calculated from DSC thermogram
Peak
Peak Height (mW)
Peak Area (mJ)
Peak Temp.(C)
H
(J/g)
C
p
1
(Cal.g C)
r
eff
(A)
I
39.26
7337.92
108.729
587.88
1.521
0.57
II
13.108
2692.72
360.214
215.728
0.508
1.55
Relation used
Atm.
Stage
Activation Energy E (eV)
Frequency Factor
1
Z (min )
Order of reaction (n)
Entropy S
1 1
(J. K .mol )
M S
N
2
I
0.912
8
9.016 x 10
1
75.562
II
2.437
14
5.197 x 10
30.512
CONCLUSIONS:
The crystals of cadmium oxalate trihydrate are thermally stable upto 540C, beyond which they begin to decompose. implies weak ionic bonds.The decomposition of the grown crystals occurs sequentially, gradually in three stages in nitrogen atmosphere while in two steps in air atmosphere. First, dehydration takes place, when the water molecules are liberated out. Then, the anhydrous material breaks down (the ionic bonding may be breaking), resulting ultimately into
the production of cadmium oxide which is seen not to be stable for high temperatures, upto around 400 0C.The activation energy (and so also the frequency factor) of the dehydration step is the smallest of all the stages of decomposition implies low energies involved for loosening of the bonded water molecules.
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