66
5 Mechanisms Involved in the Electrolytic Fabrication …
is in qualitative agreement with the two-stage mechanism for bulk dehydration of
LiCl · H 2 O [7, 9]. Additionally, it is realized from Fig. 5.3 that by increasing the
heating rate to values beyond 10 °C min
−1 , the first endothermic peak vanishes,
suggesting that the surface dehydration of LiCl proceeds in only one step at higher
heating rates. The preliminary water content of the material was found to be 2.55%
with 88% thereof being removed below 120 °C, by averaging the mass loss in samples
heated up to 600 °C with different heating rates. Three clear endothermic peaks are
apparent on the DSC curves of Fig. 5.3. The peaks seen at 93–115 °C, 608–621 °C and
1021–1195 °C are assigned to the surface dehydration, the melting and the complete
evaporation of LiCl, respectively. Along with these peaks, some small endothermic
peaks can also be seen at around 800–1000 °C. The source of these peaks can be
indicated by using thermogravimetric analysis combined with mass spectrometry
(TG-MS) analysis [7, 8].
Figure 5.3c displays the TG-MS analysis of LiCl during heating at the heating
rate of 10 °C min
−1 under an ambient air flow of 60 mL min
−1 . The mass to charge
ratios, m/z, of 18 and 44 are attributed to the H 2 O
+ and LiCl
+ ion, consisting of the
37 Cl isotopes, respectively. The m/z values of 38 and 36 are evidence of the HCl
+
ion which comprise of the
37 Cl and
35 Cl isotopes, respectively. Figure 5.3c confirms
the release of water during the surface de-hydration of LiCl at low temperatures.
Furthermore, there is also an indication of the water release at temperatures above
800 °C. The evaporation of LiCl is seen to initiate at a minor rate at temperatures
greater than around 650 °C, and then slowly accelerates up to 800 °C where a sharp
rise occurs. The release of HCl can be detected at temperatures beyond 800 °C. These
observations were used to explain the small endothermic peaks detected in the DSC
curve of Fig. 5.3. Accordingly, the first endothermic peak at around 801 °C at the
heating rate of 10 °C min
−1 is as a result of the hydrolysis of molten LiCl to LiOH,
as shown by Eq. (5.2) [8]:
LiCl + H 2 O = LiOH + HCl(g)
(5.2)
The second endothermic event at 831 °C can be assigned to the decomposition of
LiOH to form Li 2 O:
2LiOH = Li 2 O + H 2 O(g)
(5.3)
Derived from these experimental observations, nominally anhydrous LiCl readily
absorbs and retains water after exposure to air at ambient conditions, forming a
surface layer of monohydrate LiCl · H 2 O. The predominance of the water content is
removed by heating the material to temperatures higher than 120 °C. The melting
of the dehydrated LiCl takes place at temperatures very near to those indicated
in the literature. This shows that the dehydration process leaves behind relatively
pure LiCl. Substantial evaporation of LiCl takes place at temperatures far below its
nominal boiling temperature. Upon heating, the LiCl critical temperatures do not
considerably change based on the type of gas atmosphere used. However, the water
content of the atmosphere plays an important role on the evaporation behavior of
5 Mechanisms Involved in the Electrolytic Fabrication …
is in qualitative agreement with the two-stage mechanism for bulk dehydration of
LiCl · H 2 O [7, 9]. Additionally, it is realized from Fig. 5.3 that by increasing the
heating rate to values beyond 10 °C min
−1 , the first endothermic peak vanishes,
suggesting that the surface dehydration of LiCl proceeds in only one step at higher
heating rates. The preliminary water content of the material was found to be 2.55%
with 88% thereof being removed below 120 °C, by averaging the mass loss in samples
heated up to 600 °C with different heating rates. Three clear endothermic peaks are
apparent on the DSC curves of Fig. 5.3. The peaks seen at 93–115 °C, 608–621 °C and
1021–1195 °C are assigned to the surface dehydration, the melting and the complete
evaporation of LiCl, respectively. Along with these peaks, some small endothermic
peaks can also be seen at around 800–1000 °C. The source of these peaks can be
indicated by using thermogravimetric analysis combined with mass spectrometry
(TG-MS) analysis [7, 8].
Figure 5.3c displays the TG-MS analysis of LiCl during heating at the heating
rate of 10 °C min
−1 under an ambient air flow of 60 mL min
−1 . The mass to charge
ratios, m/z, of 18 and 44 are attributed to the H 2 O
+ and LiCl
+ ion, consisting of the
37 Cl isotopes, respectively. The m/z values of 38 and 36 are evidence of the HCl
+
ion which comprise of the
37 Cl and
35 Cl isotopes, respectively. Figure 5.3c confirms
the release of water during the surface de-hydration of LiCl at low temperatures.
Furthermore, there is also an indication of the water release at temperatures above
800 °C. The evaporation of LiCl is seen to initiate at a minor rate at temperatures
greater than around 650 °C, and then slowly accelerates up to 800 °C where a sharp
rise occurs. The release of HCl can be detected at temperatures beyond 800 °C. These
observations were used to explain the small endothermic peaks detected in the DSC
curve of Fig. 5.3. Accordingly, the first endothermic peak at around 801 °C at the
heating rate of 10 °C min
−1 is as a result of the hydrolysis of molten LiCl to LiOH,
as shown by Eq. (5.2) [8]:
LiCl + H 2 O = LiOH + HCl(g)
(5.2)
The second endothermic event at 831 °C can be assigned to the decomposition of
LiOH to form Li 2 O:
2LiOH = Li 2 O + H 2 O(g)
(5.3)
Derived from these experimental observations, nominally anhydrous LiCl readily
absorbs and retains water after exposure to air at ambient conditions, forming a
surface layer of monohydrate LiCl · H 2 O. The predominance of the water content is
removed by heating the material to temperatures higher than 120 °C. The melting
of the dehydrated LiCl takes place at temperatures very near to those indicated
in the literature. This shows that the dehydration process leaves behind relatively
pure LiCl. Substantial evaporation of LiCl takes place at temperatures far below its
nominal boiling temperature. Upon heating, the LiCl critical temperatures do not
considerably change based on the type of gas atmosphere used. However, the water
content of the atmosphere plays an important role on the evaporation behavior of