Gamma stimulated changes of mechanical properties of chalcogenide glasses

Journal of Non-Crystalline Solids 130 (1991) 243-247 North-Holland

243

Gamma stimulated changes of mechanical properties of chalcogenide glasses I.A. D o m o r y a d Institute for Transport Engineering, Tashkent 700045, USSR Received 13 September 1989 Revised manuscript received 4 October 1990

Gamma irradiation (Co6°) effects on the shear modulus, internal friction, microhardness, and linear dimension of chalcogenide glasses of As2S3-As2Se3, As2Se3-As2Te3, A s - S e - G e and A s - S e - I systems are reported. Gamma stimulated densification of glasses is observed. Interpretation of results is based on a model in which polymer chain segments are in statistically distributed potential wells. Changes in the lengths of these segments under irradiation are included in the model. The stabilizing influence of atoms of Ge in the A s - S e - G e system is consistent with the model. The reversibility of irradiation induced changes in the mechanical parameters in 'irradiation-annealing' cycles is noted. One of the effects of -/-irradiation of the A s - S e - I glasses is liberation of I-atoms from the matrix.

1. Introduction

Recently considerable attention has been paid to the study of physico-chemical properties of chalcogenide glassy semiconductors (ChGS) [1-3]. The influence of radiation on different properties of ChGS is important for their applications. Stability of those glasses at high radiation doses is necessary for exploitation of devices in places of intense radiation. For such applications the radiation stimulated changes of mechanical properties should be known. In addition, the study of radiation influence on mechanical parameters of inorganic polymers such as ChGS is of interest. As has been shown [4] in glassy selenium - a material with polymeric chain-like structure - high energy ,/-irradiation changes the mechanical parameters. The shear modulus and the microhardness increase while internal friction and linear dimensions decrease. This paper reports changes of mechanical parameters of some chalcogenide glasses as a function of `/-irradiation. It presents data about changes in structurally sensible parameters such as

shear modulus G, internal friction Q-~, microhardness H, linear dimensions L and chemical stability of different ChGS under 3,-irradiation.

2. Methods of measurements

Samples of glasses of systems A s 2 S 3 - A s 2 S e 3 , A s - S e - G e , A s - S e - I were the object of our investigation. Irradiation of the samples (in evacuated ampoules) was performed in the dry-channels of the -/-source C o 60 at dose rates varying between 200 and 750 R s-1. The temperature of samples in the period of radiation was 15-35 o C (the temperature was controlled); the maximum integrated dose reached 2.2 x 108 R. To study the relative changes of the shear modulus, a special device was made. It allows measurement of the period of free oscillations of samples, T, and their frequency of oscillations. The error in the measured values of T was 0.01% [5]. The evaluation of the shear-modulus changes was dependent on the frequency changes. These As2Se3-As2Te3,

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244

LA. Domoryad / Mechanical properties of chalcogenide glasses

parameters are related [6] by the equation G-

~'2L2p" 1.8754'

(1)

where L is the length of sample and/~ is the mass of the sample per unit of volume. The changes of non-elastic properties were determined from the internal friction. The calculation of the internal friction is given by Q - ' = In 2/~NT,

(2)

where T is the period of free oscillations and N is the number of oscillations before the amplitude is decreased by half. Equation (2) is applicable when Q-1 < 1, as is the case for our CHGS. The error in the measured values of internal friction was - 1%. We used the standard comparators and microhardness meters while measuring the linear parameters and microhardness of the samples. Errors in the measured values were correspondingly 0.001% and 2.5%.

3. Results and discussion

It was experimentally established that the shear modulus, microhardness, chemical stability and thermostability increased with increasing dose. The internal friction and the linear dimensions decreased with dose rate. Dependences of AG/G = fl(O), A Q - 1 / Q - 1 = f 2 ( D ) and A H / H = f 3 ( D ) appeared to be classic: initially these parameters changed linearly with increasing dose. Then they reach a constant value. Figures l ( a - c ) depict the qualitative dose dependences of these parameters for all glasses of

the above-mentioned systems. Let us analyze these dependences. Since these glasses have non-organic polymer structure, y-stimulated changes can be related to the conduct of elastic segments of polymer chains. Let every segment be found in a potential well, in which it vibrates. Let potential wells be distributed statistically at a distance from a segment. Being captured there, segments experience a delay of phase with respect to the applied tension. Then the activation energy, E, of oscillating segments would be equal to the work needed for maximum deformation of these segments. It is possible to show that, on the first approximation, this work would be presented by the summation of two terms. The first of them is connected with the 'travel' of a segment up along the energy barrier and the second term refers to the stretching of this segment:

E = ( A / l ) + Bl,

where constants A and B are defined by a stretch of segments and critical tension necessary for the 'travel' of a segment up along the barrier and l is the most probable length of a segment. In ref. [7] analytic terms for changes of a length of irradiated segment (/irr) a r e given: /irr = / 0 / ( 1

+ Qlot),

(4)

where 10 is the length of a segment before irradiation, t is the time of irradiation and Q is some coefficient. Therefore activation energy after some dose of irradiation is expressed by E= A(l+Ql0t)

lo

Bl o 1 + Qlot "

+ -

(5)

In ref. [8] analytic terms for temperature dependences of the shear moduli ( G - 1,2, where v is frequency of free oscillations) and of internal friction value for glasses with polymer non-organic chain-like structure are given. They are

[ 1,2 0.)2 1 =

Q-t Fig. 1. The qualitative dependence of relative changes of (a) shear modulus, (b) microhardness and (c) internal friction on y-irradiation dose for all glasses of As2g3-As2Se3, As2Se3As2Te 3, and As-Se-Oe systems.

(3)

A,r e x p ( 2 E / k T ) ] 7 ~o2r2 '

Ar exp( E / k T ) , 'r

(6)

¢o'ro

where ~0 is frequency of free oscillations at 0 K based on an extrapolation of temperature depen-

LA. Domoryad / Mechanicalproperties of chalcogenideglasses dence of frequency and ~- is the relaxation time of a segment in a potential well which is given by T = TOe x p ( E o / k T ).

(7)

AT = To -- "r, ( r o and T, are the times for deformation relaxation at constant tension and of the tension at constant deformation respectively). Substituting eq. (5) in eq. (6), it is possible to obtain the dose dependences of A G / G = f ( D ) and A Q - 1 / Q - 1 = f ( D ) [2]. These can be arranged into analytic terms connecting relative changes of shear moduli and internal friction with the ,/-ray dose and can be compared with experimental data. These dependences are AG G

AT e x p ( - 2 E o / k T ) T 6dZTg

×[1- exp(-2(E- E°) AQ-1-[1Q-1

exp(

E~TE° )],

where E 0 and E are activation energies before and after ~,-radiation, respectively. As E ~ E 0, both equations go to zero. To compare eq. (8) with experimental data, let us note two extreme cases. (1) E - E o / k T << 1. In this case equation (8) has the form AG G

AT e x p ( - 2 E o / k T ) ( E - E o ) T o~2T2 kT

AQ-' Q-1

'

E - E0 kT

(9)

(2) E - E o / k T >> 1. In this case we have AG G Q 1

AT e x p ( - Z E o / k T ) T

~2Tg

'

245

where E o = A l l o + Bl o (l o is the most probable length of a segment before irradiation) and the equation agrees with the experimental dependence at small doses. In the case of large doses, when Qlot >> 1, E

-

Bl o AQt + Qlo----~ ~ AQt,

(12)

where E0 << E. So the comparison of given analytic expressions for instances of small and large doses agrees with experimental dependences (fig. 1). Rather interesting in our opinion are the dependences of linear dimensions on the dose calculated from the sample. All AszSs-As2Se 3 and AszSe3-AszTe 3 glasses samples have a maximum value for A L / L = f ( D ) (fig. 2). (This maximum in A L / L is observed in the data for melted quartz and glassy Se.) The dose at which the maximum is observed depends on glass composition. Figure 2 shows that the linear dimensions of samples (As2Se3-glass, melted quartz and glassy Se) decrease at certain -/-irradiation doses. This fact proves the structural densification of samples. Gamma-stimulated densification of ChGS glasses increases chemical stability to dissolution by solvents. Experimental data showing the dissolution of y-radiated bulk glass samples and films of As-Se system in the water solutions of K O H [9] are given in fig. 3. Let us turn to fig. 4, where isodose changes of shear modulus for samples of glasses with compositions As2S3-AszSe 3 and As2Se3-AszTe 3 are presented. It is seen that A G / G decreases when atoms of sulphur are substituted by atoms of Se and also by atoms of Te. For AszS 3 glass, the maximum change in A G / G equals 5.4% and for As2Se 3 • 2As2Se 3 these changes reach only 0.52%.

(10) 1 - exp

kT

"

o,o~5 From eqs. (9) and (10), it is seen that in the initial stage of irradiation, when Qlot << 1 (case 1), A G / G and A Q - 1 / Q -1 have a linear dependence on dose. Indeed, when Qlot << 1, E - E 0 = [(A/10) - BIo] Qlot,

(11)

0,025

0,025

Fig. 2. The y-irradiation influence on (1) linear sizes of As2Se 3 glass, (2) glassy selenium (2) and (3) melted quartz.

Is-~'a'a'°°a[

LA. Domoryad / Mechanicalproperties of chalcogenideglasses

246

""

at

J

Fig. 3. The dissolution kinetics of 7-irradiation bulk and AsSe film in 30% KOH water solution. The relative time dependence for dissolution of (1) half of mass bulk sample and (2) half of film thickness on "t-radiation dose. (The error in measuring of shear modulus is 0.01%/9.) The effect of y-irradiation on these parameters of the A s - S e - G e system is less than for the As2Se 3 glasses. Figures 5(a, b) depict isodose change of microhardness and dose relative changes of shear modulus for glasses of As2SeaGe x (0.1 ~< x ~< 2.0). F r o m fig. 5 it follows that, for As2SeaGeo. 1 ( D ' = 1.8 × 108 R), A G / G = 2.5% and for glasses containing x = 2.0 G e atoms, these changes are only 0.6%. To explain the decrease of these parameters at high doses in glasses of the A s - S e - G e system, which is correlated with the increase of concentration of Ge atoms, we propose additional hypotheses. (1) The C h G S experience a radiation-stimulated stabilization connected with the breaks in polymer chains and repacking of segments of these chains. (2) The presence of atoms of G e increases the effective segment lengths. The law of change of

G

t0

I b ! ~ O,5

J¢,O ~,5 2,0 . ~

Fig. 5. The influence of "y-radiation on mechanical parameters of glasses of the As-Se-Ge system. (a) Isodose changes of microhardness of glasses according to the composition. (b) Dependence of relative changes of shear modulus As2Se3Gex (0.1 ~
6

't

.'

3

.t

./ |

Fig. 4. [so(lose changes of shear modulus for glasses of A%$~-

As2Se3 and As;Se3-As;Te3 systems.

oe.~o

;

I

."1 I

I

- - -- .N.Sa3~1

,-a --2"~s~ st.~ I

t

(~

g"l

i ~ I I I I I i

V,g

I

_ cMe~,es-

Fig. 6. Dependence of relative changes of shear modulus As2S3 and As2Se3 glasses in the 'irradiation-annealing' cycles (see text for explanation of points 1-3).

I.A. Domoryad / Mechanicalproperties of chalcogenide glasses irradiation (this dose is also the dose for maxim u m changes); point 3 corresponds to samples after annealing at a temperature close to Tg. Figure 6 shows that within the second cycle the effect of irradiation decreases. So, if the m a x i m u m relative change of shear modulus in the first cycle for As2S 3 is 5.4% then in the second cycle the change is 4.9%. A similar trend is observed for As2Se 3 glasses. The observed reversibility of the parameters m a y be predicted by the phenomenological model. As stated above, two contradictory processes, i.e. destruction and polymerization, occur during irradiation and gradual restructuring takes place. However, at each stage of irradiation the polymeric structure m a y be in one of several possible metastable states. Possibly, this metastability is the cause of recovery of the initial parameters of material while being annealed. The effect are observed in y-irradiated glassy Se [4,13]. The probability of recovery of structural transformations, peculiar to m a n y ChGS, depends on interatomic covalent connections and Van der Waals interactions. Let us indicate some individual features of glasses of A s - S e - I system under -/-radiation. It is k n o w n that the inplantation of I atoms into glasses breaks the monolithic structure and results in a decrease of thermostability and microhardness of material [11]. Iodine atoms interact with both c o m p o n e n t s of the glasses, i.e. As and Se, but the binding energy to each is different: E ( S e - I ) < E ( A s - I ) . We assume that initially under the radiation the breaking of S e - I b o n d s takes place. In these processes y-stimulated diffusion of I atoms to the surface of material and the following liberation of I atoms from glasses can occur. (It is k n o w n that some .~

80

(a) ~o

(b)1

ta9

e Fig. 7. Influence of y-radiation on (a) microhardness and (b) Tg for As2Se3Iy glasses.

247

I-containing glasses lose iodine when being kept for a long time [11].) If this supposition is correct the increase of material microhardness and corresponding increase of Tg must take place with increasing dose. Figures 7(a, b) give the dependences H = f t ( D ) and Tg = f 2 ( D ) , respectively, for As2Se3Iy (0.01 ~< y ~ 0.7) which show that the microhardness increases linearly with the dose and Tg also increases. Taking into consideration the b r o a d scope of experimental data, we conclude that y-stimulation densification of the material takes place in C h G S of various compositions when being irradiated. It provokes consequent changes of several mechanical structurally sensitive parameters. We propose that this -/-strengthening is typical of the materials with non-organic p o l y m e r structure.

References [1] V.M. Lubin, in: Proc. Int. Conf. on Amorphous Semiconductors '78, Vol. 2, ed. L. StouraE (Institute of Solid State, Prague, 1978) p. 20. [2] M.I. Klinger, in: Proc. Int. Conf. on Non-crystalline Semiconductors '89, Vol. 1, eds. V.V. Chiminetz and N.V. Dovgoshei (Uzhgorod State University, Uzhgorod, 1989) p. 4. [3] E.A. Porai-Koshits, J. Non-Cryst. Solids 123 (1991) 1. [4] I.A. Domoryad, in: Proc. Int. Conf. on Amorphous Semiconductors '84, Vol. 1, eds. E. Vateva and A. Buroff (Institute of Solid State Physics, Sofia, 1984) p. 247. [5] I.A. Domoryad, B.T. Kolomiets, Proc. ICALS, Leningrad (Nauka, Moscow, 1976) p. 155. [6] I.M. Babakov, Teoriya Kolebanii (Gostechizdat, Moscow, 1958). [7] R. Charlsby, Jadernye Izlucheniya i Polymeri (lnostrannaya Literatura, Moscow, 1962). [8] S.V. Starodubcev and I.A. Domoryad, in: Starodubcev's Collected Works, Vol. 4 (Fan, Tashkent, 1972) p. 262. [9] I.A. Domoryad, V.M. Lubin, B.T. Kolomiets and V.P. Shilo, Fiz. Claim. Stekla 11 (1985) 595. [10] I.A. Domoryad, V.G. Kudryavzev and S.Z. Dunin, Poluprovodnikovie Materiali Dlya Tverdotel'noi Elektroniki (Shiniza, Kishinev, 1982) p. 255. [11] Z.U. Borisova, Khimiya StecloobraTmichPoluprovodnikov (Chemistry of Vitreous Semiconductors) (Leningrad State University, Leningrad, 1972). [12] I.A. Domoryad, in: Amorphous Semiconductors '82 (Abstracts), eds. R. Grigorivici and A. Vancu (Central Institute of Physics, Bucharest, 1982) p. 287. [13] I.A. Domoryad, M.G. Spirina and N.I. Timochina, J. Non-Cryst. Solids 90 (1987) 525.