STUDIES ON GLASS. IX THE ELECTRICAL CONDUCTIVITY O F BORONTRIOXIDE-SODIUM BORATE GLASSES~ MONROE E. SPAGHTZ
AKD
JOHN D. CLARK
Department of Chemistry, Stanford University, California Received November 34, 1953
It has been shown quite conclusively that the conduction of an electric current by certain glasses is due to the transference of ions ( 5 ) . In other glasses, however, such as those composed of very simple inorganic and complex organic molecules, it is difficult to decide on the nature of the charge carriers. Schonborn has presented data on certain borosilicate glasses which conduct ionically ( 3 ) . His measurements show a definite break in the conductivity-temperature curve at a point corresponding to the softening region of the glass. With boron trioxide, however, no evidence of such a change in the transition region has been reported (4,6). Various boron trioxide-sodium borate systems hare also been measured, but these too showed no irregularities. This investigation represents a further study on boron trioxide-sodium borate (B2Q3-Na2B407)systems, undertaken and carried out with the aid of a new and highly refined method of measurement. APPARATUS AND PROCEDURE
The apparatus used to make the conductivity measurements was a modification of that described by Clark and Williams (1). The modifications were introduced in the interests of greater sensitivity and ease‘of control. The circuit was essentially a modified Wheatstone bridge employing precision standard resistances, variable potentials on two of the bridge arms, and a high-sensitivity quartz-suspension Compton electrometer.’ For measurements on pure boron trioxide and mixtures containing small percentages of sodium borate, a conductivity cell was constructed of Monel 1 This work is a contribution to the series of studies on glass which are being conducted by Professor George S. Parks a t Stanford University. Shell Research Fellow a t Stanford University, 1932-1933. 3 The refinements of the apparatus over that described by Clark and Williams (1) will be reported by one of us (J. D. C.) a t a later time. 833
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MONROE E. SPAGHT AND JOHN D. CLARK
metal. The outside was turned to fit a cavity in a large electrically heated copper block. In the center of the Monel piece, a cylindrical cavity of 10.5 mm. diameter and 50 mm. depth was drilled. A piece of silver wire was connected with this container, which then served as one electrode. The other electrode was a Monel rod (6.80 mm. in diameter) extending through a hole in a copper plug which was turned to fit tightly in the cone top of the copper-block cavity. This electrode was insulated from the copper plug by means of two pieces of Pyrex tubing, one fitting inside the other, the lLlonel rod fitting inside the smaller one, and the outer one fitting into the hole drilled through the copper plug. The inner electrode was thereby concentric with the cavity in the container, leaving a space of 1.85 mm. between the two cylindrical surfaces. The cell constant for the two concentric cylinders, neglecting the effect of the end surface of the r0d,4 is evaluated by the following equation:
where ,ii= specific conductivity, p = reciprocal of measured resistance, and L = length of lateral exposure of the inner electrode. For measurements on pure borax and mixtures containing high percentages of borax, the glass samples were blown in the form of tubes sealed a t one end, having an outer diameter of about 1 cm. and an inner diameter of about 0.8 cm. The inner electrode consisted of a liquid sodium amalgam which was used as the anode. The tube containing the sodium amalgam was immersed jn mercury, which served as the other electrode. A small electrically heated oven was used for temperature control. The cell constant was calculated in a manner similar t o that above. Temperatures were measured with a carefully calibrated noble-metal thermocouple and a White potentiometer. The thermocouple junction mas inserted adjacent to the conductivity cell. The samples of boron trioxide were prepared by heating in a platinum criicible a t a temperature of 1200-1500OC. for one to two hours. The boron trioxide-sodium borate mixtures which were measured in the Monel cell were prepared by fusing the two components at the above temperature until all ebullition had ceased. While in a fluid condition, the sample was poured into the NIonel container. The container was then heated to 6007OO0C., placed in the copper block, and the inner electrode was immersed while the glass was still very hot. The system was brought to thermal balance a t about 35OoC. and measurements started a t that point. The sample was then heated and cooled repeatedly over the temperature interval studied. 4 A thickness sf material between the end surface and the outer electrode was used such that a consideration of this effect is not necessary.
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STUDIES ON GLASS. IX
The glass tubes were prepared by heating and blowing to the desired shape. EXPERIMENTAL RESULTS
Two samples of boron trioxide glass were first measured from 370°C. down to 130°C. Next, three boron trioxide glasses diluted with sodium borate were measured over a similar temperature interval using the Monel cell. The percentages of sodium borate present were 2.68, 5.42, and 16.74. As a check on the consistency of the procedure, another pure boron trioxide sample was measured a t this point. Finally, pure sodium borate and 55 TABLE 1 Conductivity of a sodium borate-boron trioxide mixture Per cent of NazB40, = 5.42 TEMPERATURE
degrees C .
353.1 346.0 344.8 333.4 324.2 310 5 306.3 296 0 289 0 265.7 264 3 260.6 244 5 229 0 211 8 194 8
RECIPROCAL O F MEASURED RESISTANCE
ohm-' cm.
ohm-'
8.84 x 5.68 x 5.05 x 2.15 x 1.15 x 3.42 x 3.10 X 1.70 x 9.50 x 2.14 X 2.42 x 2.08 x 1.39 X 7.69 x 4 30 x 2.46 X
SPECIFIC CONDUCTIVITY
10-D
10-g 10-9 10-9
10-lo 10-1O 10-11 10-11 10-'1 10-12
3.71 x 2.39 X 2.12 x 9.05 X 4.84 x 1.44 x 1.30 X 7.14 x 3.98 X 8.99 x 1.01x 8.74 x 5.84 x 3.23 x 1 80 x 1.03 x
10-10 lo-" 10-l' 10-11
10-13 10-13 10-13 10-13 10-13 10-13
10-13
per cent sodium borate glasses were investigated, using a sodium amalgam electrode and the small tubes prepared from the glass samples. These last two glasses were not studied a t high temperatures because of vaporization of electrodes. Each of the three pure boron trioxide glasses studied gave conductivity values in good agreement with the others. Plotting log of the specific conductivity (as calculated after evaluation of the cell constant) against reciprocal of the absolute temperature, straight lines over the measured temperature interval were found in every case. The accurate evaluation of the cell constant was rather difficult, and owing to this fact the curves for the three samples were parallel but not identical. The deviations were
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MONROE E. SPAGHT AND J O H N D. CLARK
small and can be attributed without question t o the evaluation of the cell constant. In table 1 the recorded data on one of the three mixtures are given. Smooth curves drawn through the recorded points are shown in figure 1. The curve for pure boron trioxide represent,s an average of the three samples measured.
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-9
-I I
-It
1. SPECIFICCONDUCTIVITY OF B O R O N ' ~ R 1 O X I D E - ~ O D I U M B O R A T E GLASSES 1, Pure boron trioxide; 2, mixed glass containing 2.68 per cent sodium borate; 3, mixed glass containing 5.42 per cent sodium borate; 4, mixed glass containing 16.74 per cent sodium borate; 5 , mixed glass containing 55 per cent sodium borate; 6, pure sodium borate. FIG.
DISCUSSION
The series of curves shown in figure 1 present two marked features: (1) In all cases of mixed glasses which were measured to about 250°C., a decided and rather sharp break was found in the conductivity curve. This break occurred in each of the three glasses measured in this temperature range at a point near the t>ransitionregion of boron trioxide as dstermined from other physical data. (2) The addition of a small percentage of sodium borate to boron trioxide lowers the conductivity of the glass in the region of softening. As we go to higher temperatures, however, the mixed glass increases in conductivity more rapidly with temperature than
STUDIES O N GLASS. IX
839
does the pure boron trioxide, and a t high temperatures the mixtures are more conductive than the pure oxide. Neither of the previous investigators of these systems has reported the abrupt changes in the direction of the conductivity curves of the mixed glasses in the transition region. Nevertheless, the data reported by Thomas (6) are in fair agreement with those presented here, especially a t the higher temperatures. His values for boron trioxide-sodium borate mixtures show that at low temperatures the specific conductivities would be less than that of pure boron trioxide. Schonborn’s investigations on borosilicate glasses ( 3 ) show conductivity curves which are of the exact nature of the mixture curves in figure 1. The significance of the marked change of direction of the mixture curves is to be related to the gradual change in state of a glass in its transition region (2). It is known that a viscous liquid changes to a solid glass in a short temperature interval, and that many physical properties change in this interval with an abruptness that is suggestive of a crystallization process. The conductivity behavior of the mixed glasses seems to be further evidence of this change of state within a narrow temperature interval. The addition of sodium borate to boron trioxide introduces sodium ions, and thereby evidently brings about ionic conductivity. When a glass conducts a current ionically, a break in the conductivity curve represents a change in the binding forces acting on the conducting ions. This might be expected in every case where the conducting ions are part of the molecules or aggregates composing the glass. If conduction is electronic, or due to foreign ions which are not affected by the glass structure, the transition of a glass to a viscous liquid mould not necessarily affect the conductivity. In the case a t hand, the presence of sodium ions in the mixtures introduces ionic conductivity, and a change in the direction of the conductivity curves might well be expected in the transition region. Extrapolating the straight portions of the curves for the mixtures, points are obtained which can be considered as the “transition temperatures” of the glasses. For the three mixtures containing low percentages of borax, these x-alues are 271”C.,279”C.,and 276”C., named in the order of increasing sodium borate content. The transition temperature for pure boron trioxide, as chosen from other physical data, is in the neighborhood of 250°C. (6). The straight unbroken curve for pure boron trioxide suggests that the conduction is not carried on by ions which are part of the glass structure. Many oxides have been found to conduct electronically, and this is probably the nature of the conduction by this substance. The decrease in conductivity when sodium borate is added to boron trioxide seems to indicate that the addition of ionic conductors disturbs
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MONROE E . SPAGHT AND JOHN D. CLARK
the inherent ability of the boron trioxide to conduct electronically. As the percentage of sodium borate increases, however, the ionic conduction becomes greater, and apparently surpasses that of pure boron trioxide as higher percentages are reached. Even with small amounts of sodium borate (2 to 5 per cent), the ionic conductivity increases so rapidly with temperature that in the neighborhood of 350°C. the conductivity exceeds that of pure boron trioxide. Below the hardening region the curves are approximately parallel. Although direct current was used in the measurements, polarization effects were not noticeable in the samples containing small percentages of sodium borate. If conduction by pure boron trioxide is electronic, no polarization would be expected in any case. The glasses containing a high percentage of borax were measured with a sodium amalgam anode which acted as a source of sodium ions and thus prevented polarization. An attempt was made to measure these samples with the Monel cell, but polarization effects were evident. However, measurements were made on the 16.7 per cent sodium borate sample using the amalgam anode, and results were obtained which were in very close agreement with those obtained using the hlonel cell. This fact shows that with glasses containing low percentages of sodium borate, polarization is unimportant. SUMMARY
The electrical conductivities of boron trioxide, sodium borate, and some boron trioxide-sodium borate systems have been measured over a 240' interval with a modified Wheatstone bridge method. Pure boron trioxide shows a smooth even conductivity curve over the entire measured range, but the mixed glasses gave fairly sharp breaks in the curves in the region of transition of the glasses. These breaks are considered to be associated with the changes in many physical properties in the interval where a viscous liquid changes to a hard glass. The addition of small percentages of sodium borate to pure boron trioxide causes the conductivity to decrease, but further addition causes a steady increase.
It! is our pleasure to express appreciation l o Professor George S. Parks and Dr. F. 0. Koenig of this department for their help and advice in this work. REFERENCES (1) CLARKAND WILLIAMS:J. Phys. Chem. 37, 119 (1933). (2) PARKS AND HUFFMAN: Science 64, 364 (1926). (3) SCHONBORN: Z. Physik. 22, 305 (1924). (4) SCHTSCHUKAREW AND MULLER:Z. physik. Chem. 15OA, 439 (1930). (5) SMEKAL:Glastech. Ber. 7, 265 (1929). (6) THOMAS: J. Phys. Chem. 35, 2103 (1931). (7) THOMAS AND PARKS: J. Phys. Chem. 35, 2091 (1931).
