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Luminescence and heat evolution during a vitrification process
Journal of Non-Crystalline Solids, 13 (1973/74) 357-360. © North-HollandPublishingCompany
LUMINESCENCE AND HEAT EVOLUTION DURING A V I T R I F I C A T I O N PROCESS Koji KISHIMOTO, Hiroshi SUGA and Syfiz6 SEKI Departmen t of Chemistry, Faculty of Science, Osaka University, Toyonaka, Osaka, Japan
Received 1 August 1973
During the course of systematic investigations of glass-forming properties of simple liquids using Differential Thermal Analysis (DTA), we found a sudden heat evolution and simultaneous luminescence phenomenon near the glass transition range during the cooling process. The materials observed were ethylene chlorohydrin (C1CH2CH2OH), ethylene bromohydrin (BrCH2CH2OH), ethylene cyanohydrin (CNCH2CH2OH) and butyronitrile (CH3CH2CH2CN). Reagent grade materials supplied from Wak6 Chem. Co. were first distilled under reduced pressure, dried with a molecular sieve (Linde 3A or 4A) and finally subjected to vacuum distillation. The purities of the samples employed were determined by a gas chromatographic method; BrCH2CH2OH (99.9%), C1CH2CH2OH (99.8%), CNCH2CH2OH (99.2%), CH3CH2CH2CN (99.5%). The experimental results are summarized in fig. 1 where a small but sharp exothermic peak can be observed for each sample, immediately after glass transition takes place on cooling. This phenomenon is usually accompanied with a sound crack. The temperature at which the exothermic peak appears varies to some extent (-+ 2°C) for each run, even for the same sample. The heat effect depends on the temperature at which the exothermic peak occurs. The lower the temperature, the bigger the exothermic effect. Roughly speaking, the following series of decreasing heights of the peaks was observed; BrCH2CH2OH > C1CH2CH2OH > CNCH2CH2CH 3 > CNCH2CH2OH. We followed the process of return from the top of the exothermic peak to the base line of the thermogram. In order to understand the effect, we also took similar decay curves for the melting processes of BrCH2CH2OH and C1CH2CH2OH, as reference. Fig. 2 shows these results by plotting In AT against time, where A T is the deviation of temperature from the base line. The time required for returning to a stationary state is governed by the diffusivity of the material in heat conduction theory [1 ]. These excellent linearities, and the identical slopes among them in fig. 2, confirm that these peaks should be attributed to real heat evolutions, not to an electrical effect.
358
K. Kishimoto et al., Luminescence and heat evolution
Br(CH2)2 OH
o c~ z bJ
CI (CH 2)2 OH
1 J
~t CN (CH2) 2 CH3
-.
/-
d
y
o xhi
4
%
/,
CN(CH~2OH
I
I
90
I
II 0
I
I
I
130
I
I
1
1,50
170
F/K Fig. 1. Thermograms of four compounds using the DTA method.
"-.[-.. 4
3 X~ E
~2 C~
I
I
I
0
20
40
CI 60
80
t/s Fig. 2. Plot of In A T against time, of two compounds at exothermic decay peaks (solid lines) and at their melting points (broken lines).
K. Kishimoto et aL, Luminescence and heat evolution
359
By immersing ampules containing BrCH2CH2OH, C1CH2CH2OH and CNCH2CH2OH, respectively, in liquid nitrogen, bluish-white flashes of light were observed at a definite local position when the samples fissured in their glassy states. This luminescence is so weak in intensity that it is only visible in complete darkness. That is not the case with CNCH2CH2CH 3, although it has an exothermic peak. We examined the influence of the pressure of inert gas introduced into an ampule on the luminescence effect. The ampule containing BrCH2CH2OH was connected to a vacuum system and evacuated to 3 X 10 4 Torr. We observed the color and intensity of luminescence as the pressure of helium gas was varied. Intensities of luminescence were hardly affected in a pressure range of 1-100 Tort. At 3 × 10 - 4 Torr the intensity was found to be weaker than at 1-100 Torr. At 760 Torr the intensity became weaker again and the portion of sample emitting light became dim. After initial flashing, by rubbing the ampule with fingers, vague royal purple luminescence was observed at the point of contact. This color was the same as that of discharged helium in a Geisler tube. When nitrogen gas was used instead of helium gas, rubbing gave vague reddish-purple luminescence which was the same as that of nitrogen gas in a Geisler tube. It was concluded that luminescence had occurred simultaneously with a real heat evolution on cooling a supercooled liquid through the glass transition region. This phenomenon is not confined to the compounds described above and was also found in tartaric acid, 4 - 3 0 H M B B A [2] and aqueous solutions of Ca(NO3) 2 (10 mol%), Th(NO3) 4 (15 tool %), NiC12 ( 6 - 1 0 tool %) and iso-propylbenzene [3]. However, in all cases except iso-propylbenzene only exothermic peaks were found, and luminescence was not observed. In the case of iso-propylbenzene, when the specimen for the heat capacity measurement was evacuated and thoroughly outgassed to a pressure of 4 X 10 -5 Torr, a green flash was observed. On the contrary, when the pressure in the ampule which contained the specimen reached around 10 -3 Tort, a bluish-white flash, which was quite similar to that of BrCH2CH2OH etc., was observed. This phenomenon, which has never been reported, reminds us of triboluminescence In the usual case of triboluminescence, an external mechanical force acts on a crystal, resulting in crystal breakdown. Triboluminescence is believed to be due to the discharge of triboelectricity [4] generated by charge separation between cleaved crystal faces when the crystal is ground. Another type of triboluminescence was discovered during the phase change of aragonite to calcite crystal [5], and the phase change of a to/3 form of methanol crystal [6]. In both cases, the phase change results in crystal breakdown and the generation of triboelectricity. Although we have no direct evidence, the following inference may be plausible. Rapid quenching of the liquid sample below Tg introduces strain which may arise from the difficulty for molecules of readjusting themselves to equilibrium in their frozen-in state. When an accumulated strain energy exceeds a critical value, the glass cracks: this results in the liberation of part of the strain energy and the simultaneous occurrence of triboelectricity.
360
K. Kishhimoto et al., Luminescence and heat evolution
Evidence in support of this explanation may be shown by the following experiment. An electrode dipped in the sample of ethylene bromohydrin, which was contained in an evacuated ampule, was connected to a vibrating capacitor electrometer (Keithley Instruments, model 640). When the sample cracked the electrometer momentarily registered several volts. Anyhow, the accumulated distortion energy may be released in the combined forms of thermal, sound and light energies. No observation of any exothermic peaks in the heating curves of DTA in fig. 1 is in accordance with this explanation.
Acknowledgement The authors would like to express their gratitude to Prof. Ishida and Mr. Uemura for using the vibrating capacitor electrometer°
References [1] [2] [3] [4] [5] [6]
M.J. Void, Anal. Chem. 21 (1946) 683. M. Sorai and S. Seki, Mol. Cryst. Liq. Cryst., in press. K. Kishimoto, H. Suga and S. Seki, Bull. Chem. Soe. Japan, in press. I.N. Stranski, E. Strauss and G. Wolff, Z. Electrochem. 59 (1955) 341. N.M. Johnson and F. Danieis, J. Chem. Phys. 34 O961) 1434. G.J. Trout, D.E. Moore and J.G. Hawke, Nature 235 (1972) 174.
LUMINESCENCE AND HEAT EVOLUTION DURING A V I T R I F I C A T I O N PROCESS Koji KISHIMOTO, Hiroshi SUGA and Syfiz6 SEKI Departmen t of Chemistry, Faculty of Science, Osaka University, Toyonaka, Osaka, Japan
Received 1 August 1973
During the course of systematic investigations of glass-forming properties of simple liquids using Differential Thermal Analysis (DTA), we found a sudden heat evolution and simultaneous luminescence phenomenon near the glass transition range during the cooling process. The materials observed were ethylene chlorohydrin (C1CH2CH2OH), ethylene bromohydrin (BrCH2CH2OH), ethylene cyanohydrin (CNCH2CH2OH) and butyronitrile (CH3CH2CH2CN). Reagent grade materials supplied from Wak6 Chem. Co. were first distilled under reduced pressure, dried with a molecular sieve (Linde 3A or 4A) and finally subjected to vacuum distillation. The purities of the samples employed were determined by a gas chromatographic method; BrCH2CH2OH (99.9%), C1CH2CH2OH (99.8%), CNCH2CH2OH (99.2%), CH3CH2CH2CN (99.5%). The experimental results are summarized in fig. 1 where a small but sharp exothermic peak can be observed for each sample, immediately after glass transition takes place on cooling. This phenomenon is usually accompanied with a sound crack. The temperature at which the exothermic peak appears varies to some extent (-+ 2°C) for each run, even for the same sample. The heat effect depends on the temperature at which the exothermic peak occurs. The lower the temperature, the bigger the exothermic effect. Roughly speaking, the following series of decreasing heights of the peaks was observed; BrCH2CH2OH > C1CH2CH2OH > CNCH2CH2CH 3 > CNCH2CH2OH. We followed the process of return from the top of the exothermic peak to the base line of the thermogram. In order to understand the effect, we also took similar decay curves for the melting processes of BrCH2CH2OH and C1CH2CH2OH, as reference. Fig. 2 shows these results by plotting In AT against time, where A T is the deviation of temperature from the base line. The time required for returning to a stationary state is governed by the diffusivity of the material in heat conduction theory [1 ]. These excellent linearities, and the identical slopes among them in fig. 2, confirm that these peaks should be attributed to real heat evolutions, not to an electrical effect.
358
K. Kishimoto et al., Luminescence and heat evolution
Br(CH2)2 OH
o c~ z bJ
CI (CH 2)2 OH
1 J
~t CN (CH2) 2 CH3
-.
/-
d
y
o xhi
4
%
/,
CN(CH~2OH
I
I
90
I
II 0
I
I
I
130
I
I
1
1,50
170
F/K Fig. 1. Thermograms of four compounds using the DTA method.
"-.[-.. 4
3 X~ E
~2 C~
I
I
I
0
20
40
CI 60
80
t/s Fig. 2. Plot of In A T against time, of two compounds at exothermic decay peaks (solid lines) and at their melting points (broken lines).
K. Kishimoto et aL, Luminescence and heat evolution
359
By immersing ampules containing BrCH2CH2OH, C1CH2CH2OH and CNCH2CH2OH, respectively, in liquid nitrogen, bluish-white flashes of light were observed at a definite local position when the samples fissured in their glassy states. This luminescence is so weak in intensity that it is only visible in complete darkness. That is not the case with CNCH2CH2CH 3, although it has an exothermic peak. We examined the influence of the pressure of inert gas introduced into an ampule on the luminescence effect. The ampule containing BrCH2CH2OH was connected to a vacuum system and evacuated to 3 X 10 4 Torr. We observed the color and intensity of luminescence as the pressure of helium gas was varied. Intensities of luminescence were hardly affected in a pressure range of 1-100 Tort. At 3 × 10 - 4 Torr the intensity was found to be weaker than at 1-100 Torr. At 760 Torr the intensity became weaker again and the portion of sample emitting light became dim. After initial flashing, by rubbing the ampule with fingers, vague royal purple luminescence was observed at the point of contact. This color was the same as that of discharged helium in a Geisler tube. When nitrogen gas was used instead of helium gas, rubbing gave vague reddish-purple luminescence which was the same as that of nitrogen gas in a Geisler tube. It was concluded that luminescence had occurred simultaneously with a real heat evolution on cooling a supercooled liquid through the glass transition region. This phenomenon is not confined to the compounds described above and was also found in tartaric acid, 4 - 3 0 H M B B A [2] and aqueous solutions of Ca(NO3) 2 (10 mol%), Th(NO3) 4 (15 tool %), NiC12 ( 6 - 1 0 tool %) and iso-propylbenzene [3]. However, in all cases except iso-propylbenzene only exothermic peaks were found, and luminescence was not observed. In the case of iso-propylbenzene, when the specimen for the heat capacity measurement was evacuated and thoroughly outgassed to a pressure of 4 X 10 -5 Torr, a green flash was observed. On the contrary, when the pressure in the ampule which contained the specimen reached around 10 -3 Tort, a bluish-white flash, which was quite similar to that of BrCH2CH2OH etc., was observed. This phenomenon, which has never been reported, reminds us of triboluminescence In the usual case of triboluminescence, an external mechanical force acts on a crystal, resulting in crystal breakdown. Triboluminescence is believed to be due to the discharge of triboelectricity [4] generated by charge separation between cleaved crystal faces when the crystal is ground. Another type of triboluminescence was discovered during the phase change of aragonite to calcite crystal [5], and the phase change of a to/3 form of methanol crystal [6]. In both cases, the phase change results in crystal breakdown and the generation of triboelectricity. Although we have no direct evidence, the following inference may be plausible. Rapid quenching of the liquid sample below Tg introduces strain which may arise from the difficulty for molecules of readjusting themselves to equilibrium in their frozen-in state. When an accumulated strain energy exceeds a critical value, the glass cracks: this results in the liberation of part of the strain energy and the simultaneous occurrence of triboelectricity.
360
K. Kishhimoto et al., Luminescence and heat evolution
Evidence in support of this explanation may be shown by the following experiment. An electrode dipped in the sample of ethylene bromohydrin, which was contained in an evacuated ampule, was connected to a vibrating capacitor electrometer (Keithley Instruments, model 640). When the sample cracked the electrometer momentarily registered several volts. Anyhow, the accumulated distortion energy may be released in the combined forms of thermal, sound and light energies. No observation of any exothermic peaks in the heating curves of DTA in fig. 1 is in accordance with this explanation.
Acknowledgement The authors would like to express their gratitude to Prof. Ishida and Mr. Uemura for using the vibrating capacitor electrometer°
References [1] [2] [3] [4] [5] [6]
M.J. Void, Anal. Chem. 21 (1946) 683. M. Sorai and S. Seki, Mol. Cryst. Liq. Cryst., in press. K. Kishimoto, H. Suga and S. Seki, Bull. Chem. Soe. Japan, in press. I.N. Stranski, E. Strauss and G. Wolff, Z. Electrochem. 59 (1955) 341. N.M. Johnson and F. Danieis, J. Chem. Phys. 34 O961) 1434. G.J. Trout, D.E. Moore and J.G. Hawke, Nature 235 (1972) 174.