Crystallization of iron phosphate glasses

Abstract

Differential thermal analysis (DTA), thermogravimetric analysis (TG), X-ray diffraction and M6ssbauer spectroscopy were employed in the investigation of crystalline products of FeOx--P~O~ glasses generated by various heat treatments. In glasses with a high value of e = Fe~+/(Fe 2+ q-FEZ+), absorption of oxygen occurs in a broad temperature range identified by TG. Depending on the value of e, two exotherms appear in the DTA curves, the low-temperature one corresponding to crystallization of the Fe3(PO4) ~ type regions, and the high-temperature one being related to various phases with dominating FePOa. Each exotherm has its own transformation region, identical in absolute value. The MSssbauer spectra of glasses which underwent thermal treatment at higher temperatures exhibit some indication of phases of the types Fe3(PO~)~ " xH20 and FePO4 " xHeO.It is assumed that the transport of charge in iron phosphate glasses is due to the migration of charge carriers between Fe ions of various valencies. In our recent paper devoted to the analysis of FeO~-P20 5 glasses, using the thermostimulated depolarization method (TSD) we predicted the presence of microscopic regions of the dispersed phase inserted in the homogeneous matrix of the glass [1]. The mechanism of devitrification, studied in this connection by means of transmission electron microscopy [2], as well as some indirect structural conclusions based on ac and dc characteristics measurements [3] and considerations of the phase separation of phosphate glasses [4,5], assume the existence of several phases.It is the aim of this paper to investigate the crystallization of FeOx-P205 glasses in a non-isothermal way (using differential thermal analysis, DTA, and thermogravimetry, TG), to identify crystalline phases generated by heat treatment (using X-ray analysis and M6ssbauer spectroscopy), and to judge the possibility of liquid-liquid phase separation. Experimental Sample preparationReagent-grade P~Os, F%Oz and dextrose were carefully mixed in the desired proportion and then approximately 15 g of the mixture was placed in an SlOe crucible and melted in an electric muffle furnace. An argon atmosphere under normal conditions was present during the preparation of glasses with a > 0.5

The glass formation and crystallization studies on iron phosphate–silicate glasses

“…Thermal characteristics of phosphate glasses from the simple P 2 O 5 -Fe 2 O 3 system [16] indicate that the crystallization of iron phosphate glasses runs in two stages, and the Fe 2? /Fe 3?…”

Vitrified metal finishing wastes

“…The major DTA peak at 635-675 • C may be due to the formation of ferrous/ferric pyrophosphate, Fe 3 (P 2 O 7 ) 2 or Fe 2+ Fe 3+ 2 (P 2 O 7 ) 2 [14,25], or to similar Q 0 , Q 1 and Q 2 crystalline species (where Q = number of bridging oxygens bonds per PO 4 group) [28]. It may also be due to the formation of FePO 4 , as occurred during heat treatment of P 2 O 5 Fe 2 O 3 Na 2 O glasses in oxygen [45] and P 2 O 5 Fe 2 O 3 glasses in argon [46]. In addition to the chemical composition, the Fe 2+ / Fe redox ratio can influence the temperature at which crystallisation occurs [14,16,25,31,46], and increasing the Fe 2+ / Fe ratio can reduce crystallisation temperature [46].…”

“…It may also be due to the formation of FePO 4 , as occurred during heat treatment of P 2 O 5 Fe 2 O 3 Na 2 O glasses in oxygen [45] and P 2 O 5 Fe 2 O 3 glasses in argon [46]. In addition to the chemical composition, the Fe 2+ / Fe redox ratio can influence the temperature at which crystallisation occurs [14,16,25,31,46], and increasing the Fe 2+ / Fe ratio can reduce crystallisation temperature [46]. It is therefore important, in order to optimise high temperature glass stability, to have a low Fe 2+ / Fe ratio.…”

Effect of controlled crystallization on polaronic transport in phosphate‐based glass‐ceramics

“…The focus of so far published studies on induced crystallization of iron phosphate glasses was mainly on the structural changes which depend on glass composition and crystallization conditions but less on their influence on the electrical transport . Our recent investigations, however, show that the controlled crystallization appears to be a powerful tool for tailoring electrical properties of these materials .…”

“…7,20 The focus of so far published studies on induced crystallization of iron phosphate glasses was mainly on the structural changes which depend on glass composition and crystallization conditions but less on their influence on the electrical transport. 8,[21][22][23][24][25] Our recent investigations, however, show that the controlled crystallization appears to be a powerful tool for tailoring electrical properties of these materials. 26,27 For instance, our study on the 10ZnO-30Fe 2 O 3 -60P 2 O 5 (mol%) glass system 26 shows that in the initial stage of crystallization, the electrical conductivity decreases, whereas with further crystallization progress it rises as a consequence of an increase in the size of crystal grains and their different distribution within the glass matrix.…”