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Unique physical properties and fragility of 50CuOx-50P2O5 glasses
]OURNAb OF
ELSEVIER
Journal of Non-Crystalline Solids 201 (1996) 222-230
Unique physical properties and fragility of 50CuOx-50P205 glasses Ryuji Sato a,*, Takayuki Komatsu b, Kazumasa Matusita c a Departmentof Material Engineering, Tsuruoka National College of Technology, Tsuruoka, Yamagata-ken997, Japan b Departmentof Chemisto; Nagaoka UniversiD'of Technology, Kamitomioka-cho, Nagaoka 940-21, Japan ¢ Departmentof Civil and EnvironmentalEngineering, Nagaoka University of Technology, Kamitomioka-cho, Nagaoka 940-21, Japan
Received 6 July 1995; revised 11 September 1995
Abstract 50CuOx-50P205 glasses with different copper valence states (0 < Cu + / ( C u 2 + ~- C u + ) < 0.61) were prepared by adding glucose during glass melting, and some physical properties and fragility of these glasses were examined. The density of 50CuO~-50PzO5 glasses decreases linearly with increasing Cu + content from 3.18 to 3.01 g / c m 3. Although the glass transition temperature decreases with increasing Cu + content from 437 to 196°C, the viscosity and the activation energy for viscous flow also decrease with increasing Cu + content. From the concept of fragility, it was found that 50CuOx-50P205 supercooled liquids become strong with increasing Cu + content due to the formation of strong Cu +-O bonds. The properties of 50CuO~-50P205 glasses were compared with those of xNaOo.5-(50 - x)MgO-50P205 glasses, and the results suggest that the former glasses exhibit unique properties and fragility due to a unique coordination state of Cu +.
1. Introduction It is well known that some oxide glasses containing copper ions are semiconductors showing hopping conduction and superionic conductors [1-6]. Copper can exist in glasses as metallic (Cu°), cuprous (Cu ÷) or cupric (Cu 2+) ions, although most oxide glasses are usually assumed not to contain metallic copper. It is known that the valence state in glasses, Cu +/Y'.Cu, affects electrical properties because conduction is due to electron hopping from Cu ÷ to Cu 2+. Studies on other physical properties in glasses containing different copper ions are not numerous.
* Corresponding author. Tel.: + 81-235 223 030; fax: + 81-235 241 840.
The effect of the copper valence state on structure and properties of copper aluminosilicate glasses has received much attention, because these glasses have very low thermal expansion, as well as low liquidus temperatures and viscosities [7-9]. It is recognized that Cu ÷ ions play an important role in the unique properties of copper aluminosilicate glasses. Recently, Bae and Weinberg [10] investigated the effect of the valence state of copper in copper metaphosphate glass on the crystallization behavior and glass transition temperature. They reported that the glass transition temperature of the glass increases with increasing Cu 2+ content. Zheng et al. [11] and Sato and co-workers [12,13] reported that the thermal stability of B i - S r - C a - C u - O glasses is sensitive to copper valence state. Sato et al. [14] also reported that both the viscosity and the activation energy for
0022-3093/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S0022-3093(96)00137-8
R. Sato et al. / Journal of Non-Cr3,stalline Solids 201 (1996) 222-230
viscous flow of Bi 2Sr2CaCu20 x supercooled liquids decrease with increasing Cu + content. Generally, for example, in single alkali-silicate supercooled liquids, a decrease in viscosity involves an increase in activation energy for viscous flow [15]. That is, a decrease in viscosity involving a decrease in activation energy for viscous flow is one of the unique properties in copper containing glasses and supercooled liquids. For glass science and technology, it is very important to clarify the origin of unique properties in copper containing glasses. In this paper, 50CuOz50P205 glasses with different copper valence states were prepared by adding glucose during glass melting, and some physical properties of these glasses were examined. Particularly, we focus our attention on the effect of the copper valence state on glass transition, thermal expansion and viscosity, because these properties are important for understanding of the unique properties in the copper containing glasses and supercooled liquid. The study of the viscosity of CUOx-P205 glasses with different copper valence states is very few, although electric properties have been extensively studied so far [1-4]. We also consider the 50CuOx-50P205 glasses from the concept of fragility. Further, the properties of 50CuO~-50 P205 glasses were compared with those of xNaO0.5-(50 -x)MgO-50 P205 glasses, because ion radius of Cu ÷ and Cu 2÷ is nearly equal to Na ÷ and Mg 2+ respectively.
2. Experimental
223
heated to 180°C. The bulk glasses were annealed at glass transition temperature for 3 h in nitrogen atmosphere to release internal stress. 2.2. Preparation of xNaOo.5-(50 - x)MgO-50P 205 glasses
The nominal compositions examined in the present study are xNaO0.5-(50 - x ) M g O - 5 0 P 2 0 5 (x = 0, 10, 20 and 30). A mixture of reagent quality of MgO, Na2CO 3 and H3PO 4 was reacted and dried at 300°C for 2 h and melted in an alumina crucible at 1200-1300°C for 30 min in an electric furnace. The batch weight was 30 g. The melts were poured onto an iron plate heated to 180°C. The bulk glasses were annealed at glass transition temperature for 3 h to release internal stress. 2.3. Characterization
Glass transition temperatures, Tg, were determined by using differential scanning calorimetry (DSC) at various heating rates. Viscosies in the glass transition region (107-1011 Pa s) were measured by a penetration method within an accuracy of _+5%. Thermal expansion coefficients from 50 to 200°C were measured by a dilatometer within an accuracy _+5%. Densities were determined by the pycnometer method using kerosene. Chemical compositions of the glasses were analyzed by inductively coupled plasma emission spectroscopy (ICPS) within an accuracy of _+0.1%. Cu + contents in the glasses were analyzed by the cerate titration [16,17] within an accuracy of _+5%.
2.1. Preparation of 50CuOx-50P 205 glass
The nominal compositions examined in the present study are 50CuOx-50P2Os. A mixture of reagent quality of CuO and H3PO 4 was reacted and dried at 300°C for 2 h and melted in an alumina crucible at 1200°C for 30 min in an electric fumace. The batch weight was 30 g. The melts were poured onto an iron plate and pressed quickly to a thickness of 1-2 mm. In order to control copper valence state, the glasses were ground and glucose (0, 0.5, 1.0, 2.0, 3.0 wt%) was added. The mixture was remelted in a covered alumina crucible at 1200°C for 10 min in an electric furnace. The melts were casted on an iron plate
3. Results 3.1. Chemical composition and density
The chemical compositions determined by ICPS and Cu+/(Cu2+ + Cu +) ratios analyzed by a cerate titration method of the 50CUOx-50205 glasses are given in Table 1. In all glasses, the change in chemical compositions during the melting is very small. Contamination of alumina from the crucible was observed. The fraction of Cu + increases with increasing glucose. For more than 3 wt% added
224
R. Sato et al. / Journal of Non-Crystalline Solids 201 (1996) 222-230
Table 1 Chemical compositions determined by ICPS and Cu+/(Cu z+ + Cu + ) ratio analyzed by cerate titration of 50CuO~-50P205 glasses Glass
Glucose (wt%)
Composition
Cu +/(Cu z + + Cu + )
CuO
Cu20
PzO5
AI203
A B C D E
0.0 0.5 1.0 2.0 3.0
50.31 44.86 41.02 28.08 24.11
0.13 3.77 6.61 15.29 17.66
48.36 50.24 51.24 55.47 56.82
1.20 1.16 1.13 1.15 1.41
glucose, carbon remained in the glass, bubbles were observed, and homogeneous glasses could not be obtained. For xNaO0.5-(50 - x)MgO-50P205 glasses, nominal batch compositions have been used in this paper. The densities are shown in Fig. 1 as a function of R + / ( R ' 2 + + R +) ratio, where R + is Cu + or Na ÷ and R'2÷ is Cu 2÷ or Mg 2÷. In the 50CuO/-50P205 glasses, the density decreases almost linearly with increasing Cu ÷ content. A slight increase in density with increasing Na ÷ content is observed in xNaO0.5-(50-x)MgO-50P205 glasses. Fig. 2 shows the values of molar volume for 50CuO x50P205 glasses and xNaO0.5-(50- x)MgO-50P205 glasses as a function of R+/(R'2÷+ R ÷) ratio. It is observed that the molar volume increases with increasing R + / ( R ' 2 + + R ÷) in both glass systems. However, a change in the molar volume for
< 0.01 0.14 0.24 0.52 0.61
+ + + +
0.2 0.3 0.3 0.4
50CuO~-50PzO 5 glasses is larger than that for xNaO0.5-(50 - x)MgO-50PzO 5 glasses. 3.2. Thermal properties Fig. 3 shows the heating rate dependence of glass transition temperature, Tg, for 50CUOx-50P205 glass with Cu+/(Cu2++ Cu ÷) <0.01 and 20 NaO0.5-30 MgO-50P205 glass. In both glasses, the value of Tg decreases with decreasing heating rate. In the present study, Tg' was determined by extrapolating to 0 K/min. Fig. 4 shows the values of Tg determined by the method mentioned above for 50CuOx-50P205 glasses and xNaO0.5-(50- x)MgO-50P205 glasses as a function of R+/(R'2++ R ÷) ratio. The values reported by Bae et al. [10] are also shown in Fig. 4. In the 50CuOx-50P205 glasses, Tg decreases from 43
3.18q
2.60
3.14
2.56
42
~41 40
~ 3.10 .~
2.s2
~39 _=3 8
,
"~ 3.06(
2.48
GI
O
_~ 371 o
3.02
2.44
'5 36 35
2.98
J
0
0.1
' • ' 0.2 0.3
' 0.4
' 0.5
' " 2.40 0.6 0.7
R*/(R' 2++R*) Fig. 1. Values of the density for 50CuO x -50P205 glasses ( O ) with different Cu + contents and xNaOo. 5 - ( 5 0 - x)MgO-50P205 glasses (O).
34
t
i
0.1
0.2
.
i
0.3
.
i
i
i
0.4
0.5
0.6
=
0.7
R+/(R' 2*+R+) Fig. 2. Values of the molar volume for 50 CuO x-50P205 glasses ( O ) and xNaO 0 5 - ( 5 0 - x)MgO-50P205 glasses (©) as a function of R + / ( R '2;c + R + ) ratio.
R. Sato et al. / Journal of Non-Crystalline Solids 201 (1996) 222-230 45O
140
448
130
446
225
120
444
%
o 442
x
110
100
440
~q
438
80 (
436 434
I
0
2
=
I
=
I
=
4 6 8 Heating rate (K/min)
7O
=
10
Fig. 3. Heating rate dependence of glass transition temperature, Tg, for 50 CuOx-50P205 glass (O) with Cu+/(Cu 2+ +Cu + ) < 0.01 and 20 NaO0.5--30MgO-50P205 glass (O).
437 to 196°C as the Cu+/(Cu2++ Cu +) ratio increases from about 0.0 to about 0.6. It is also observed that the value of Tg decreases from 531 to 392°C as the Na+/(Mg2+ + Na ÷) ratio raises to 0.6. The change in Tg in xNaO0.5-(50 - x)MgO-50P205 glasses, however, is smaller than that of 50CuO x50P205 glasses. Thermal expansion coefficients, or, for 50CuO x50P205 glasses and xNaO0.5-(50- x)MgO-50P205 glasses are shown in Fig. 5 as a function of
I
I
0.1
0.2
i
.
i
i
0.3 0.4 0.5 R+ / (R'e*+R +)
|
0.6
.
0.7
Fig. 5. Thermal expansion coefficients, et, for 50CuOx-50P205 glasses ( 0 ) and xNaO0.5-(50- x)MgO-50P205 glasses (O) as a function of R + / ( R '2+ + R ÷ ) ratio.
R + / ( R ' 2 + + R +) ratio. In the x N a O 0 . 5 - ( 5 0 x)MgO-50P205 glasses, ct increases monotonicly from 8 0 × 10 -7 to 130× 10 -7 K -l. Values of et for 50 CUOx50P20 s glasses with Cu+/(Cu 2+ + Cu ÷) ratio < 0.3 are almost the same, i.e., ~ = 90 × 10 -7 K -1, and for the glasses with Cu+/(Cu2÷+ Cu +) ratio > 0.4, a slight increase is observed with increasing Cu÷/(Cu2+ + Cu ÷) ratio. These results indicate that monovalent copper ions play a different role from alkali ions in the phosphate glasses. 3.3. Viscosity at glass transition region
550 500 (
400
30o
250 200 [] 150 0.0
,
0
0.2
0.4 R+/(R' 2++R+)
0.6
0.8
Fig. 4. Values of the glass transition temperature, Tg, for 50CuOx-50P205 glasses ( 0 ) and xNaO0.5-(50-x)MgO50P205 glasses (O) as a function of R + / ( R '2+ + R +) ratio. n : 50CuO~-50P205 glasses [10].
The temperature dependence of viscosity at the glass transition region for 50CuOx-50P205 glasses are shown in Fig. 6. The temperature range for the viscosity measurements was about 50°C at near Tg. The viscosities ranging from 107 tO 1011 Pa s, and the plot of log xl against 1 / T shows a straight line because of the narrow temperature range, where ~i is a viscosity and T is a temperature. The viscosity data for xNaO0.5-(50- x)MgO-50P205 glasses are not shown here. The activation energy E'q for viscous flow for each glass was evaluated by using the Andrade equation, xl = A e x p ( E ~ / R T ) , where R is the gas constant and A is a constant. The values obtained for 50CuOx-50P205 glasses and xNaO0. 5(50 - x)MgO-50P205 glasses are shown in Fig. 7 as a function of R+/(R'2++ R ÷) ratio. In xNaOo. 5-
R. Sato et al. / Journal of Non-Crystalline Solids 201 (1996) 222-230
226 11.0
11.0
10.5
10.5
.g io.o
10.0
g
9.5
n
9.5
8
9.o
~
8.5
~> 8.5
-
8.0
8.0
7.5
7.5
7.0 ' • 1.20 1.30 1.40 1.51) 1.60 1.70 1.80 1.90 2.00 2.10
7.0
IO00/T
0.80
, 0.85
0.90
0.95
1.00
Tg/T
(K-1)
Fig. 6. Temperature dependence of the viscosity at the glass transition region for 50CuOx-50P;O 5 glasses. Q: C u + / ( C u 2÷ + C u + ) < 0 . 0 1 , C): C u + / ( C u 2+ + C u + ) = 0.14, • : C u + / ( C u 2+ + C u + ) = 0.24, []: C u + / ( C u 2+ + Cu + ) = 0.52, • : C u + / ( C u 2+ + C u + ) = 0.61.
( 5 0 - x)MgO-50PeO 5 glasses values of Ex I range from 697 kJ/mol for 50MgO-50PeO 5 to 504 kJ/mol for 30NaO0.5-20 MgO-50P205 glass, and E'q linearly decreases with increasing Na +/(Mg2+ + Na ÷). In the 50CUOx-50P205 glasses with Cu÷/(Cu2++ Cu +) < 0.3, the value of E'q decreases rapidly, but a slight decrease is observed in glasses with Cu+/(Cu 2÷ + Cu ÷) > 0.4.
Fig. 8. Viscosity data for 50CuOx-50 P205 glasses in a scaled Arrhenius form normalized at Tg. Q: C u + / ( C u 2+ + C u + ) < 0.01, O: C u + / ( C u 2+ + C u + ) = 0.14, • : C u + / ( C u 2+ + C u + ) = 0.24, []: C u + / ( C u e+ + C u + ) = 0.52, • : C u + / ( C u 2+ + C u + ) = 0.61.
Fig. 8 shows the viscosity for 50CuOx-50P20 s glasses plotted in a reduced Arrhenius form, where the normalizing temperature is Tg. This plot is called 'Angell plot' [18,19]. The slope of the plots in this viscosity range, E'q/2.303RTg, is a measure of the 'fragility', m, of supercooled liquids [18] because the non-Arrhenius character in the temperature dependence of the viscosity for supercooled liquids corresponds to the slope, Erl/2.303RTg, at around Tg.
700 11.0
10.5
6OO
10.0
~6o
#_
9.5
'~ 9.0 o
tu 400
.~_
~> 8.5
3OO
200 0
8.0 ' 0.1
' • ' 0.2 0.3
' 0.4
' - ' 0.5 0.6
7.5 0.7
R+/(R' 2*+R+)
Fig. 7. Values of the activation energy, ETI, for viscous flow estimated from Arrhenius plot for 50CuO.-50P205 glasses ( Q ) and x N a O o . s - ( 5 0 - x)MgO-50P205 glasses ( © ) as a function of R + / ( R '2+ + R + ) ratio.
7.0
0.80
L
0.85
0.90 Yg/T
0.95
1.00
Fig. 9. Viscosity data for xNaOo. 5 - ( 5 0 - x)MgO-50P205 glasses in a scaled Arrhenius form normalized at Tg. O: x = 0.0, O: x = 1 0 , • : x = 2 0 , rq: x=30.
R. Sato et aL/Journal of Non-Crystalline Solids 201 (1996)222-230 46
42
E38 36 34 32 30 0
i . i 0.1 0,2
i
,
0.3
t
i
t
0.4
0.5
0.6
,
0.7
R+/(R ' 2++R+) F i g . 10, R + / ( R ' 2 + + R + ) ratio d e p e n d e n c e
of fragility, m, for
50CuO~-50P205 glasses (0) and xNaO0.5-(50-x)MgO50P205 glasses(©).
The slope of the plot decreases with increasing Cu+/(Cu2++Cu+). The same plot for xNaO0. 5( 5 0 - x)MgO-50P205 glasses is also shown in Fig. 9. The R+/(R'2++ R ÷) dependence of fragility, m = E'q/2.303RTg, for 50CUOx-50P205 glasses and xNaO0.5-(50- x)MgO-50P205 glasses is shown in Fig. 10. In the xNaO0.s-(50-x)MgO-50P205 glasses, fragility decreases slightly with increasing Na ÷ content, however, a rapid decrease in fragility occurs in the 50CUOx-50P205 glasses with Cu+/(Cu2+ + Cu ÷) < 0.3. These results again highlight the difference between Cu ÷ and Na ÷ ions.
4. D i s c u s s i o n
4.1. Chemical glasses
composition
of 50CuOx-50P 205
As shown in Table 1, there is A1203 contamination in all glasses from alumina crucibles. Generally, an introduction of trivalent ions, A13+ or B 3+, into phosphate glasses strengthens the glass network [20,21]. Since the amount of A1203 in all glasses are almost the same, it is possible to clarify the effect of copper valence state on the physical properties of the glasses. The CuO phase transforms into the Cu20 phase above 1025°C in air. However, Cu ions in the
227
melt of 50CUOx-50P205 composition at 1200°C may exist almost completely as Cu 2÷ ions due to strong acidity of phosphate melts [22]. Also as shown in Table 1, Cu ÷ content in the glass increases with increasing glucose content obviously because glucose reduced Cu 2÷ ions into Cu ÷ ions. However, a homogeneous glass with Cu+/(Cu2++ Cu ÷) > 0.7 was not obtained. In order to examine the effect of the copper valence state on physical properties of 50CuO x50P20 s glasses, it is necessary to control widely the copper valence. Bae and Weinberg [10] succeeded in preparing of 50CuOx-50P20~ glass with C u + / ( C u 2 + + C u + ) = 0 . 7 6 6 by changing melting time. Tsuchiya and Moriya [22] prepared 50CuO x50P205 glasses with different copper valence states by adding carbon, but they could not obtain the glass with Cu+/(Cu2+ + Cu ÷) > 0.7. The results obtained in the present study and by Tsuchiya and Moriya imply that the 50CuO~-50P20 s glass with C u + / ( C u Z + + C u + ) > 0 . 7 cannot be prepared by adding reducing agents. 4.2. Properties of 5OCuOx-5OP205 glasses
Molar volume for 50CuOx-50P205 glasses and xNaO05-(50 - x)MgO-50P205 glasses increase linearly with increasing R ÷ content as shown in Fig. 2. In both glasses, the glass composition deviates from metaphosphate composition increasing with RO0 s content. As a result, a bridging oxygen is introduced into glass network and a network structure becomes more open than that of meta-composition glass. In 50CuOx-50P:O 5 glasses the large increase occurred compared with that in x N a O 0 5 - ( 5 0 - x ) M g O 50P:O 5 glasses, implying an introduction of a more loose structure. It is known that the coordination state of Cu ÷ in CueO is linear CuO z and that of Cu 2÷ in CuO is planar C u O 4. The density of C u 2 0 and CuO is 6.04 and 6.31 g / c m 3, respectively. If the oxygen coordination state of Cu ions in copper phosphate glasses is similar to that in the crystals, i.e., linear CuO 2 for Cu ÷ ions and planar CuO 4 for Cu 2÷ ions, a large increase in the molar volume would be expected. Drake and Scanlan [23] proposed that in CuOP205 glasses the Cu ÷ coordination is similar in Cu20. Nakagawa et al. [24] investigated a local
R. Sato et al. / Journal of Non-Crystalline Solids 201 (1996) 222-230
228
structure around Cu ions in a Bi2SrzCaCu20 x glass by a Cu extended X-ray absorption fine structure (EXAFS) analysis and proposed that some fraction of Cu ions exists as Cu ÷ surrounded by two oxygen atoms. Sato et al. [14] reported that the density of the Bi 2SrECaCu 2Ox glass decreases with increasing Cu + content indicating that Cu + ions are surrounded by two oxygen atoms. Kamiya et al. [25] reported from EXAFS experiments that Cu + ions in Cu20" A 1 2 0 3 • 4SiO 2 glass are surrounded by two oxygen atoms through covalent Cu+-O bonds and this is the primary reason for its low thermal expansion coefficient. The large increase in the molar volume obtained in the present study also indicates that in 50CuO~-50P205 glasses, Cu ÷ ions are surrounded by two oxygen atoms in CuOx-P205 glass. The decrease in Tg with increasing Cu ÷ content in CUOx-P205 glasses has been reported by Bae and Weinberg [10]. As shown in Fig. 4, the tendency of decrease in their study agrees with that in the present study. They mentioned that the intermolecular force of Cu 2÷ is higher than that of Cu ÷ in phosphates, and thus glasses containing more Cu 2÷ possess higher glass transition temperature. In the present study, we consider the relationship between glass transition temperature and single bond strength proposed by Sun [26]. Fig. 11 shows the value of Tg for alkali and alkaline-earth [27] metaphosphate glasses as a function of single bond strength. There is a tendency for the value of Tg to 600
II
|
5OO A
o *" 400
0 300
2 0 0
,
80
i
90
,
i
,
i
,
i
.
|
.
i
I
i
.
100 110 120 130 140 150 160 Single bond strength (kJ/mol)
Fig. 11. Values of Tg for alkali and alkaline-earth metaphosphate glasses as a function of single bond strength. O: LiOo. 5, ©: NaOo 5, I1: MgO, El: CaO, A: SrO, r,: BaO.
decrease with decreasing the single bond strength, although NaO0.5 deviates slightly from this. This indicates that the single bond strength is one of the origins to control the glass transition temperature. Although the single bond strength of copper oxides was not reported by Sun, the single bond strength of monovalent and divalent copper oxide can be calculated from thermodynamic data [28]. The single bond strength of CuO and CuO0.5 is 186 and 274 kJ/mol respectively. The single bond strength of Cu+-O is higher than that of CuZ+-O, thus the tendency of the decrease in Tg observed in 50CuO x50P205 glasses is not explained. Very recently, Jamnicky et al. reported that a decrease in a packing density in the glass structure leads to decrease in Tg and is the main factor affecting Tg in CUEO-P205MoO 3 glasses [29]. The open structure which is introduced by Cu + ions surrounded two oxygens may be an origin of the decrease in Tg in 50CuOx50P205 glasses. It is well known that the thermal expansion coefficient of oxide glasses containing copper ions such a s C U E O - A 1 2 0 3 .4SiO 2 depends strongly on the copper valence state and decreases with increasing Cu+/(Cu2+ + Cu +) ratio [30]. It is well recognized that Cu ÷ ions play an important role in the low thermal expansion coefficient. However, in the 50CuO~-50P205 glasses with Cu+/(Cu 2++ Cu +) < 0.3, the values of about 90 × 10 - 7 a r e insensitive to the copper valence state and the thermal expansion coefficient for the glasses with Cu+/(Cu2++ Cu +) > 0.4 increase slightly with increasing Cu + content as shown in Fig. 5. These differences indicate that the low thermal expansion observed in C u 2 0 . A l 2 0 3 - 4 S i O 2 glasses is not only a consequence of Cu + ion present, but also depends on glass composition. In comparison with the xNaO0. 5( 5 0 - x)MgO-50P205 glasses, the increase in ct is small in the 50 CUOx-50 P205 glasses. These results would reflect a peculiarity of Cu ÷ ions, i.e., a loose structure introduced by a low oxygen coordination and a high single bond strength, 274 kJ/mol, which is classified into an intermediate by Sun's classification [26]. 4.3. Effect of copper valence on fragility
From the fragility concept, in which the logarithm of viscosity is plotted against reduced temperature
229
R. Sato et al. / Journal of Non-Crystalline Solids 201 (1996) 222-230
Tg/T, one can say that the glasses with low E-q at near Tg or with high Tg in a given glass system tend to be more strong than those with high E'q or low Tg. As shown in Fig. 4 the value of Tg decreases with increasing monovalent ion content in 50CuO x50P205 glasses and xNaO0.5-(50- x)MgO-50P205 glasses. However, fragility, m, decreases rapidly with increasing Cu ÷ content in the 50CuO~-50P205 supercooled liquids as shown in Fig. 10, thus indicating that the 50CuOx-50P205 supercooled liquids become strong with increasing Cu ÷ content. Although the fragility in x N a O 0 . 5 - ( 5 0 - x ) M g O 50P205 supercooled liquids also decreases, the degree of decrease is small. This small decrease is due to the deviation from the meta-composition with increasing Na + content. That is, a bridging oxygen increases due to the deviation from the meta-composition and a three dimensional expanse becomes large. Since the viscosity is controlled by flow unit size and free volume, the variation of the liquid viscosity with temperature is known to correspond to the intermediate range structure of liquids. AngeU suggested that the distinction between strong and fragile characters is to be associated with the differences in the stability of intermediate range order against temperature-induced degradation [18,31]. Further, AngeU pointed out that changes in heat capacity at Tg in strong glasses are extremely small compared with those in fragile glasses [18,31]. Therefore, it is considered that strong glasses have 'liquid-like' structure preserved at low temperatures and a more homogeneous open structure. The result obtained in the present study suggests that the 50CuOx-50P205 glasses become more and more liquid like with a more homogeneous open structure with increasing Cu + content. The glass structure model of 50CuOx-50P205 glasses which has been proposed in this paper, are shown in Fig. 12. In our present model, Cu 2+ ions act as modifier ions in the phosphate; thus, non bridging oxygens are introduced as shown in Fig. 12(A). Cu ÷ ions are surrounded by two oxygen atoms and bridge two phosphate chains; thus, a large correlation length is expected in the glass structure. Further, the coordination of Cu ÷ ions is explained by using sp hybrid orbital. In other words, a Cu +-O bond might be covalent and its bond would be
A
0
II ..... 0-.1~1 -0
0 I ..... o-
.....
n_ ~
0 U
. . .o ..
-o
9
-oI
o'
....O - ~ - - O .....
B
O
II ..... O--~1 - O .....
I
----O-~L-O ..... Fig. 12. Structure model of 50CuOx-50P205 glasses. A: CuOP20~, B: CuO0.5-P205.
strong, as shown by the value of single bond strength, 274 kJ/mol. In the 50CuOx-50P205 supercooled liquids with high Cu 2+ contents, an ionic Cu2+-O bond breaks easily with increasing temperature and the correlation length becomes small. In the 50CUOx-50P205 supercooled liquids with high Cu ÷ contents, a large correlation length due to the strong Cu÷-O bond would be stable. It is concluded that the decrease in fragility, m, with increasing Cu ÷ ion content observed in 50CUOx-O 5 glasses is due to the improvement in stability of intermediate range order. The result obtained in this study implies that Cu ÷ ions are not a modifier such as alkali and alkalineearth ions, but a intermediate or network former in some oxide glasses.
5. C o n c l u s i o n
Density of 50CuOx-50PzO 5 glasses decreased rapidly with increasing Cu+/(Cu2+ + Cu ÷) indicating that the glass structure becomes loose. The glass transition temperature decrease with increasing C u + / ( C u 2 + + C u ÷) may be due to the different coordination between Cu ÷ and Cu 2÷. The viscosity and activation energy for viscous flow of 50CuO x-
230
R. Sato et al. / Journal of Non-Crystalline Solids 201 (1996) 222-230
50P205 glasses decreased with increasing Cu+/(CuZ++ Cu+). From the fragility concept, the 50CuOx-50P205 supercooled liquids may become strong with increasing Cu+/(Cu2++ Cu ÷) due to improvement in the thermal stability of intermediate range order.
References [1] M. Sayer and A. Mansing, Phys. Rev. B6 (1972) 4629. [2] C. Ananthamohan and C.A. Hogarth, J. Mater. Sci. Lett. 9 (1990) 85. [3] A. Duran, J.R. Jourado and J.M.F. Navarro, J. Non-Cryst. Solids 79 (1986) 333. [4] A. Duran, J.R. Jourado and J.M.F. Navarro, J. Non-Cryst. Solids 79 (1986) 353. [5] C. Liu and C.A. Angell, Solid State Ionics 13 (1984) 105. [6] N. Machida and T. Minami, J. Am. Ceram. Soc. 71 (1988) 784. [7] K. Matusita and J.D. Mackenzie, J. Non-Cryst. Solids 30 (1979) 285. [8] K. Matusita, S. Sakka and T. Shouji, J. Am. Ceram. Soc. 66 (1983) 33. [9] A. Makishima, T. Utsugi and T. Sakaino, J. Am. Ceram. Soc. 62 (1979) 30. [10] B.S. Bae and M.C. Weinberg, J. Am. Ceram. Soc. 76 (1993) 1395. [11] H. Zheng, M.W. Colby and J.D. Mackenzie, J. Non-Cryst. Solids 127 (199l) 143. [12] R. Sato, T. Komatsu, Y. Kuken, K. Matusita, K. Sawada and M. Hiraoka, J. Non-Cryst. Solids 152 (1993) 150.
[13] R. Sato, T. Komatsu and K. Matusita, J. Non-Cryst. Solids 181 (1995) 64. [14] R. Sato, T. Komatsu and K. Matusita, J. Non-Cryst. Solids 160 (1993) 180. [15] S.K. Lee, M. Tatsumisaga and T. Minami, J. Ceram. Soc. Jpn. 101 (1993) 1018. [16] W.P. Close and F. Tillman, Glass Technol. 10 (1969) 134. [17] E.F. Riebling, Inorg. Chem. 12 (1973) 2213. [18] C.A. Angell, J. Non-Cryst. Solids 131-133 (1991) 13. [19] S.K. Lee, M. Tatsumisaga and T. Minami, Phys. Chem. Glasses 35 (1994) 226. [20] S. Sakka, Yogyo-Kyoukai-shi 85 (1977) 299. [21] N.J. Kreidl and W.A. Weyl, J. Am. Ceram. Soc. 24 (1941) 372. [22] T. Tsuchiya and T. Moriya, Glass Ceramic Bull. 22 (1975) 55. [23] C.F. Drake and I.F. Scanlan, J. Non-Cryst. Solids 4 (1970) 234. [24] Z. Nakagawa, H. Morikawa, T.H. Sung, M. Yoshimura, F. Maruno and Y. Udagawa, J. Chem. Soc. Jpn. 12 (1989) 2085. [25] K. Kamiya, K. Okasaka, M. Wada and H. Nasu, J. Am. Ceram. Soc. 75 (1992) 477. [26] K.H. Sun, J. Am. Ceram. Soc. 30 (1947) 277. [27] Hand Book of Glass Data, Part B (Elsevier, Amsterdam, 1985). [28] M.W. Chase Jr., C.A. Davies, J.R. Downey Jr., D.J. Frurip, R.A. McDonald and A.N. Syverud, JANAF Thermochemical Tables, 3rd Ed. (The American Chemical Society and The American Institute of Physics for the National Bureau of Standards, Washington, D.C., 1985) [29] M. Jamnicky, P. Znasik, D. Tunega and M.D. Ingram, J. Non-Cryst. Solids 185 (1995) 151. [30] T. Yoko, K. Kamiya and S. Sakka, J. Non-Cryst. Solids 71 (1985) 245. [31] C.A. Angell, J. Non-Cryst. Solids 73 (1985) 1.
ELSEVIER
Journal of Non-Crystalline Solids 201 (1996) 222-230
Unique physical properties and fragility of 50CuOx-50P205 glasses Ryuji Sato a,*, Takayuki Komatsu b, Kazumasa Matusita c a Departmentof Material Engineering, Tsuruoka National College of Technology, Tsuruoka, Yamagata-ken997, Japan b Departmentof Chemisto; Nagaoka UniversiD'of Technology, Kamitomioka-cho, Nagaoka 940-21, Japan ¢ Departmentof Civil and EnvironmentalEngineering, Nagaoka University of Technology, Kamitomioka-cho, Nagaoka 940-21, Japan
Received 6 July 1995; revised 11 September 1995
Abstract 50CuOx-50P205 glasses with different copper valence states (0 < Cu + / ( C u 2 + ~- C u + ) < 0.61) were prepared by adding glucose during glass melting, and some physical properties and fragility of these glasses were examined. The density of 50CuO~-50PzO5 glasses decreases linearly with increasing Cu + content from 3.18 to 3.01 g / c m 3. Although the glass transition temperature decreases with increasing Cu + content from 437 to 196°C, the viscosity and the activation energy for viscous flow also decrease with increasing Cu + content. From the concept of fragility, it was found that 50CuOx-50P205 supercooled liquids become strong with increasing Cu + content due to the formation of strong Cu +-O bonds. The properties of 50CuO~-50P205 glasses were compared with those of xNaOo.5-(50 - x)MgO-50P205 glasses, and the results suggest that the former glasses exhibit unique properties and fragility due to a unique coordination state of Cu +.
1. Introduction It is well known that some oxide glasses containing copper ions are semiconductors showing hopping conduction and superionic conductors [1-6]. Copper can exist in glasses as metallic (Cu°), cuprous (Cu ÷) or cupric (Cu 2+) ions, although most oxide glasses are usually assumed not to contain metallic copper. It is known that the valence state in glasses, Cu +/Y'.Cu, affects electrical properties because conduction is due to electron hopping from Cu ÷ to Cu 2+. Studies on other physical properties in glasses containing different copper ions are not numerous.
* Corresponding author. Tel.: + 81-235 223 030; fax: + 81-235 241 840.
The effect of the copper valence state on structure and properties of copper aluminosilicate glasses has received much attention, because these glasses have very low thermal expansion, as well as low liquidus temperatures and viscosities [7-9]. It is recognized that Cu ÷ ions play an important role in the unique properties of copper aluminosilicate glasses. Recently, Bae and Weinberg [10] investigated the effect of the valence state of copper in copper metaphosphate glass on the crystallization behavior and glass transition temperature. They reported that the glass transition temperature of the glass increases with increasing Cu 2+ content. Zheng et al. [11] and Sato and co-workers [12,13] reported that the thermal stability of B i - S r - C a - C u - O glasses is sensitive to copper valence state. Sato et al. [14] also reported that both the viscosity and the activation energy for
0022-3093/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S0022-3093(96)00137-8
R. Sato et al. / Journal of Non-Cr3,stalline Solids 201 (1996) 222-230
viscous flow of Bi 2Sr2CaCu20 x supercooled liquids decrease with increasing Cu + content. Generally, for example, in single alkali-silicate supercooled liquids, a decrease in viscosity involves an increase in activation energy for viscous flow [15]. That is, a decrease in viscosity involving a decrease in activation energy for viscous flow is one of the unique properties in copper containing glasses and supercooled liquids. For glass science and technology, it is very important to clarify the origin of unique properties in copper containing glasses. In this paper, 50CuOz50P205 glasses with different copper valence states were prepared by adding glucose during glass melting, and some physical properties of these glasses were examined. Particularly, we focus our attention on the effect of the copper valence state on glass transition, thermal expansion and viscosity, because these properties are important for understanding of the unique properties in the copper containing glasses and supercooled liquid. The study of the viscosity of CUOx-P205 glasses with different copper valence states is very few, although electric properties have been extensively studied so far [1-4]. We also consider the 50CuOx-50P205 glasses from the concept of fragility. Further, the properties of 50CuO~-50 P205 glasses were compared with those of xNaO0.5-(50 -x)MgO-50 P205 glasses, because ion radius of Cu ÷ and Cu 2÷ is nearly equal to Na ÷ and Mg 2+ respectively.
2. Experimental
223
heated to 180°C. The bulk glasses were annealed at glass transition temperature for 3 h in nitrogen atmosphere to release internal stress. 2.2. Preparation of xNaOo.5-(50 - x)MgO-50P 205 glasses
The nominal compositions examined in the present study are xNaO0.5-(50 - x ) M g O - 5 0 P 2 0 5 (x = 0, 10, 20 and 30). A mixture of reagent quality of MgO, Na2CO 3 and H3PO 4 was reacted and dried at 300°C for 2 h and melted in an alumina crucible at 1200-1300°C for 30 min in an electric furnace. The batch weight was 30 g. The melts were poured onto an iron plate heated to 180°C. The bulk glasses were annealed at glass transition temperature for 3 h to release internal stress. 2.3. Characterization
Glass transition temperatures, Tg, were determined by using differential scanning calorimetry (DSC) at various heating rates. Viscosies in the glass transition region (107-1011 Pa s) were measured by a penetration method within an accuracy of _+5%. Thermal expansion coefficients from 50 to 200°C were measured by a dilatometer within an accuracy _+5%. Densities were determined by the pycnometer method using kerosene. Chemical compositions of the glasses were analyzed by inductively coupled plasma emission spectroscopy (ICPS) within an accuracy of _+0.1%. Cu + contents in the glasses were analyzed by the cerate titration [16,17] within an accuracy of _+5%.
2.1. Preparation of 50CuOx-50P 205 glass
The nominal compositions examined in the present study are 50CuOx-50P2Os. A mixture of reagent quality of CuO and H3PO 4 was reacted and dried at 300°C for 2 h and melted in an alumina crucible at 1200°C for 30 min in an electric fumace. The batch weight was 30 g. The melts were poured onto an iron plate and pressed quickly to a thickness of 1-2 mm. In order to control copper valence state, the glasses were ground and glucose (0, 0.5, 1.0, 2.0, 3.0 wt%) was added. The mixture was remelted in a covered alumina crucible at 1200°C for 10 min in an electric furnace. The melts were casted on an iron plate
3. Results 3.1. Chemical composition and density
The chemical compositions determined by ICPS and Cu+/(Cu2+ + Cu +) ratios analyzed by a cerate titration method of the 50CUOx-50205 glasses are given in Table 1. In all glasses, the change in chemical compositions during the melting is very small. Contamination of alumina from the crucible was observed. The fraction of Cu + increases with increasing glucose. For more than 3 wt% added
224
R. Sato et al. / Journal of Non-Crystalline Solids 201 (1996) 222-230
Table 1 Chemical compositions determined by ICPS and Cu+/(Cu z+ + Cu + ) ratio analyzed by cerate titration of 50CuO~-50P205 glasses Glass
Glucose (wt%)
Composition
Cu +/(Cu z + + Cu + )
CuO
Cu20
PzO5
AI203
A B C D E
0.0 0.5 1.0 2.0 3.0
50.31 44.86 41.02 28.08 24.11
0.13 3.77 6.61 15.29 17.66
48.36 50.24 51.24 55.47 56.82
1.20 1.16 1.13 1.15 1.41
glucose, carbon remained in the glass, bubbles were observed, and homogeneous glasses could not be obtained. For xNaO0.5-(50 - x)MgO-50P205 glasses, nominal batch compositions have been used in this paper. The densities are shown in Fig. 1 as a function of R + / ( R ' 2 + + R +) ratio, where R + is Cu + or Na ÷ and R'2÷ is Cu 2÷ or Mg 2÷. In the 50CuO/-50P205 glasses, the density decreases almost linearly with increasing Cu ÷ content. A slight increase in density with increasing Na ÷ content is observed in xNaO0.5-(50-x)MgO-50P205 glasses. Fig. 2 shows the values of molar volume for 50CuO x50P205 glasses and xNaO0.5-(50- x)MgO-50P205 glasses as a function of R+/(R'2÷+ R ÷) ratio. It is observed that the molar volume increases with increasing R + / ( R ' 2 + + R ÷) in both glass systems. However, a change in the molar volume for
< 0.01 0.14 0.24 0.52 0.61
+ + + +
0.2 0.3 0.3 0.4
50CuO~-50PzO 5 glasses is larger than that for xNaO0.5-(50 - x)MgO-50PzO 5 glasses. 3.2. Thermal properties Fig. 3 shows the heating rate dependence of glass transition temperature, Tg, for 50CUOx-50P205 glass with Cu+/(Cu2++ Cu ÷) <0.01 and 20 NaO0.5-30 MgO-50P205 glass. In both glasses, the value of Tg decreases with decreasing heating rate. In the present study, Tg' was determined by extrapolating to 0 K/min. Fig. 4 shows the values of Tg determined by the method mentioned above for 50CuOx-50P205 glasses and xNaO0.5-(50- x)MgO-50P205 glasses as a function of R+/(R'2++ R ÷) ratio. The values reported by Bae et al. [10] are also shown in Fig. 4. In the 50CuOx-50P205 glasses, Tg decreases from 43
3.18q
2.60
3.14
2.56
42
~41 40
~ 3.10 .~
2.s2
~39 _=3 8
,
"~ 3.06(
2.48
GI
O
_~ 371 o
3.02
2.44
'5 36 35
2.98
J
0
0.1
' • ' 0.2 0.3
' 0.4
' 0.5
' " 2.40 0.6 0.7
R*/(R' 2++R*) Fig. 1. Values of the density for 50CuO x -50P205 glasses ( O ) with different Cu + contents and xNaOo. 5 - ( 5 0 - x)MgO-50P205 glasses (O).
34
t
i
0.1
0.2
.
i
0.3
.
i
i
i
0.4
0.5
0.6
=
0.7
R+/(R' 2*+R+) Fig. 2. Values of the molar volume for 50 CuO x-50P205 glasses ( O ) and xNaO 0 5 - ( 5 0 - x)MgO-50P205 glasses (©) as a function of R + / ( R '2;c + R + ) ratio.
R. Sato et al. / Journal of Non-Crystalline Solids 201 (1996) 222-230 45O
140
448
130
446
225
120
444
%
o 442
x
110
100
440
~q
438
80 (
436 434
I
0
2
=
I
=
I
=
4 6 8 Heating rate (K/min)
7O
=
10
Fig. 3. Heating rate dependence of glass transition temperature, Tg, for 50 CuOx-50P205 glass (O) with Cu+/(Cu 2+ +Cu + ) < 0.01 and 20 NaO0.5--30MgO-50P205 glass (O).
437 to 196°C as the Cu+/(Cu2++ Cu +) ratio increases from about 0.0 to about 0.6. It is also observed that the value of Tg decreases from 531 to 392°C as the Na+/(Mg2+ + Na ÷) ratio raises to 0.6. The change in Tg in xNaO0.5-(50 - x)MgO-50P205 glasses, however, is smaller than that of 50CuO x50P205 glasses. Thermal expansion coefficients, or, for 50CuO x50P205 glasses and xNaO0.5-(50- x)MgO-50P205 glasses are shown in Fig. 5 as a function of
I
I
0.1
0.2
i
.
i
i
0.3 0.4 0.5 R+ / (R'e*+R +)
|
0.6
.
0.7
Fig. 5. Thermal expansion coefficients, et, for 50CuOx-50P205 glasses ( 0 ) and xNaO0.5-(50- x)MgO-50P205 glasses (O) as a function of R + / ( R '2+ + R ÷ ) ratio.
R + / ( R ' 2 + + R +) ratio. In the x N a O 0 . 5 - ( 5 0 x)MgO-50P205 glasses, ct increases monotonicly from 8 0 × 10 -7 to 130× 10 -7 K -l. Values of et for 50 CUOx50P20 s glasses with Cu+/(Cu 2+ + Cu ÷) ratio < 0.3 are almost the same, i.e., ~ = 90 × 10 -7 K -1, and for the glasses with Cu+/(Cu2÷+ Cu +) ratio > 0.4, a slight increase is observed with increasing Cu÷/(Cu2+ + Cu ÷) ratio. These results indicate that monovalent copper ions play a different role from alkali ions in the phosphate glasses. 3.3. Viscosity at glass transition region
550 500 (
400
30o
250 200 [] 150 0.0
,
0
0.2
0.4 R+/(R' 2++R+)
0.6
0.8
Fig. 4. Values of the glass transition temperature, Tg, for 50CuOx-50P205 glasses ( 0 ) and xNaO0.5-(50-x)MgO50P205 glasses (O) as a function of R + / ( R '2+ + R +) ratio. n : 50CuO~-50P205 glasses [10].
The temperature dependence of viscosity at the glass transition region for 50CuOx-50P205 glasses are shown in Fig. 6. The temperature range for the viscosity measurements was about 50°C at near Tg. The viscosities ranging from 107 tO 1011 Pa s, and the plot of log xl against 1 / T shows a straight line because of the narrow temperature range, where ~i is a viscosity and T is a temperature. The viscosity data for xNaO0.5-(50- x)MgO-50P205 glasses are not shown here. The activation energy E'q for viscous flow for each glass was evaluated by using the Andrade equation, xl = A e x p ( E ~ / R T ) , where R is the gas constant and A is a constant. The values obtained for 50CuOx-50P205 glasses and xNaO0. 5(50 - x)MgO-50P205 glasses are shown in Fig. 7 as a function of R+/(R'2++ R ÷) ratio. In xNaOo. 5-
R. Sato et al. / Journal of Non-Crystalline Solids 201 (1996) 222-230
226 11.0
11.0
10.5
10.5
.g io.o
10.0
g
9.5
n
9.5
8
9.o
~
8.5
~> 8.5
-
8.0
8.0
7.5
7.5
7.0 ' • 1.20 1.30 1.40 1.51) 1.60 1.70 1.80 1.90 2.00 2.10
7.0
IO00/T
0.80
, 0.85
0.90
0.95
1.00
Tg/T
(K-1)
Fig. 6. Temperature dependence of the viscosity at the glass transition region for 50CuOx-50P;O 5 glasses. Q: C u + / ( C u 2÷ + C u + ) < 0 . 0 1 , C): C u + / ( C u 2+ + C u + ) = 0.14, • : C u + / ( C u 2+ + C u + ) = 0.24, []: C u + / ( C u 2+ + Cu + ) = 0.52, • : C u + / ( C u 2+ + C u + ) = 0.61.
( 5 0 - x)MgO-50PeO 5 glasses values of Ex I range from 697 kJ/mol for 50MgO-50PeO 5 to 504 kJ/mol for 30NaO0.5-20 MgO-50P205 glass, and E'q linearly decreases with increasing Na +/(Mg2+ + Na ÷). In the 50CUOx-50P205 glasses with Cu÷/(Cu2++ Cu +) < 0.3, the value of E'q decreases rapidly, but a slight decrease is observed in glasses with Cu+/(Cu 2÷ + Cu ÷) > 0.4.
Fig. 8. Viscosity data for 50CuOx-50 P205 glasses in a scaled Arrhenius form normalized at Tg. Q: C u + / ( C u 2+ + C u + ) < 0.01, O: C u + / ( C u 2+ + C u + ) = 0.14, • : C u + / ( C u 2+ + C u + ) = 0.24, []: C u + / ( C u e+ + C u + ) = 0.52, • : C u + / ( C u 2+ + C u + ) = 0.61.
Fig. 8 shows the viscosity for 50CuOx-50P20 s glasses plotted in a reduced Arrhenius form, where the normalizing temperature is Tg. This plot is called 'Angell plot' [18,19]. The slope of the plots in this viscosity range, E'q/2.303RTg, is a measure of the 'fragility', m, of supercooled liquids [18] because the non-Arrhenius character in the temperature dependence of the viscosity for supercooled liquids corresponds to the slope, Erl/2.303RTg, at around Tg.
700 11.0
10.5
6OO
10.0
~6o
#_
9.5
'~ 9.0 o
tu 400
.~_
~> 8.5
3OO
200 0
8.0 ' 0.1
' • ' 0.2 0.3
' 0.4
' - ' 0.5 0.6
7.5 0.7
R+/(R' 2*+R+)
Fig. 7. Values of the activation energy, ETI, for viscous flow estimated from Arrhenius plot for 50CuO.-50P205 glasses ( Q ) and x N a O o . s - ( 5 0 - x)MgO-50P205 glasses ( © ) as a function of R + / ( R '2+ + R + ) ratio.
7.0
0.80
L
0.85
0.90 Yg/T
0.95
1.00
Fig. 9. Viscosity data for xNaOo. 5 - ( 5 0 - x)MgO-50P205 glasses in a scaled Arrhenius form normalized at Tg. O: x = 0.0, O: x = 1 0 , • : x = 2 0 , rq: x=30.
R. Sato et aL/Journal of Non-Crystalline Solids 201 (1996)222-230 46
42
E38 36 34 32 30 0
i . i 0.1 0,2
i
,
0.3
t
i
t
0.4
0.5
0.6
,
0.7
R+/(R ' 2++R+) F i g . 10, R + / ( R ' 2 + + R + ) ratio d e p e n d e n c e
of fragility, m, for
50CuO~-50P205 glasses (0) and xNaO0.5-(50-x)MgO50P205 glasses(©).
The slope of the plot decreases with increasing Cu+/(Cu2++Cu+). The same plot for xNaO0. 5( 5 0 - x)MgO-50P205 glasses is also shown in Fig. 9. The R+/(R'2++ R ÷) dependence of fragility, m = E'q/2.303RTg, for 50CUOx-50P205 glasses and xNaO0.5-(50- x)MgO-50P205 glasses is shown in Fig. 10. In the xNaO0.s-(50-x)MgO-50P205 glasses, fragility decreases slightly with increasing Na ÷ content, however, a rapid decrease in fragility occurs in the 50CUOx-50P205 glasses with Cu+/(Cu2+ + Cu ÷) < 0.3. These results again highlight the difference between Cu ÷ and Na ÷ ions.
4. D i s c u s s i o n
4.1. Chemical glasses
composition
of 50CuOx-50P 205
As shown in Table 1, there is A1203 contamination in all glasses from alumina crucibles. Generally, an introduction of trivalent ions, A13+ or B 3+, into phosphate glasses strengthens the glass network [20,21]. Since the amount of A1203 in all glasses are almost the same, it is possible to clarify the effect of copper valence state on the physical properties of the glasses. The CuO phase transforms into the Cu20 phase above 1025°C in air. However, Cu ions in the
227
melt of 50CUOx-50P205 composition at 1200°C may exist almost completely as Cu 2÷ ions due to strong acidity of phosphate melts [22]. Also as shown in Table 1, Cu ÷ content in the glass increases with increasing glucose content obviously because glucose reduced Cu 2÷ ions into Cu ÷ ions. However, a homogeneous glass with Cu+/(Cu2++ Cu ÷) > 0.7 was not obtained. In order to examine the effect of the copper valence state on physical properties of 50CuO x50P20 s glasses, it is necessary to control widely the copper valence. Bae and Weinberg [10] succeeded in preparing of 50CuOx-50P20~ glass with C u + / ( C u 2 + + C u + ) = 0 . 7 6 6 by changing melting time. Tsuchiya and Moriya [22] prepared 50CuO x50P205 glasses with different copper valence states by adding carbon, but they could not obtain the glass with Cu+/(Cu2+ + Cu ÷) > 0.7. The results obtained in the present study and by Tsuchiya and Moriya imply that the 50CuO~-50P20 s glass with C u + / ( C u Z + + C u + ) > 0 . 7 cannot be prepared by adding reducing agents. 4.2. Properties of 5OCuOx-5OP205 glasses
Molar volume for 50CuOx-50P205 glasses and xNaO05-(50 - x)MgO-50P205 glasses increase linearly with increasing R ÷ content as shown in Fig. 2. In both glasses, the glass composition deviates from metaphosphate composition increasing with RO0 s content. As a result, a bridging oxygen is introduced into glass network and a network structure becomes more open than that of meta-composition glass. In 50CuOx-50P:O 5 glasses the large increase occurred compared with that in x N a O 0 5 - ( 5 0 - x ) M g O 50P:O 5 glasses, implying an introduction of a more loose structure. It is known that the coordination state of Cu ÷ in CueO is linear CuO z and that of Cu 2÷ in CuO is planar C u O 4. The density of C u 2 0 and CuO is 6.04 and 6.31 g / c m 3, respectively. If the oxygen coordination state of Cu ions in copper phosphate glasses is similar to that in the crystals, i.e., linear CuO 2 for Cu ÷ ions and planar CuO 4 for Cu 2÷ ions, a large increase in the molar volume would be expected. Drake and Scanlan [23] proposed that in CuOP205 glasses the Cu ÷ coordination is similar in Cu20. Nakagawa et al. [24] investigated a local
R. Sato et al. / Journal of Non-Crystalline Solids 201 (1996) 222-230
228
structure around Cu ions in a Bi2SrzCaCu20 x glass by a Cu extended X-ray absorption fine structure (EXAFS) analysis and proposed that some fraction of Cu ions exists as Cu ÷ surrounded by two oxygen atoms. Sato et al. [14] reported that the density of the Bi 2SrECaCu 2Ox glass decreases with increasing Cu + content indicating that Cu + ions are surrounded by two oxygen atoms. Kamiya et al. [25] reported from EXAFS experiments that Cu + ions in Cu20" A 1 2 0 3 • 4SiO 2 glass are surrounded by two oxygen atoms through covalent Cu+-O bonds and this is the primary reason for its low thermal expansion coefficient. The large increase in the molar volume obtained in the present study also indicates that in 50CuO~-50P205 glasses, Cu ÷ ions are surrounded by two oxygen atoms in CuOx-P205 glass. The decrease in Tg with increasing Cu ÷ content in CUOx-P205 glasses has been reported by Bae and Weinberg [10]. As shown in Fig. 4, the tendency of decrease in their study agrees with that in the present study. They mentioned that the intermolecular force of Cu 2÷ is higher than that of Cu ÷ in phosphates, and thus glasses containing more Cu 2÷ possess higher glass transition temperature. In the present study, we consider the relationship between glass transition temperature and single bond strength proposed by Sun [26]. Fig. 11 shows the value of Tg for alkali and alkaline-earth [27] metaphosphate glasses as a function of single bond strength. There is a tendency for the value of Tg to 600
II
|
5OO A
o *" 400
0 300
2 0 0
,
80
i
90
,
i
,
i
,
i
.
|
.
i
I
i
.
100 110 120 130 140 150 160 Single bond strength (kJ/mol)
Fig. 11. Values of Tg for alkali and alkaline-earth metaphosphate glasses as a function of single bond strength. O: LiOo. 5, ©: NaOo 5, I1: MgO, El: CaO, A: SrO, r,: BaO.
decrease with decreasing the single bond strength, although NaO0.5 deviates slightly from this. This indicates that the single bond strength is one of the origins to control the glass transition temperature. Although the single bond strength of copper oxides was not reported by Sun, the single bond strength of monovalent and divalent copper oxide can be calculated from thermodynamic data [28]. The single bond strength of CuO and CuO0.5 is 186 and 274 kJ/mol respectively. The single bond strength of Cu+-O is higher than that of CuZ+-O, thus the tendency of the decrease in Tg observed in 50CuO x50P205 glasses is not explained. Very recently, Jamnicky et al. reported that a decrease in a packing density in the glass structure leads to decrease in Tg and is the main factor affecting Tg in CUEO-P205MoO 3 glasses [29]. The open structure which is introduced by Cu + ions surrounded two oxygens may be an origin of the decrease in Tg in 50CuOx50P205 glasses. It is well known that the thermal expansion coefficient of oxide glasses containing copper ions such a s C U E O - A 1 2 0 3 .4SiO 2 depends strongly on the copper valence state and decreases with increasing Cu+/(Cu2+ + Cu +) ratio [30]. It is well recognized that Cu ÷ ions play an important role in the low thermal expansion coefficient. However, in the 50CuO~-50P205 glasses with Cu+/(Cu 2++ Cu +) < 0.3, the values of about 90 × 10 - 7 a r e insensitive to the copper valence state and the thermal expansion coefficient for the glasses with Cu+/(Cu2++ Cu +) > 0.4 increase slightly with increasing Cu + content as shown in Fig. 5. These differences indicate that the low thermal expansion observed in C u 2 0 . A l 2 0 3 - 4 S i O 2 glasses is not only a consequence of Cu + ion present, but also depends on glass composition. In comparison with the xNaO0. 5( 5 0 - x)MgO-50P205 glasses, the increase in ct is small in the 50 CUOx-50 P205 glasses. These results would reflect a peculiarity of Cu ÷ ions, i.e., a loose structure introduced by a low oxygen coordination and a high single bond strength, 274 kJ/mol, which is classified into an intermediate by Sun's classification [26]. 4.3. Effect of copper valence on fragility
From the fragility concept, in which the logarithm of viscosity is plotted against reduced temperature
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Tg/T, one can say that the glasses with low E-q at near Tg or with high Tg in a given glass system tend to be more strong than those with high E'q or low Tg. As shown in Fig. 4 the value of Tg decreases with increasing monovalent ion content in 50CuO x50P205 glasses and xNaO0.5-(50- x)MgO-50P205 glasses. However, fragility, m, decreases rapidly with increasing Cu ÷ content in the 50CuO~-50P205 supercooled liquids as shown in Fig. 10, thus indicating that the 50CuOx-50P205 supercooled liquids become strong with increasing Cu ÷ content. Although the fragility in x N a O 0 . 5 - ( 5 0 - x ) M g O 50P205 supercooled liquids also decreases, the degree of decrease is small. This small decrease is due to the deviation from the meta-composition with increasing Na + content. That is, a bridging oxygen increases due to the deviation from the meta-composition and a three dimensional expanse becomes large. Since the viscosity is controlled by flow unit size and free volume, the variation of the liquid viscosity with temperature is known to correspond to the intermediate range structure of liquids. AngeU suggested that the distinction between strong and fragile characters is to be associated with the differences in the stability of intermediate range order against temperature-induced degradation [18,31]. Further, AngeU pointed out that changes in heat capacity at Tg in strong glasses are extremely small compared with those in fragile glasses [18,31]. Therefore, it is considered that strong glasses have 'liquid-like' structure preserved at low temperatures and a more homogeneous open structure. The result obtained in the present study suggests that the 50CuOx-50P205 glasses become more and more liquid like with a more homogeneous open structure with increasing Cu + content. The glass structure model of 50CuOx-50P205 glasses which has been proposed in this paper, are shown in Fig. 12. In our present model, Cu 2+ ions act as modifier ions in the phosphate; thus, non bridging oxygens are introduced as shown in Fig. 12(A). Cu ÷ ions are surrounded by two oxygen atoms and bridge two phosphate chains; thus, a large correlation length is expected in the glass structure. Further, the coordination of Cu ÷ ions is explained by using sp hybrid orbital. In other words, a Cu +-O bond might be covalent and its bond would be
A
0
II ..... 0-.1~1 -0
0 I ..... o-
.....
n_ ~
0 U
. . .o ..
-o
9
-oI
o'
....O - ~ - - O .....
B
O
II ..... O--~1 - O .....
I
----O-~L-O ..... Fig. 12. Structure model of 50CuOx-50P205 glasses. A: CuOP20~, B: CuO0.5-P205.
strong, as shown by the value of single bond strength, 274 kJ/mol. In the 50CuOx-50P205 supercooled liquids with high Cu 2+ contents, an ionic Cu2+-O bond breaks easily with increasing temperature and the correlation length becomes small. In the 50CUOx-50P205 supercooled liquids with high Cu ÷ contents, a large correlation length due to the strong Cu÷-O bond would be stable. It is concluded that the decrease in fragility, m, with increasing Cu ÷ ion content observed in 50CUOx-O 5 glasses is due to the improvement in stability of intermediate range order. The result obtained in this study implies that Cu ÷ ions are not a modifier such as alkali and alkalineearth ions, but a intermediate or network former in some oxide glasses.
5. C o n c l u s i o n
Density of 50CuOx-50PzO 5 glasses decreased rapidly with increasing Cu+/(Cu2+ + Cu ÷) indicating that the glass structure becomes loose. The glass transition temperature decrease with increasing C u + / ( C u 2 + + C u ÷) may be due to the different coordination between Cu ÷ and Cu 2÷. The viscosity and activation energy for viscous flow of 50CuO x-
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50P205 glasses decreased with increasing Cu+/(CuZ++ Cu+). From the fragility concept, the 50CuOx-50P205 supercooled liquids may become strong with increasing Cu+/(Cu2++ Cu ÷) due to improvement in the thermal stability of intermediate range order.
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