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Estimation of Glass-Forming Ability and Glass Stability of Sb2S3-As2S3-Sb2Te3 Glasses by Thermal Properties



K. N`Dri, J. Sei , D. Houphouet-Boigny , G. Kra and J.C. Jumas
 
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ABSTRACT

Glass forming ability parameters defined by Trg =T8/Tm and Kh = (-Tg)/(Tm-), glass stability parameters such as Kw = (Txh-Tg)/Tm, ΔT = Txh-Tg and Kll = Txh/(Tg+Tm) and the degree of undercooling ΔTr = (Tm-T)/Tm are analysed for glasses of the ternary system Sb2S3-As2S3-Sb2Te3 as function of the As2S3 concentration. These parameters are formulated by different combinations of the following characteristic Differential Scanning Calorimetry (DSC) temperatures: the glass transition temperature (Tg), the onset crystallization temperature (Txh), the peak crystallization temperature (Tch) and the melting temperature (Tm). Variations of the above parameters indicate that the studied glasses can vitrify easily and become increasingly stable when the concentration of As2S3 increases. Good correlations between ΔTr and Kh and between ΔTr and parameters (Kw, ΔT and Kll), are found implying a low frequency of homogeneous nucleation in the thermally stable glasses. The degree of undercooling ΔTr is an important parameter for the glass forming ability and the glass stability of Sb2S3-Sb2Te3-As2S3 glasses.

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K. N`Dri, J. Sei , D. Houphouet-Boigny , G. Kra and J.C. Jumas , 2007. Estimation of Glass-Forming Ability and Glass Stability of Sb2S3-As2S3-Sb2Te3 Glasses by Thermal Properties. Journal of Applied Sciences, 7: 3167-3176.

DOI: 10.3923/jas.2007.3167.3176

URL: https://scialert.net/abstract/?doi=jas.2007.3167.3176

INTRODUCTION

The ability of substances to vitrify on cooling from the melt is known as glass-forming ability (GFA). Glass formation of materials containing one or more elements sulphur (S), selenium (Se) or tellurium (Te) in combination with elements from IVth and Vth group of the periodic table is relatively easy. Many kinds of these materials have been prepared by means of melt quenching method (Saffarini and Saiter, 2006; Repkova et al., 2006; Singh et al., 2006; Soliman and El-Den, 2007; El-Mokhtar, 2007). Several parameters or criteria have been proposed to reflect the relative GFA among bulk glasses on the basis of different calculation methods (Lu and Liu, 2002, 2003). One is the reduced glass transition temperature Trg = Tg/Tm which is the ratio between the glass transition temperature Tg and the melting temperature Tm of the corresponding glass-forming system (Kumar et al., 2006; Chol-Lyong et al., 2006). Another parameter Kh = (-Tg)/(Tm-), where is the onset crystallization temperature, is also used as a measure of the glass-forming tendency of materials by Farid (2002), Aljihmani et al. (2003) and Nikhil Sur et al. (2006).

Once a glass is made for instance by fast quenching a melt, its stability can be easily investigated. Thus the supercooled liquid range ΔT = -Tg (Kumar et al., 2006), the parameter Kw = (-Tg)/Tm(Avramov et al., 2003; Nasciemento et al., 2005) and a new criterion Kll =/(Tg+Tm) of Lu and Liu (2002, 2003) are used to evaluate the glass stability against crystallization on heating.

The thermal stability of the metastable supercooled liquid obtained at temperatures between Tg and Tm or liquids temperature Tl can be discussed from the kinetics aspect because it would be informative. In supercooled liquids, the frequency of homogeneous nucleation depends on the degree of undercooling ΔTr = (Tm-T)/Tm as predicted by the classical theory of homogeneous nucleation.

The aim of this study is to calculate the values and interpret the variations of the glass-forming parameters (Trg and Kh), the glass stability parameters (ΔT, Kh and Kll) and the degree of undercooling (ΔTr) for vitreous samples of the ternary system Sb2S3-As2S3-Sb2Te3 as function of As2S3 concentration. Correlations between these parameters will be made in order to show the importance of the degree of undercooling for glass forming ability and glass stability and a possible relationship between glass-forming ability and glass stability for glasses of the above system will be verified.

MATERIALS AND METHODS

Glasses of Sb2S3-As2S3-Sb2Te3 system were prepared by our vitreous semiconductors group in &laqvo;Laboratoire de Chimie des Matériaux Inorganiques&raqvo;, by direct synthesis from pure starting elements such as As, Sb, Te and S. Quartz ampoules were filled with ~ 0.3 g of the mixed elements and then evacuated to ~10–3torr, sealed and heated to 900°C at the rate of 1°C min‾1. The tubes were held at this temperature for 24 h and quenched in ice-water. The glassy state in the quenched samples was confirmed by X-ray diffraction (as no sharp peak was observed) at room temperature using the Cu-Kα radiation (λ =1.5405 Å). The shaded area representing the domain of glass formation in the Sb2S3-As2S3-Sb2Te3 system is shown in Fig. 1.

The thermal characteristic temperatures such as the glass transition temperature (Tg), the onset crystallization temperature (), the peak crystallization temperature () and the melting temperature (Tm) were measured in &laqvo;Laboratoire des Agrégats Moléculaires et des Matériaux Inorganiques&raqvo;, by using DSC 121 Setaram apparatus at a heating rate of 5°C –1 in the studied temperature range (from 25 to 650°C). For studied glasses of the ternary system Sb2S3-Sb2Te-As2S3, glass-forming ability was estimated using the following numerical parameters: The reduced glass transition temperature, Trg= Tg/Tm and Kh() = (-)/(-) or Kh() = (-Tg)/(Tm-) parameters. The glass stability parameters were also estimated by Kw() = (-Tg)/Tm or Kw() = (-Tg)/Tm, Kll () = /(Tg+Tm) or Kll () = /(Tg+Tm) and ΔT() = -Tg. The degree of undercooling was evaluated by ΔTr() = (Tm-)/Tm (Komatsu et al., 1997).

Fig. 1: Zone of glass formation in the Sb2S3-Sb2Te3-As2S3 system

In this study we alternatively substitute and (except for ΔTr() and ΔT()) and calculate the values of glass-forming ability and glass stability parameters given by above expressions and analysis will be made for them.

RESULTS AND DISCUSSION

Positions of the glasses with their numbering used throughout this study are shown in (Fig. 2). Several glasses of Sb2S3-As2S3 (0 mol% Sb2Te3) binary system and a series of samples with the constant Sb2Te3 concentration of 20 mol% in the Sb2S3-As2S3-Sb2Te3 ternary system were investigated by Differential Scanning Calorimetry (DSC) Table 1.

Glass-forming ability parameters: Using the data shown in Table 1, the reduced glass transition temperature (Trg) values for glasses of Sb2S3-As2S3 system increase from 0.607 (10 mol% As2S3) to 0.714 (50 mol% As2S3). Trg calculation is not possible for some glasses of this system having As2S3 concentration beyond 50 mol% because of the absence of the melting temperature Tm. On the ternary system Sb2S3-As2S3-Sb2Te3 with a constant concentration of 20 mol% Sb2Te3, the evolution of Trg is not linear. Trg has an optimal value equal to 0.765 (30 mol% As2S3) and decreased values are observed when increasing As2S3 concentration is up 30 mol% (Table 1).

The parameters Kh() and Kh(), calculated with the onset and peak crystallization temperatures and , respectively (Table 1), increase from 0.039 to 0.710 on Sb2S3-As2S3 system (Fig. 3). But Kh() and Kh() can not be calculated when As2S3 concentration is beyond 50% because glasses exhibit no crystallization temperature. The crystallization and melting temperatures absence has been observed in the Sb2S3-As2S3 system by Durand et al. (1997) and in other systems such as Ge-Se- Te-Sn, Sb-Se-Ge-Ga and TeSe3IAs4 by Feng et al. (1999) and Jean-Luc Adam (2001), respectively.

Fig. 2: Positions within the Sb2S3-Sb2Te3-As2S3 ternary system

Table 1: Composition of glasses, thermal properties (Tg,Txh, Tch and Tm) from DSC curves, values of glass forming ability parameters (Trg, Kh (Txh) or Kh (Thc)), values of glass stability parameters (Kw (Txh) or Kw (Tch), (Kl l(Txh) or Kll (Tch) and ΔT (Tch)) and degree of undercooling ΔTr (Txh) for studied glasses and the ternary system Sb2S3-As2S3-Sb2Te3
---: Absence of thermal properties (Tg,Txh, Tch and Tm) from DSC curves, values of glass- forming ability parameters (Trg, Kh (Txh) or Kh (Tch)) and those of glass stability parameters (Kw () or Kw (Tch), (Kll (Txh) or Kll (Tch) and ΔT (Txh)) and value of degree of undercooling ΔTrg () can’t be calculated

Fig. 3: Variation of Kh () and Kh () with the As2S3 concentration in the Sb2S3-As2S3 binary system

The glass-forming ability of Sb2S3-As2S3 glasses having As2S3 concentration beyond 50 mol% is higher than GFA of their counterparts of the aforesaid system because they do not exhibit any crystallization and melting temperature (Table 1). The absence of these thermal characteristics can be associated with the glass-forming ability because glass without crystallization temperature can be a good glass former and can have a good glass-forming ability. For Sb2S3-As2S3-Sb2Te3 glasses containing a constant Sb2Te3 concentration of 20 mol%, Kh () and Kh () parameters increase from 0.018 to 0.826 and from 0.077 to 1.066, respectively as shown in Fig. 4. In spite of the differences between the values of Kh () and Kh (), they show similar trends in all studied cases (Fig. 3, 4). even if values of Kh () are higher than those of Kh ().

Fig. 4: Variation of Kh () and Kh () with the As2S3 concentration in the Sb2S3-As2S3-Sb2Te3 ternary system at constant Sb2Te3 concentration of 20 mol%

Variations in Trg and Kh parameters are largely due to the variations in the thermal properties of glasses. These parameters seem to depend on compositions. Even if Trg does not give the width of the temperature interval it also determines how close to the liquidus temperature the decreasing mobility in the liquid starts to reduce the nucleation rate. Trg plays a crucial role in determining the glass-forming ability of an alloy because the higher is the ratio, the higher is the Glass-Forming Ability (GFA) according Uhlmann (1977) and Davies (1975). It has been confirmed that Trg = 2/3, the two thirds rule, holds well generally for wide variety of inorganic glass forming substances (Sakka and Mackenzie, 1971). Thus for the liquids having Trg equal to or more raised than 2/3, the formation of glasses would be easy because it leads to low nucleation rates. As pointed out by Turnbull (1969), these liquids are good glass formers. It is clearly shown that the two thirds rule holds well for the studied glasses of Sb2S3-As2S3-Sb2Te3 system.

The increase of Kh(), Kh() and Trg (except of Trg of 20% Sb2Te3) with increasing As2S3 concentration can enable to suggest that As2S3 (covalent compound) incorporation supports the Glass-Forming Ability (GFA). In other words, GFA increases with increase in As2S3 concentration. For these glasses Kh(), Kh() and Trg reflect the GFA effectively. GFA in the Sb2S3-As2S3-Sb2Te3 system is conditioned by the presence of a glass forming compound As2S3. This indicates that As2S3 glass is the best glass forming system among the vitreous samples of the above system. There is in this case an obvious correlation between covalence and glass-forming ability. It is possible to suggest that the presence of covalent bondings gives flexibility (elasticity) to the structure that is a necessary factor for the topological disordering of the structure during glass formation. The enough large flexibility that permits to the elementary components (atoms, cations, coordination polyhedra) to occupy different positions one relative to another, which fact does not create long range order and does not lead to the simultaneous appearance of strains, that destroy the structure, gives the ability to oxide and chalcogenide systems to form glasses. Trg evolution of Sb2S3-As2S3-Sb2Te3 glasses containing 20 mol% Sb2Te3 is not linear like Kh(), Kh() and Trg seen above. For these glasses, Trg cannot reflect the GFA. This behaviour was found in many bulk metallic glasses (Lu and Liu, 2002) and phosphate glass systems (Ouchetto et al., 1991).

Glass stability parameters: In this study, values of the supercooled liquid region ΔT(Txh), Kw() or Kw() parameter and Lu-Liu parameter Kll (Txh) or Kll (Tch), used for stability assessment of Sb2S3-As2S3-Sb2Te3 glasses, Table 1. Generally, the difference ΔT (Txh) = Txh-Tg, gives a measure of thermal stability of the glass (Kamboj and Thangaraj, 2003). For the present study, its values are found to be in the range 12-81 K on the binary system Sb2S3-As2S3 and 05-71 K on the ternary system Sb2S3-As2S3-Sb2Te3 (containing 20 mol% Sb2Te3). Values of Kw() or Kw () parameters and Lu-Liu parameters Kll (Txh) or Kll (Tch) increase when As2S3 concentration increases on the binary system Sb2S3-As2S3 (Fig. 5, 6) and on the ternary system Sb2S3-As2S3-Sb2Te3 with 20 mol% Sb2Te3 (Fig. 7, 8). When the concentration of As2S3 is beyond 50 mol%, ΔT, Kw or Kll can not be calculated for certain glasses of Sb2S3-As2S3 system because these glasses exhibit Tg but no crystallization temperature (Table 1).

Fig. 5: Variation of Kw () and Kw () with the As2S3 concentration in the Sb2S3-As2S3 binary system

Fig. 6: Variation of Kll () and Kl l() with the As2S3 concentration in the Sb2S3-As2S3 binary system

There is consequently no melting temperature of the crystallized species. Very stable against devitrification, these glasses can be pulled into optical fibers (Jean Luc Adams, 2001). All values of Kw () and Kll () obtained from () are superior to those of Kw () and Kll() calculated with . The alternative use of instead of in these expressions does not result in significant differences in these glass stability parameters. Even if Kw and Kll obtained at and don’t give the width of the temperature interval like ΔT (), they can be suggested to represent the glass stability because they have similar trends when they are plotted versus As2S3 concentration (Fig. 5-8). It is obvious that the thermally stable glasses in the above systems are obtained when the As2S3 concentration increases. In other words, the thermal stability of these glasses against crystallization increases with increase in As2S3 concentration.

Fig. 7: Variation of Kw () and Kw () with the As2S3 concentration in the Sb2S3-As2S3-Sb2Te3 ternary system at constant Sb2Te3 concentration of 20 mol%

As can be shown in Table 1 the increases in ΔT () in the two systems mainly result from the increase of . This implies that the added As2S3 acts effectively as an inhibitor of crystallization. The onset-crystallization can serve as an important factor estimating the stability of glass (Hen et al., 1991). On the other hand, it is reasonable to assign that As2S3, a covalent compound, acts as network breaking agents to decrease Tg and Tm because both of them decrease with increasing As2S3 concentration as shown in Table 1.

Increased values of ΔT(Txh) may indicate that the supercooled liquid can remain stable in a wide temperature range without crystallization. Thus, As2S3 incorporation has a stabilizing effect because ΔT (Txh) becomes wide when its concentration increases. It is desirable to have ΔT as large as possible in order to achieve a large working range during operations such as perform preparation for fibre drawing according Feng et al. (1999). The partial replacement of Sb2S3 (less covalent) by As2S3 (covalent), on Sb2S3-As2S3 and Sb2S3-As2S3-Sb2Te3 (with 20 mol% Sb2Te3) systems, can exhibit a predominantly character which increases the resistance of the glass to devitrification. Thus, As2S3 involves probably the formation of high stable network structure which may be due to the presence of covalent bondings. That means that As2S3 acts to increase the homogeneity of glass and strengthening the glass network. In the same time it acts to increase the covalence character by forming stable units which ensure effectively the stability of the glasses. According to a previous study (El-Idrissi Raghni et al., 1995), the Raman and IR bands of Sb2S3-As2S3 glasses are attributed to AsS3 and SbS3, the trigonal pyramidal units in which As and Sb obey the 8-N rule (N is the number of electrons needed to complete its valence shell).

Fig. 8: Variation of Kll () and Kll () with the As2S3 concentration in the Sb2S3-As2S3-Sb2Te3 ternary system at constant Sb2Te3 concentration of 20 mol%

Results of 121Sb Mössbauer spectroscopy of 20 and 40 mol% Tl2S sections of Sb2S3-As2S3-Tl2S glasses (Durand et al., 1997) and those of 10 mol% Sb2Te3 section of Sb2Se3-As2Se3-Sb2Te3 glasses (Leh Deli et al., 2005), indicate that the isomer shifts are negative in all vitreous compounds studied by the above researchers. These isomer shifts are found to be between -5.06 and -4.56 mm sec‾1 for Sb2S3-As2S3-Tl2S glasses and between -5.16 and -4.79 mm sec‾1 for those of Sb2Se3-As2Se3-Sb2Te3. Antimony (Sb) in these glasses exists only as Sb(III) species. So its coordination is pyramidal (SbS3). As there is no available information on the structure of Sb2S3-As2S3-Sb2Te3 glasses containing a constant concentration of 20 mol% Sb2Te3, we can suggest that these glasses also consist of mixed glass networks with the combination of SbS3 and AsS3, the trigonal pyramidal units as seen in the case of Sb2S3-As2S3 glasses. There is obvious correlation between covalence and glass stability.

Degree of undercooling parameter: The calculated values of ΔTr(Txh) at the onset crystallization temperature (Txh) for examined glasses, are shown in Table 1. It is indicated that ΔTr (Txh) values depend on the thermal characteristics such as and Tm. increases but Tm decreases when As2S3 concentration increases on Sb2S3-As2S3 binary system and on 20 mol% Sb2Te3 section of Sb2S3-As2S3-Sb2Te3 ternary system. As indicated in Fig. 9 and 10, Txh decreasing and Tm increasing imply the increasing of ΔTr (Txh). So at the higher value of (lower value of Tm) in each system, corresponds to the smaller value of ΔTr (Txh) and a small undercooled region (Tm-Txh) is observed. At the smaller value of (higher value of Tm) corresponds to the higher value of ΔTr (Txh) and a wide undercooled region is also observed.

Fig. 9:
Evolution of the onset crystallization temperature , the melting temperature Tm and the undercooled liquid region Tm-versus the degree of undercooling Δtr () on the Sb2S3-As2S3 binary system

Fig. 10:
Evolution of the onset crystallization temperature Txh, the melting temperature Tm and the undercooled liquid region Tm-Txh versus the degree of undercooling ΔTr () on the Sb2S3-As2S3-Sb2Te3 ternary system at constant Sb2Te3 concentration of 20 mol%

The degree of undercooling depends on the extended undercooled region which depends on Txh and Tm. The variation of the degree of undercooling, ΔTr (Txh), as a function of As2S3 concentration is shown in Fig. 11. ΔTr (Txh) decreases linearly on Sb2S3-As2S3 system but it decreases exponentially on the Sb2S3-As2S3-Sb2Te3 (containing 20 mol% Sb2Te3) with increasing of As2S3 concentration. In the two cases, it is seen that small values of ΔTr () are obtained when the concentration of As2S3 increases.

Fig. 11: Variation of ΔTr () with the As2S3 concentration in the Sb2S3-As2S3-Sb2Te3 ternary system at constant Sb2Te3 concentrations of 0 and 20 mol%

The decreased values of ΔTr () can be explained by progressive substitution of Sb2S3 by As2S3 as it is seen above in the cases of glass-forming ability and glass stability. Thus, added As2S3 can diminish the degree of undercooling and the undercooled region. These results imply that the glasses with high concentration of As2S3 have a possibility of a low frequency of homogeneous nucleation because ΔTr () decreases in respect to the classical theory of homogenous nucleation. A similar trend was reported in (30-x)K2O-xNb2O5-70TeO2 glasses (Komatsu et al., 1997) when Nb2O5 concentration increases.

Correlations between the degree of undercooling, glass-forming ability and glass stability: Similar trends were observed when glass-forming parameters (Kh (), Kh ()) and glass-stability parameters (Kw ( or ) and Kll ( or ) are plotted versus As2S3 concentration. So, we will use only values of Kw () and Kll () obtained with temperature during correlations in which these parameters are used.

Correlation between ΔTr () and Kh(), for studied glasses containing 0 and 20 mol% Sb2Te3, is shown in Fig. 12 which indicates that ΔTr () decreases when Kh () increases. The same behaviour is observed between ΔTr () and the other glass-forming ability parameter Trg (Fig. 13) of Sb2S3-As2S3 (0 mol% Sb2Te3) binary system. In other words, the frequency of homogeneous nucleation becomes lower when GFA becomes higher. According to Nasciemento et al. (2005), a high glass forming ability is associated with slow crystallization rate.

Fig. 12: Correlation between ΔTr () and Kh () in the Sb2S3-As2S3-Sb2Te3 ternary system at constant Sb2Te3 concentrations of 0 and 20 mol%

Fig. 13: Correlation between ΔTr() and Trg in the Sb2S3-As2S3-Sb2Te3 ternary system at constant Sb2Te3 concentrations of 0 and 20 mol%

From this assertion, it can be suggested that an undercooled liquid having high glass-forming ability (high value of Kh () or Trg) can be a good glass forming material. Because it can be easily quenched into a glass by using small cooling rate. But an undercooled liquid having small glass-forming ability (low value of Kh (Tch) or Trg) cannot be a good glass-forming material. Because it requires high cooling rate to be quenched into a glass. For the studied glasses, ΔTr () can be an important parameter for the glass-forming ability because a good correlation is obtained.

Correlation between ΔTr (Txh) and ΔT (Txh) of glasses with 0 and 20 mol% Sb2Te3 shows that ΔTr (Txh) decreases when ΔT (Txh) increases (Fig. 14). Supercooled liquid having wide supercooled liquid region (ΔT () is wide) is characterized by a low degree of undercooling ((ΔTr () is low).

Fig. 14: Correlation between ΔTr () and ΔT () in the Sb2S3-As2S3-Sb2Te3 ternary system at constant Sb2Te3 concentrations of 0 and 20 mol%

Fig. 15: Correlation between ΔTr () and Kw () in the Sb2S3-As2S3-Sb2Te3 ternary system at constant Sb2Te3 concentrations of 0 and 20 mol%

That means that the supercooled liquid is stable in wide temperature range without crystallization and with high resistance to the nucleation and growth of crystalline phases (Kapaklis et al., 2003). The same behaviour is observed when ΔTr () is correlated with the other glass stability parameters such as Kw () and Kll () (Fig. 15 and 16). At the higher values of ΔT(), Kw () and Kll () corresponds with lower value of ΔTr (). Thus, when the glass stability parameters become higher, the frequency of homogeneous nucleation becomes lower (ΔTr () decreases) and vice versa. The relationship between ΔTr () and parameters (ΔT(), Kw () and Kll ()) shown in Fig. 14-16 indicates a good correlation. ΔTr () is one of the key parameter for the glass stability of As2S3 based glasses.

Fig. 16: Correlation between ΔT () and Kll () in the Sb2S3-As2S3-Sb2Te3 ternary system at constant Sb2Te3 concentrations of 0 and 20 mol%

Fig. 17: Correlation between ΔT () and Kh () in the Sb2S3-As2S3-Sb2Te3 ternary system at constant Sb2Te3 concentrations of 0 and 20 mol%

Both ΔT () and Kh () of glasses with 0 mol% and 20 mol% Sb2Te3 increase with the increasing in As2S3 content (Fig. 17). In another words, a wide supercoooled liquid region shows a high glass-forming ability. A large ΔT () value may indicate that the supercooled liquid is stable in a wide temperature without crystallization, this leads to a larger GFA of the alloy (Inoue et al., 1993). There is a correlation between glass-forming ability and glass stability. As discussed above, the overall liquid phase stability is positively related to the quantity of ΔT () = -Tg while the crystallization resistance is proportional to . The increase of ΔT () can lead to an increase of liquid phase stability at metastable state and hence an increase in the GFA.

Fig. 18: Correlation between Kw () and Kh () in the Sb2S3-As2S3-Sb2Te3 ternary system at constant Sb2Te3 concentrations of 0 and 20 mol%

Fig. 19: Correlation between Kll () and Kh () in the Sb2S3-As2hS3-Sb2Te3 ternary system at constant Sb2Te3 concentrations of 0 and 20 mol%

Therefore, the GFA is positively associated with the ΔT () for glasses of Sb2S3-As2S3-Sb2Te3 system. So Kh () can be used to represent ΔT () or Trg and vice versa for these glasses based on As2S3. That means ΔT () is a good criterion for characterization of GFA for these glasses used in the experiment. This speculation has been well confirmed in several glass-forming alloy systems in which the supercooled liquid region correlates reasonably well with the GFA of alloys (Shen and Schawrz, 1999). The same trend is observed between Kh (Thc) and the other parameters of glass stability (Kw (Thc) and Kll (Thc) of 0 and 20 mol% Sb2Te3 (Fig. 18, 19). According to Hruby (1972), the higher is the value of Kh for certain glass, the higher its stability against crystallization on heating and presumably, the higher the glass ability to vitrify on cooling. Glass which vitrifies easily (high Kh ()) is a thermally stable glass (high ΔT ()). Glass forming ability Kh () governs the thermal stability of studied glasses. This behaviour is observed when each glass stability parameter is plotted as a function of Trg in Sb2S3-As2S3 binary system but not in the case of Trg of 20% Sb2Te3 in Sb2S3-As2S3-Sb2Te3 system. Thus Trg is not a good indicator of glass-forming ability for these glasses containing 20 mol% Sb2Te3.

CONCLUSION

Glass forming ability parameters (except of Trg of 20 mol% Sb2Te3) and glass stability parameters increase but the degree of undercooling of glasses decreases when the content of As2S3 increases in Sb2S3-As2S3-Sb2Te3 system. This implies that glasses can be obtained easily and they can become most stable against crystallization. A low frequency of homogeneous nucleation can be suggested. The correlations between the degree of undercooling, glass forming ability and glass stability have shown that the degree of undercooling is an important parameter for the studied glasses.

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