DSC studies on the phase transformation of hydrazonium sulfate

Materials Chemistry 7 (1982) 577 - 586

DSC STUDIES ON THE PHASE TRANSFORMATION OF HYDRAZONIUM SULFATE

K.C. PATIL, J.P. VITTAL Department o f Inorganzc and Physical Chemistry, Indzan lnstttute o f Science B A N G A L O R E - 5 6 0 0 1 2 -lndia. Received 9 March 1982; accepted 27 April 1982 Abstract - The hxgh temperature phase transformation of hydrazonium sulfate, N2H6804 has been studied using DSC. The enthalpy of phase transition is found to be 3.63 + 0.1 k J mole "1 . The phase transition temperature ~s found to decrease with the increase of particle size. It appears that the strata energy and not surface energy, is responsible for the phase transformation. The molar volume o f the salt increases during the transformation as found by the dilatometnc experiment revolving percentage of linear thermal expansion. On cooling, the transformation from the high temperature modification to orthorhombic form is incomplete and extends over a wide range of temperature.

INTRODUCTION Hydrazonlum sulfate, N2H6SO 4 is known to exist in orthorhombic and monoclinic modifications at room temperature 1. The crystal structure o f the orthorhombic form has been studied extensively by X-ray and neutron diffraction methods 2"4. The neutron diffraction studies o f orthorhombic N2H6SO 4 by Power et al a revealed the existence o f a phase change at - 5 0 ° C which was later examined m detad by Caville using Raman spectra s. The low temperature modification was found to be monoclinic 6 . One more phase o f N2H6SO 4 is known to exist above 208oc 7 -9. However, the crystal structure o f the high temperature modification 0390-60351821050577-0952.00/0 Copyright © 1982 by CENFOR S.R.L. All rishts of reproductionin any form reserved

578 is not known 6. Okamoto et al 1° have studied the hxgh temperature as well as low temperature changes by X-ray powder diffraction, NMR and DTA experiments. The phase transition temperatures were reported to be sensitive to particle size. The particle size effect for the low temperature transition was discussed on the basis of the thermodynamic nucleation theory taking into account the interfacial energy between the high and low temperature phases. In the case of the phase transition which occurs at 208°C, the DTA peak temperature was found to displace to the higher temperature side as the particle size increases. This could not be explained on the basis of the equation employed by Okamoto et all 0 Further, the high temperature phase transformation of N2H6SO 4 was found to be reversible and show thermal hysteresis 1o. During the course of the study on the phase transformation of N2H6SO 4 by DSC, it has been found that, on cooling, the transformation of high temperature form to room temperature orthorhomblc form is partaal and extends over a wzde range of temperature. It was thought, therefore, interesting to reinvestigate this phase transformation m greater detail.

EYLPERIMENTAL Hydrazonium sulfate, N2H6SO4, analar sample (BDH product)was recrystallized from hot aqueous solution and dried in air. The purity of NzH6SO 4 was checked by chemical analysis. The crystals were ground well and sieved to different range of particle sizes (40-400/am) using STM standard sieves. Differential Scanning Calorimetric (DSC) experiments were carried out on a Du Pont 990 Thermal Analyzer fitted with the 910 DSC accessory module. The DSC cell was calibrated as gwen in the manufacturer's instruction manual for accurate temperature and enthalpy measurements. All the experiments were carried out in crimped aluminium cups and at a heating rate of 10°C.min "1 unless otherwise specified. Cooling curves of DSC were obtained without controlled cooling; but the cooling rate was found to be somewhat linear viz., 6 to 8°C.min q from 250 to 100°C. The temperature reported for the transformations are accurate to within -+ I°C. The heat of transformation did not vary more than + 5*/, from run to run on any one particular system. The rate of phase transition from high temperature form to room temperature form was followed by heating 100 mg of NzH6SO 4 (particle size, 40-53/am) beyond the phase transition temperature and then quenching it by sudden cooling to room temperature (23 -+ 2°C). A-

579 bout 10 mg of this preheated sample was used for DSC experiments to determine the percentage of orthorhombic N2H6SO 4 formed (from the AH measurements) as a function of time. Thermal expansion measurements were carried out employing a dilatometer model LKB 3185, with a constant heating rate of 4°C.mlnq and a pellet sample size o f 10 x 12.5 mm.

RESULTS AND DISCUSSION

The DSC experimental results showing the effect of particle size on the phase transition temperature, the enthalpy of phase transition, etc., are summarized in Table 1 The transformation temperature is taken as the imtaal temperature which is the intersection of the low temperature side of the peak with the base line. It can be seen (from Table 1) that the phase transformation telnperature decreases as the pamcle size increases. The particle size seems to have

Table 1. - Dependence of phase transition temperature and the heat of phase transition (AH) on the particle size.

Particle size range lam

Amount of the sample mg

1

40-53

2

S1 No

Phase transition Temperature, °C Imtlal

Enthalpy* of phase transition k J.mole "1

Tl

Peak Tp

7.560

219.6

228.0

3.60

53-75

8.015

218.0

228.4

3.63

3

125-150

8.055

216 2

228.0

3.56

4

250-300

8.750

214.6

221.1

3.61

5

300-400

12.260

213.5

219.0

3.74

6

Pellet

18.810

213.8

222.9

3.72

(250 KP.cm -z Pressure) * Enthalpy of phase transformation, AH = 3.63 -+0 1 kJ mole-1

580 no effect on the heat of phase transition. The heat or enthalpy of phase transition was found to be 3.63 -+ 0.1 kJ.mole "I (average of fifteen experiments). Normally, for the solid state reactions, smaller the particle size greater will be its reactivity. Hence, the reaction would be faster and the initial reaction temperature would be lowered. Similarly, for the phase transformation of solids one can expect that the phase transformation temperature (T]) should decrease as the particle size decreases due to increase m surface area or dislocations produced during grinding. In the present study, contrary to the expectations, the phase transition temperature decreases as the particle size increases (Table 1). Similar observation has been reported by Okamoto et al 1o using DTA technique. A plot of phase transition temperature versus inverse (average) particle size gives a straight line 220

1

i

i

i

u 218

~L E

216

~ o

21l, E

o {3 .E

210

t

~

~

0 Inverse

~

I ~ ~ ~ i I 10 20 p a r t f c l e s i z e , 1 0 - 3 / j m -1

Fig. 1 - Dependence o f phase transitton temperature on parttcle size for N 2 H 6 S O 4

(Fig. 1) which can be represented by a linear re . . . . . . . . . c

Ta-To =--

(1)

r

where r, the radius of the particle is taken as half of the particle size (d) assuming the particles to be spherical. To is the phase transition temperature when r = oo and c is a constant. Equation (1) has the form that has been derived for the first order phase transition from the thermodynamic consideration. There the coefficient, c depends totally on the "model" of the interface 11

581 Considering the melting theory o f Reiss and Wilson 12 to phase transformation, the relation between the particle size and the phase transition tempera',ire may be given as, 2 To rO

-- T1 -

7/XH

(')'1

v l - 72 v2 )

(2)

where Vl and v 2 are the molar volumes and 3'1 and 72 are the surface energies of phase 1 and 2 respectively. /XH is the heat of phase transition. For the phase transformation observed in N2H6SO4, T O ( T l and therefore 71

--

<

')'2

v2

(3)

V2

The surface composition of the crystal faces may change for finer particle size in such a way as to increase the surface energy. Hence, when r 1 ( r 2 , then 71 > 3'2. According to Rao et al 1 3 the heat o f phase transition observed for palticular particle size is given by the expression AHob s = AHreal + 3A 3, (r I + r2) A r / r 3

(4)

where AHobs and AHreal are respectively, the observed and real volumetric heat o f phase transformation for the particle size, r and A~/ is the difference in the surface energy o f the two phases and Ar = r 1 - r 2. In the case of the phase transition o f N2H6SO4, AH seems to be independent o f particle size, then r I ,~ r2 and hence "/1 ~'~"/'2. The expression (3) is approximated to v2>vl

(5)

The plausible reason for the particle size dependence o f the phase transition is that when the surface energy o f the two phases being approximately equal, the molar volume increases or the density decreases on going from orthorhombic to high temperature form. Since the change in volume is acoompanied by phase transformation, strain energy is introduced in the phase. The strain energy maintams a coherent interface which usually has a low surface energy 14. It appears that the crystal structure o f the parent and the product phases happen to match well across the common interface, nucleation o f new phase can be greatly facilitated as a result o f low interfacial energy I s

582 The results of dilatometric experiment of N2H6SO 4 involving the linear thermal expansion measurements are shown in Fig. 2. The salt hydrazonium sulfate undergoes a sudden increase of percentage of linear expansion in the temperature range of 216-238°C. Momm et a116 have shown that the linear variation m the i

,

I

'

I

,

[

i

1o I

,

C 0 Q.

~05 o

g g_

I

50

A

J

1O0

i

I

150 Temperature, "C

~

I

200

i

250

Ftg. 2 - Plot o f percentage o f hnear expanszon versus temperature for N 2 H 6 S O 4.

cell parameters and the percentage of linear expansion are in the same trend. Hence, the thermal expansion experiment appears to support the view that the molar volume increases during the phase change from orthorhombic to high temperature form and v2 > v I . The heating and cooling curves of DSC of N2H6SO4 (particle size 250-300 #m) are shown in Fig. 3. On cooling, the exothermic peak consists of a large number of very small exotherms extending over a wide temperature range. On repeating the heating and cooling cycles, similar effects were observed. Observation of thermal hysteresis, AT, over a wide range of temperature is due to the absence of thermodynamic equilibrium at all stages of transformation. Ubbelohde 17 has pointed that hysteresis is the necessary consequence of the co-existance mechanism of continuous transformations. The main factor controlling hysteresis is the

583 strain energy, e and the magnitude of e that can be stored m the hybrid crystals. In the neighbourhood of the transformation temperature the surface of the two phases are supposed to get "thicker", their intersection, being intermediate over a narrow range of temperature, is likely to be a metastable equilibrium. The termination of this metastable region in the two directions marks the width of hysteresis. Hysteresis can also occur for kinetic reasons. Though the free energies are equal for the two phases at the phase transinon temperature, the new phase cannot nucleate because of the exlstance of the kinetic barriers. The nucleation barrlers should be higher and hence the hysteresis AT, greater for involving larger volumes of AV and hence the strain energy accompanying transformation 17

o x L~J

~ - -

0

(b}

*

4

I 05mcal

~

Ca)

s -1

C LIJ

i

150

L

I

175 200 T~mperatur¢, °C

I

225

Fzg. 3 - D S C o f orthorhombtc N 2 H 6 S 0 4 (parttcle stze. 2 5 0 - 3 0 0 larnJ. (a) Heatzng curve (heatmg rate l O ° C - m m -1), (b) Cooling curve.

When the particle size is smaller, the high temperature phase gets stabihzed, ie., the phase transition appears to be thermally irreversible for the smaller particle size. When high temperature form of NzH6SO4 is aged at room temperature, it is slowly converted into orthorhombic form. The rate of the conversion of high temperature form to orthorhomblc form of at room temperature (23 + 2°C) has been followed by DSC as described in the experimental section. The plot of fraction of orthorhombic form versus time in hours is given in Fig. 4. The conversion is exponential in nature and incomplete. This behaviour is attributed to kinetic factors. For such transformations, the kinetics will be a function of

N2H6SO4

584 75

Z

~' 50

I

/--

I

I

f

2

I

0

25

I

50 Time, hours

I

75

100

Fig. 4 - Rate o f conversion o f high temperature modification (parttcle size" 25. -40 lam) to orthorhombic N 2 H 6 S 0 4 . both time and temperature. When the particle size is small or the surface area is large, they shall have additional degrees o f freedom and hence the observed behaviour.

Acknowledgements The authors wish to thank Dr. Momin, Bhabha A t o m i c Research Centre, Bombay for the thermal expansion experiment and Prof. Pai Verneker for hzs interest and encouragement.

REFERENCES

1. 2.

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585 3. 4.

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7. 8. 9. 10.

11. 12. 13. 14. 15. 16. 17. 18.

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52, 1619, 1979. S.J. PEPPIATT, J.R. SAMBLES - Proc. Roy. Soc. Set A, 3 4 5 , 3 8 7 , 1975. H. REISS, I.B. WILSON - J. Collold. Sct., 3 , 5 5 1 , 1948. M. N A T A R A J A N , A.R. DAS, C.N.R. RAO - Trans. Farad. Soc., 65, 3081, 1969. V. RAGHAVAN, M. COHEN - Changes o f State, of Treahse on Sohd State Chemistry, Vol. 5, ed. N.B. Hannay, Plenum Press, New York, 1975, Ch.2. K.J. RAO, C.N.R. RAO - J Materzal Scl., 1, 238, 1966. A.C. MOMIN, M.D. MATHEWS - Indian J. Chem., 15A, 1977, 1096 and Ibid., 9, 582, 1971. A.R. UBBELOHDE - Quart. Revs., 11, 246, 1957. D.G. THOMAS, L.A.K. STAVELEY - J. Chem. Soc., 1420 and 2572, 1951