Vidros fosfatos com tungstênio: incorporação de cátions alcalinos e sua influência nas propriedades estruturais e condutoras
Abstract
Many glasses have been studied as solid
electrolytes in behalf of replacing commercial liquid electrolytes. However, even
for the best glass compositions available in the literature, values of conductivity
similar to those presented by liquids are far beyond to be achieved. This work
intends to show how changes in the structure, by means of composition, can help
to increase the conductivity in oxide glasses. Samples with composition x WO3 -
0.30 (NaPO3)n - (0.70-x) NaF, with 0,30 ≤ 𝑥 ≤ 0,60;, were produced by melting
quenching methodology. Substitution of tungsten for sodium fluoride generates a
linear decrease of glass transition temperature while the stability remains high
enough up to 𝑾 = 𝟒𝟎 %𝒎𝒐𝒍 and decreasing for higher WO3 content. Raman and
31P and 19F 1D MAS-NMR spectroscopies data shows that the less NaF leads to a
depolymerization of main phosphate chains, due to formation of 𝑷 − 𝑶 − 𝑾
bonds. In addition, fluorine atoms prefer to bind in the 𝑷 − 𝑭 form, with low
tungsten content, or in 𝑾 − 𝑭, with high proportion All samples showed same
calculated values of Ea for ionic conductivity and drop of σ0. For higher fluoride
proportion, it was observed a rise in the conductivity of around one order of
magnitude. However, UV-Vis absorption curves demonstrate more presence of
W5+ species in samples with 𝑾 ≥ 𝟓𝟎 %𝒎𝒐𝒍. This leads to a mixed conductivity
of these materials.
In order to obtain maximum ionic conductivity, NaF and (NaPO3)n were replaced
by Li2O and (LiPO3)n respectively. Samples with composition x WO3 – 0.40
(LiPO3)n – (0.60-x) Li2O, with 0.30 ≤ 𝑥 ≤ 0.50; 0.40 WO3 – y (LiPO3)n – (0.60-
y) Li2O, with 0.30 ≤ 𝑦 ≤ 0.50 and z WO3 – (0.80-z) (LiPO3)n – 0.20 Li2O, with
0.30 ≤ 𝑧 ≤ 0.50; were also produced using the melting quenching methodology.
The three series of samples presented linear increase of Tg with their respective
substitutions, besides having good thermal stability (> 100 °𝐶). Raman and 31P
1D MAS-NMR spectroscopies data exhibited the influence of WO3 content on the
depolymerization of phosphate chains, as well as the formation of WO6 unit
clusters, when much tungsten is present. Although UV-Vis absorption spectra
indicate mixed conductivities in some samples, calculated values of ionic
conductivity show that the most conductive sample without presence of reduced
species reached 6.3 10−4Ω−1𝑐𝑚−1, comparable to conductive crystalline
samples.