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Mechanical damping of silver-containing SiAsChalcogenide glasses
Journal of Non-Crystalline Solids, 16 (1974) 46-54. © North-Holland Publishing Company
MECHANICAL DAMPING OF SILVERCONTAINING Si-As-CHALCOGENIDE GLASSES Eva-Maria AMRHEIN*, Delbert E. DAY and Norbert J. KREIDL Department of Ceramic Engineering, The University of Missouri-Rolla, Rolla, Mo. 65401, USA Received 1 April 1974 Mechanical damping measurements were used to study the structural conditions responsible for the increase in thermal stability that has been observed when Ag and Se are substituted simultaneously in Si35As25_x Agx Te40_y Sey glasses, Glasses containing Ag exhibited a mechanical damping peak whose magnitude was approximately proportional to the Ag content and which was absent in the more completely cross-linked Si3s As2s Te40 base glass. This peak shifted to lower temperatures and split into two overlapping peaks with increasing Se content. The splitting of the peak has been tentatively attributed to phase separation which has been postulated to occur at higher Se contents.
1. Introduction An investigation of infrared-transmittant chalcogenide glasses has shown [I, 2] that the simultaneous introduction of Ag and Se into a Si35As25Te40 base glass resuits in a considerable improvement in thermal stability, particularly after controlled heat treatment. The high softening temperatures of some of these glasses ( > 650°C for log r/= 8.4) have been attributed to the introduction of Se with its higher bond strength, which becomes compatible with the base glass when Ag is present, and to the initiation and development of glass-glass phase separation, with the matrix containing the stronger bonds in a compositional phase not obtainable in a single phase bulk glass. It is difficult to experimentally demonstrate this plausible interpretation, however. The electron density difference of the two phases appears to be so small that electron microscopy has thus far, pending the development of more specific etching techniques, only given an indication of phase separation. Similarly, the spatial distribution of the two phases is apparently too small for successful analysis by electron microprobe. Thermal expansion curves frequently show two break points characteristic of phase separation [1 ] and DTA gives an indication of the higher Tg of the rheologically dominant phase and its development with heat treatment. It was expected, however, that mechanical and dielectric loss measurements might provide information * Present address: Schoenstatt Center, W. 284 N. 698 Cherry Lane, Waukesha, WI 53186.
E.M. Amrhein et al., Mechanical damping
47
on the immediate surrounding of the Ag atom and its contribution to the structural changes associated with the higher thermal stability. This paper presents the results of mechanical loss measurements on glasses in the system Si35 As25_x Agx Te40_y Sey. The glasses are identified by x and y, which represent the atomic percent Ag and Se, respectively. For example, glass 8 - 1 5 has the composition Si35 ASl7 Ag 8 Te25 Sel5.
2. Experimental procedure Considerable experimental difficulty was encountered in preparing samples of suitable shape for the mechanical loss measurements from melts that can not be exposed to oxygen and, in most cases, form very brittle glasses. Samples were obtained by remelting previously prepared glasses in evacuated fused silica vials of the desired shape and subsequently crushing, grinding or etching away the vial. Silica capillaries were used for obtaining thin rods. Three different techniques were employed to measure the mechanical loss over an extended frequency and temperature range. The methods, frequency range and sample configurations were: (1) An inverted torsion pendulum operating at 0 . 5 - 5 Hz, using rods less than 1 mm dia. (2) A Forster-type sonic apparatus [3], utilizing vibrations at 2 - 2 0 kHz on rods 5 - 7 mm dia. and 5 - 7 cm long. (3) A composite quartz oscillator [4] operating at 33 kHz. Half-wavelength samples of 5 mm dia. ( 3 . 5 - 4 . 5 cm long, depending upon the velocity of sound in the samples) were attached to the vibrating quartz assembly by a thin film of vacuum grease or bee's wax. Bee's wax was used for the low temperature measurements to avoid additional temperature dependent damping caused by the contact grease. Since the accuracy of this method is high and sample preparation as well as attachment were relatively easy, most of the data were obtained by this method.
3. Results
Examples of the measured values of mechanical loss versus temperature are shown in fig. 1 for the three measuring techniques. The background losses decreased in the order torsion pendulum > sonic ~> composite oscillator and this difference in background loss accounts for the relative vertical displacement of the curves in fig. 1. However, consistent results were obtained with the three methods concerning the presence or absence of a mechanical loss peak; compare curves B and D for glass 5-05. The change in the resonance frequency of the composite oscillator, as illustrated for glass 8 - 1 0 in fig. 2, depicts the relaxation of the elastic modulus normally associated with the mechanical loss peak observed in this glass (fig. 3).
E.M. Amrhein et al., Mechanical damping
48
5G
40
30 A ~
b 2C
10
0
i
-200
I
~
-I00
I
i
I
0
i
100 TEMP.
/
200
I
300
*C
Fig. 1. Internal friction versus temperature curves obtained by three different measuring techniques. Curve A, glass 8 - 05, torsion pendulum, f = 1.1 Hz; Curve B, glass 5 - 0 5 , sonic apparatus, f = 2460 Hz;Curve C, base glass, sonic apparatus, f = 5085 Hz, Curve D, glass 5 - 0 5 , composite oscillator, f = 33.6 kHz.
33,7
v
-
33.6
33.5
I
33.41 -200
f
I -I00
I
I 0
TEMP.
,
I I00
°C
Fig. 2. Resonance frequency of composite oscillator versus temperature for glass 8 - 1 0 .
E.M. Amrhein et al., Mechanical damping
49
Table 1 Internal friction measured with composition oscillator. Sample Quartz driver and gage crystal assembly Driver and gage crystal plus fused silica extension rod Si35As25Te4o base glass (measured between - 180 and -50°C) Glass5-15
Q-1 2.8 × 10 -5 6 - 7 × 10 -5 (in air) 3.7 X 10 -5 9.2 X 10 -4
O
Frequency = 33 kHz, 2 0 - 2 5 C, pressure ~ 10 -5 torr. The results obtained with the composite oscillator are plotted in subsequent figures in terms of the voltage ratio Vd/Vg, where Vd and Vg represent the voltage on the driving and gage crystal, respectively. This ratio is related to the internal friction, Q - l , by the mass ratio and resonance frequencies [4]. Since these latter two factors vary little over the temperature range investigated, the equation
Q-I=(~-~)(Vd/VX
10 - 4 )
(1)
is accurate within a few percent. Table 1 gives some examples for the absolute values of Q-1 measured with the composite oscillator. The change in the mechanical loss with the introduction o f Ag and Se into the base glass is shown in figs. 3 and 4. The data points have been omitted from these figures for better clarity, but there was very little scatter in the composite oscillator data as depicted by curve D in fig. 1. In general, one main mechanical loss peak was observed which became larger with increasing Ag content at constant Se content (fig. 3). With increasing replacement of Te by Se, this peak shifted to lower temperatures and showed evidence of splitting into two peaks (fig. 4). The activation energy for this peak is given in table 2, for those glasses where it could be calculated. Table 2 Activation energy of mechanical damping peak in Si3s As25-x Agx Teao-y Sey glasses. Glass x y
Measuring frequencies
Calculated act. energy (kcal/mol)
8
5
33.4 kHz
1.1 kHz
8
10
33.6 kHz
4.34 kHz
3.2
same sample, both frequencies
5
5
33.7 kHz
2.46 kHz
11.6
same sample, both frequencies
5
15
33.7 kHz
4.35 kHz 4.38 kHz
5.4 3.8
Remarks
12
duplicate low frequency measurement on same sample
50
E.M. Amrhein et al., Mechanical damping
50 3(3
8-05
2O
lC
/
0-00
(3
40 > ao >,,
20 10
"
5-~0
0 ...... 4O 203010 ~~------ 10-20 0 ...... -200
I
J
I
I
-100
i
0 TEMP.
I 100
I 200
°C
Fig. 3. Mechanical damping of silver-selenium doped Si-As-chalcogenide glasses. Frequency = 33.6 kHz.
4. Discussion The data for the mechanical losses are summarized for all glasses investigated in figs. 5 and 6. The difference in the resonance frequency, Af, between - 1 8 0 ° C and room temperature (fig. 2) has been used as a measure of the overall magnitude of the losses in fig. 5. Because of the splitting and overlap of the loss peaks in the glasses of higher Se content, the difference in the resonance frequency was found to be a more reliable measure of the total losses than the peak heights which could only be estimated. The points for the base glass in fig. 5 depict the normal temperature dependence of the elastic modulus. From the compositional dependence of the loss peak upon Ag content we con-
E.M. Amrhein et al., Mechanical damping
51
80_
60
5 -20
50
>~ >-o
40
30 20 10 0 -200
i
I -100
I
l i 0 TEMP. ° C
I 100
i
I 200
Fig. 4. Mechanical damping o f S i - A s - c h a l c o g e n i d e glasses containing 5% Ag with increasing substitution of Se for Te. Arrows designate a m o u n t each curve has been displaced vertically for improved clarity.
clude that this peak is due to the relaxation of Ag in these glasses. To the first approximation, the magnitude of the losses is linearly dependent upon the Ag content for a constant Se content (fig. 5A). Similarly, the activation energy for the mechan30C
300
A
/
20C
' ~ 100
/7 Base
Glass
-
I,iiif
0
5
10 AT. % A g
200
x
100
-
I
I
15
5
i
10 AT.%
i
15 Se
I
i
20
25
Fig. 5. (A) Dependence o f the dispersion height o f the mechanical loss peak on silver content. × = 5 % Se, • = 10 % Se, ® = 20 % Se. (B) Dependence of the dispersion height o f the mechanical loss peak on selenium content. × = 5% Ag, ®= 8% Ag, • = 10% Ag.
E.M. Amrhein et al., Mechanical damping
52
I
I..) O
I
I
I
I
-50
•t•
-100
x
I-.
-150 0
I
I
I
I
I
5
10
15
20
25
AT. PERCENT
Se
Fig. 6. Change in temperature of the loss maxima with Se content, f = 33.6 kHz. X = 5% Ag, ® = 8% Ag, • = 10% Ag. Two temperatures are shown for those glasses where two loss maxima were observed. ical loss peak in glasses of low Se content is close to that measured for d.c. conductivity. The activation energy for d.c. conductivity of 1 0 - 0 5 and 8 - 0 0 glasses varied from 13 to 15.6 kcal/mol, respectively, compared with 11 - 1 2 kcal/mol for the mechanical loss peak for 8 - 0 5 and 5 - 0 5 glasses (table 2). The mobility of Ag in these glasses can be envisioned to be relatively high assuming that Ag destroys the cross linking of the glass structure. Small additions of Se, less than ~ 10%, result in a small increase in the magnitude of the loss peak (fig. 5B). With further additions of Se the loss peak becomes smaller, but, more significantly, the peak shifts markedly to lower temperatures and shows evidence of splitting into two peaks (fig. 6). There is also a sharp reduction in the activation energy (table 2). The splitting of the peak is not attributed to the difference between Te and Se bonds; neither peak shows any linear dependence on Se content. Since peak heights and widths vary somewhat with sample preparation techniques this dependence must be studied in more detail. Nevertheless, the splitting of the loss peaks is tentatively attributed to increasing phase separation which is believed to occur with increasing Se content and heat treatment. Each peak is ascribed, therefore, to the Ag present in each of the two compositional phases. No mechanical loss due to S e - A s , S e - S e , T e - T e chains, as is found in pure I I I - V compounds [5], was detected. This can be explained by the action of Si as a cross-linking agent in the three dimensional network of the base glass. The role of Ge [6] and Si in chalcogenide glasses consists in forming four coordinated groupings initiating such a network. The mechanical loss of the base glass is nearly as low as that of vitreous SiO 2. The shoulder in curves A and B, (fig. 1), between
E.M. Amrhein et aL, Mechanical damping
53
100 and 200°C corresponding to Tmax _~ 0.7 Tg may be caused by the relaxation of chain segments formed when Ag is present, however.
5. Conclusions Mechanical loss measurements supplemented by other data [7] for S e - A s - T e glasses, in which Ag allows the introduction of substantial amounts of Se, suggest the following picture of the structural condition responsible for their high softening temperatures. The base glass Si35 As25 Te40 owes its high softening range to a cross-linked three dimensional network structure in an essentially homogeneous glass. The substitution of Ag for As leads to the formation of chain segments. The relative amount of available chalcogenide bonds is increased because of the lower bonding requirements of Ag and the softening temperature decreases. Increasing replacement of Te by Se causes a considerable increase in thermal stability not only by an increase in the average bond strength, but more so, by phase separation, with one connected phase having a very high viscosity. This is indicated by the reduction of the background losses and the splitting of the main loss maximum into two peaks when the Se content exceeds ~ 10%. The sharpening of one of the doublet peaks indicates a tendency to crystallization in one of the phases, which was also found in far infrared absorption measurements [8]. The additional increase in softening temperature with heat treatment is believed to be associated with the onset of crystallization. In chain structures small crystallites can drastically increase the viscosity because they restrict the motion of the chain segments [9]. There is no direct evidence that one of the phases is higher in Ag than the other. This may explain the lack of resolution in electron microscopy. This factor also shows the major role of microstructure, particularly in the matrix phase, in the achieved thermal stability. However, the Ag atom is able to change sites thereby producing a mechanical loss peak whose activation energy is comparable to that for d.c. conductivity.
Acknowledgements The support of the Office of Naval Research under grant N00014-69-A-0003 is thankfully acknowledged. We are indebted to Dr. A.E. Schwaneke, U.S. Bureau of Mines, Rolla Metallurgy Center for permission to use the quartz composite oscillator system and for their interest and help in this work. We also acknowledge the assitance of Carlton Sorrell in performing many of the experimental measurements.
54
E.M. Amrhein et al., Mechanical damping
References [1] H.E. Anthonis, N.J. Kreidl and W. Ratzenbock, J. Non-Crystalline Solids 11 (1972) 257. [2] E.M. Amrhein, D.E. Day and N.J. Kreidl, Thermally Resistant Chalcogenide Transparencies, Phys. Chem. Glass, submitted. [3] R. Forster, Z Metallkunde 29 (1937) 109. [4] A.E. Schwaneke, Construction and Operation of a Quartz Composite Oscillator, Rept. on Investigations 6419 (U.S. Bureau of Mines, 1964). [5] M. Imaoka and H. Sakemura, Asahi Garasu Gijutsu Shoreiaki Kenuy Hokoku 19 (1972) 1. [6] F. Betts and A. Bienenstock, J. Non-Crystalline Solids 8 - 1 0 (1972) 56. [7] J.P. De Neufville, J. Non-Crystalline Solids 8 - 1 0 (1972) 56. [8] E.M. Amrhein, D.E. Day, N.J. Kreidl and J.H. Weaver, Far Infrared Spectra of Polynary Chalcogenide Glasses, J. Non-Crystalline Solids, submitted. [9] P.J. Flory, Principle of Polymer Chemistry (CorneU University Press, 1953) 47-50.
MECHANICAL DAMPING OF SILVERCONTAINING Si-As-CHALCOGENIDE GLASSES Eva-Maria AMRHEIN*, Delbert E. DAY and Norbert J. KREIDL Department of Ceramic Engineering, The University of Missouri-Rolla, Rolla, Mo. 65401, USA Received 1 April 1974 Mechanical damping measurements were used to study the structural conditions responsible for the increase in thermal stability that has been observed when Ag and Se are substituted simultaneously in Si35As25_x Agx Te40_y Sey glasses, Glasses containing Ag exhibited a mechanical damping peak whose magnitude was approximately proportional to the Ag content and which was absent in the more completely cross-linked Si3s As2s Te40 base glass. This peak shifted to lower temperatures and split into two overlapping peaks with increasing Se content. The splitting of the peak has been tentatively attributed to phase separation which has been postulated to occur at higher Se contents.
1. Introduction An investigation of infrared-transmittant chalcogenide glasses has shown [I, 2] that the simultaneous introduction of Ag and Se into a Si35As25Te40 base glass resuits in a considerable improvement in thermal stability, particularly after controlled heat treatment. The high softening temperatures of some of these glasses ( > 650°C for log r/= 8.4) have been attributed to the introduction of Se with its higher bond strength, which becomes compatible with the base glass when Ag is present, and to the initiation and development of glass-glass phase separation, with the matrix containing the stronger bonds in a compositional phase not obtainable in a single phase bulk glass. It is difficult to experimentally demonstrate this plausible interpretation, however. The electron density difference of the two phases appears to be so small that electron microscopy has thus far, pending the development of more specific etching techniques, only given an indication of phase separation. Similarly, the spatial distribution of the two phases is apparently too small for successful analysis by electron microprobe. Thermal expansion curves frequently show two break points characteristic of phase separation [1 ] and DTA gives an indication of the higher Tg of the rheologically dominant phase and its development with heat treatment. It was expected, however, that mechanical and dielectric loss measurements might provide information * Present address: Schoenstatt Center, W. 284 N. 698 Cherry Lane, Waukesha, WI 53186.
E.M. Amrhein et al., Mechanical damping
47
on the immediate surrounding of the Ag atom and its contribution to the structural changes associated with the higher thermal stability. This paper presents the results of mechanical loss measurements on glasses in the system Si35 As25_x Agx Te40_y Sey. The glasses are identified by x and y, which represent the atomic percent Ag and Se, respectively. For example, glass 8 - 1 5 has the composition Si35 ASl7 Ag 8 Te25 Sel5.
2. Experimental procedure Considerable experimental difficulty was encountered in preparing samples of suitable shape for the mechanical loss measurements from melts that can not be exposed to oxygen and, in most cases, form very brittle glasses. Samples were obtained by remelting previously prepared glasses in evacuated fused silica vials of the desired shape and subsequently crushing, grinding or etching away the vial. Silica capillaries were used for obtaining thin rods. Three different techniques were employed to measure the mechanical loss over an extended frequency and temperature range. The methods, frequency range and sample configurations were: (1) An inverted torsion pendulum operating at 0 . 5 - 5 Hz, using rods less than 1 mm dia. (2) A Forster-type sonic apparatus [3], utilizing vibrations at 2 - 2 0 kHz on rods 5 - 7 mm dia. and 5 - 7 cm long. (3) A composite quartz oscillator [4] operating at 33 kHz. Half-wavelength samples of 5 mm dia. ( 3 . 5 - 4 . 5 cm long, depending upon the velocity of sound in the samples) were attached to the vibrating quartz assembly by a thin film of vacuum grease or bee's wax. Bee's wax was used for the low temperature measurements to avoid additional temperature dependent damping caused by the contact grease. Since the accuracy of this method is high and sample preparation as well as attachment were relatively easy, most of the data were obtained by this method.
3. Results
Examples of the measured values of mechanical loss versus temperature are shown in fig. 1 for the three measuring techniques. The background losses decreased in the order torsion pendulum > sonic ~> composite oscillator and this difference in background loss accounts for the relative vertical displacement of the curves in fig. 1. However, consistent results were obtained with the three methods concerning the presence or absence of a mechanical loss peak; compare curves B and D for glass 5-05. The change in the resonance frequency of the composite oscillator, as illustrated for glass 8 - 1 0 in fig. 2, depicts the relaxation of the elastic modulus normally associated with the mechanical loss peak observed in this glass (fig. 3).
E.M. Amrhein et al., Mechanical damping
48
5G
40
30 A ~
b 2C
10
0
i
-200
I
~
-I00
I
i
I
0
i
100 TEMP.
/
200
I
300
*C
Fig. 1. Internal friction versus temperature curves obtained by three different measuring techniques. Curve A, glass 8 - 05, torsion pendulum, f = 1.1 Hz; Curve B, glass 5 - 0 5 , sonic apparatus, f = 2460 Hz;Curve C, base glass, sonic apparatus, f = 5085 Hz, Curve D, glass 5 - 0 5 , composite oscillator, f = 33.6 kHz.
33,7
v
-
33.6
33.5
I
33.41 -200
f
I -I00
I
I 0
TEMP.
,
I I00
°C
Fig. 2. Resonance frequency of composite oscillator versus temperature for glass 8 - 1 0 .
E.M. Amrhein et al., Mechanical damping
49
Table 1 Internal friction measured with composition oscillator. Sample Quartz driver and gage crystal assembly Driver and gage crystal plus fused silica extension rod Si35As25Te4o base glass (measured between - 180 and -50°C) Glass5-15
Q-1 2.8 × 10 -5 6 - 7 × 10 -5 (in air) 3.7 X 10 -5 9.2 X 10 -4
O
Frequency = 33 kHz, 2 0 - 2 5 C, pressure ~ 10 -5 torr. The results obtained with the composite oscillator are plotted in subsequent figures in terms of the voltage ratio Vd/Vg, where Vd and Vg represent the voltage on the driving and gage crystal, respectively. This ratio is related to the internal friction, Q - l , by the mass ratio and resonance frequencies [4]. Since these latter two factors vary little over the temperature range investigated, the equation
Q-I=(~-~)(Vd/VX
10 - 4 )
(1)
is accurate within a few percent. Table 1 gives some examples for the absolute values of Q-1 measured with the composite oscillator. The change in the mechanical loss with the introduction o f Ag and Se into the base glass is shown in figs. 3 and 4. The data points have been omitted from these figures for better clarity, but there was very little scatter in the composite oscillator data as depicted by curve D in fig. 1. In general, one main mechanical loss peak was observed which became larger with increasing Ag content at constant Se content (fig. 3). With increasing replacement of Te by Se, this peak shifted to lower temperatures and showed evidence of splitting into two peaks (fig. 4). The activation energy for this peak is given in table 2, for those glasses where it could be calculated. Table 2 Activation energy of mechanical damping peak in Si3s As25-x Agx Teao-y Sey glasses. Glass x y
Measuring frequencies
Calculated act. energy (kcal/mol)
8
5
33.4 kHz
1.1 kHz
8
10
33.6 kHz
4.34 kHz
3.2
same sample, both frequencies
5
5
33.7 kHz
2.46 kHz
11.6
same sample, both frequencies
5
15
33.7 kHz
4.35 kHz 4.38 kHz
5.4 3.8
Remarks
12
duplicate low frequency measurement on same sample
50
E.M. Amrhein et al., Mechanical damping
50 3(3
8-05
2O
lC
/
0-00
(3
40 > ao >,,
20 10
"
5-~0
0 ...... 4O 203010 ~~------ 10-20 0 ...... -200
I
J
I
I
-100
i
0 TEMP.
I 100
I 200
°C
Fig. 3. Mechanical damping of silver-selenium doped Si-As-chalcogenide glasses. Frequency = 33.6 kHz.
4. Discussion The data for the mechanical losses are summarized for all glasses investigated in figs. 5 and 6. The difference in the resonance frequency, Af, between - 1 8 0 ° C and room temperature (fig. 2) has been used as a measure of the overall magnitude of the losses in fig. 5. Because of the splitting and overlap of the loss peaks in the glasses of higher Se content, the difference in the resonance frequency was found to be a more reliable measure of the total losses than the peak heights which could only be estimated. The points for the base glass in fig. 5 depict the normal temperature dependence of the elastic modulus. From the compositional dependence of the loss peak upon Ag content we con-
E.M. Amrhein et al., Mechanical damping
51
80_
60
5 -20
50
>~ >-o
40
30 20 10 0 -200
i
I -100
I
l i 0 TEMP. ° C
I 100
i
I 200
Fig. 4. Mechanical damping o f S i - A s - c h a l c o g e n i d e glasses containing 5% Ag with increasing substitution of Se for Te. Arrows designate a m o u n t each curve has been displaced vertically for improved clarity.
clude that this peak is due to the relaxation of Ag in these glasses. To the first approximation, the magnitude of the losses is linearly dependent upon the Ag content for a constant Se content (fig. 5A). Similarly, the activation energy for the mechan30C
300
A
/
20C
' ~ 100
/7 Base
Glass
-
I,iiif
0
5
10 AT. % A g
200
x
100
-
I
I
15
5
i
10 AT.%
i
15 Se
I
i
20
25
Fig. 5. (A) Dependence o f the dispersion height o f the mechanical loss peak on silver content. × = 5 % Se, • = 10 % Se, ® = 20 % Se. (B) Dependence of the dispersion height o f the mechanical loss peak on selenium content. × = 5% Ag, ®= 8% Ag, • = 10% Ag.
E.M. Amrhein et al., Mechanical damping
52
I
I..) O
I
I
I
I
-50
•t•
-100
x
I-.
-150 0
I
I
I
I
I
5
10
15
20
25
AT. PERCENT
Se
Fig. 6. Change in temperature of the loss maxima with Se content, f = 33.6 kHz. X = 5% Ag, ® = 8% Ag, • = 10% Ag. Two temperatures are shown for those glasses where two loss maxima were observed. ical loss peak in glasses of low Se content is close to that measured for d.c. conductivity. The activation energy for d.c. conductivity of 1 0 - 0 5 and 8 - 0 0 glasses varied from 13 to 15.6 kcal/mol, respectively, compared with 11 - 1 2 kcal/mol for the mechanical loss peak for 8 - 0 5 and 5 - 0 5 glasses (table 2). The mobility of Ag in these glasses can be envisioned to be relatively high assuming that Ag destroys the cross linking of the glass structure. Small additions of Se, less than ~ 10%, result in a small increase in the magnitude of the loss peak (fig. 5B). With further additions of Se the loss peak becomes smaller, but, more significantly, the peak shifts markedly to lower temperatures and shows evidence of splitting into two peaks (fig. 6). There is also a sharp reduction in the activation energy (table 2). The splitting of the peak is not attributed to the difference between Te and Se bonds; neither peak shows any linear dependence on Se content. Since peak heights and widths vary somewhat with sample preparation techniques this dependence must be studied in more detail. Nevertheless, the splitting of the loss peaks is tentatively attributed to increasing phase separation which is believed to occur with increasing Se content and heat treatment. Each peak is ascribed, therefore, to the Ag present in each of the two compositional phases. No mechanical loss due to S e - A s , S e - S e , T e - T e chains, as is found in pure I I I - V compounds [5], was detected. This can be explained by the action of Si as a cross-linking agent in the three dimensional network of the base glass. The role of Ge [6] and Si in chalcogenide glasses consists in forming four coordinated groupings initiating such a network. The mechanical loss of the base glass is nearly as low as that of vitreous SiO 2. The shoulder in curves A and B, (fig. 1), between
E.M. Amrhein et aL, Mechanical damping
53
100 and 200°C corresponding to Tmax _~ 0.7 Tg may be caused by the relaxation of chain segments formed when Ag is present, however.
5. Conclusions Mechanical loss measurements supplemented by other data [7] for S e - A s - T e glasses, in which Ag allows the introduction of substantial amounts of Se, suggest the following picture of the structural condition responsible for their high softening temperatures. The base glass Si35 As25 Te40 owes its high softening range to a cross-linked three dimensional network structure in an essentially homogeneous glass. The substitution of Ag for As leads to the formation of chain segments. The relative amount of available chalcogenide bonds is increased because of the lower bonding requirements of Ag and the softening temperature decreases. Increasing replacement of Te by Se causes a considerable increase in thermal stability not only by an increase in the average bond strength, but more so, by phase separation, with one connected phase having a very high viscosity. This is indicated by the reduction of the background losses and the splitting of the main loss maximum into two peaks when the Se content exceeds ~ 10%. The sharpening of one of the doublet peaks indicates a tendency to crystallization in one of the phases, which was also found in far infrared absorption measurements [8]. The additional increase in softening temperature with heat treatment is believed to be associated with the onset of crystallization. In chain structures small crystallites can drastically increase the viscosity because they restrict the motion of the chain segments [9]. There is no direct evidence that one of the phases is higher in Ag than the other. This may explain the lack of resolution in electron microscopy. This factor also shows the major role of microstructure, particularly in the matrix phase, in the achieved thermal stability. However, the Ag atom is able to change sites thereby producing a mechanical loss peak whose activation energy is comparable to that for d.c. conductivity.
Acknowledgements The support of the Office of Naval Research under grant N00014-69-A-0003 is thankfully acknowledged. We are indebted to Dr. A.E. Schwaneke, U.S. Bureau of Mines, Rolla Metallurgy Center for permission to use the quartz composite oscillator system and for their interest and help in this work. We also acknowledge the assitance of Carlton Sorrell in performing many of the experimental measurements.
54
E.M. Amrhein et al., Mechanical damping
References [1] H.E. Anthonis, N.J. Kreidl and W. Ratzenbock, J. Non-Crystalline Solids 11 (1972) 257. [2] E.M. Amrhein, D.E. Day and N.J. Kreidl, Thermally Resistant Chalcogenide Transparencies, Phys. Chem. Glass, submitted. [3] R. Forster, Z Metallkunde 29 (1937) 109. [4] A.E. Schwaneke, Construction and Operation of a Quartz Composite Oscillator, Rept. on Investigations 6419 (U.S. Bureau of Mines, 1964). [5] M. Imaoka and H. Sakemura, Asahi Garasu Gijutsu Shoreiaki Kenuy Hokoku 19 (1972) 1. [6] F. Betts and A. Bienenstock, J. Non-Crystalline Solids 8 - 1 0 (1972) 56. [7] J.P. De Neufville, J. Non-Crystalline Solids 8 - 1 0 (1972) 56. [8] E.M. Amrhein, D.E. Day, N.J. Kreidl and J.H. Weaver, Far Infrared Spectra of Polynary Chalcogenide Glasses, J. Non-Crystalline Solids, submitted. [9] P.J. Flory, Principle of Polymer Chemistry (CorneU University Press, 1953) 47-50.