Georgios D. Chryssikos

Infrared reflectance spectra of lithium borate glasses

Abstract The infrared reflectance spectra of lithium borate glasses (0–73 mol% Li 2 O) have been measured over a broad and continuous spectral range (30–4000 cm −1 ) in an attempt to study systematically their structure. The reflectance data have been analyzed by Kramers-Kronig inversion to obtain the optical and dielectric properties of the materials. The study of the deconvoluted mid-infrared spectra has revealed a glass network built up of various boron-oxygen groups, with structure and concentration strongly dependent on the Li 2 O content. Of particular interest was the far-infrared asymmetric absorption profile, which could be well simulated by two broad bands attributed to vibrations of lithium cations in two distinct network environments. The presence of these two Li + -host site distributions was found to be independent of the details of the borate network structure and has been taken to imply a more general characteristics of ionic conducting oxide glasses.

Time domain reflection methods for dielectric measurements to 10 GHz

Total reflection methods and instrumentation for their use are described for measurements of dielectric permittivity and loss at frequencies to 10 GHz or more. Several cell designs are shown, together with analyses of their performance. Procedures are given for correcting effects of wave propagation in the cells and residual reflections in the cells by bilinear analysis with calibrations using dielectrics of known permittivity. Representative results are presented for highly polar liquids, dilute solutions of polar molecules in nonpolar solvents, electrolyte solutions, and ionic glasses with appreciable ohmic conduction.

Borate glass structure by Raman and infrared spectroscopies

Abstract Raman and infrared spectroscopies have been employed to probe the continuous evolution of borate glass structures as a function of the nature and concentration of alkali oxide modifier. At relatively low alkali contents, the glasses contain covalent networks consisting of interconnected units bearing BO − 4 tetrahedra. Further addition of alkali oxide causes the progressive depolymerization of the network as a result of the formation of non-bridging oxygen atoms. Eventually, complete network disruption into small and highly charged borate units results in “ionic” glasses. The nature of the alkali cation determines the structure of these glasses by affecting several high- temperature isomerization or disproportionation equilibria.

Infrared reflectance investigation of alkali diborate glasses

Alkali diborate glasses have been investigated by infrared reflectance spectroscopy to demonstrate the potential of this technique for a quantitative description of the glass structure. Kramers-Kronig transformation was performed on the reflectance data to obtain the optical and dielectric properties of glasses over a broad and continuous spectral range (30–4000 cm−1). Analysis of the mind infrared spectra shows that the normalized absorption area attributed to boron-oxygen tetrahedra decreases upon increasing alkali cation ionic radius, and scales linearly with the fraction of four-coordinated boron atoms determined recently from NMR spectra. The far infrared asymmetric absorption profiles were deconvoluted into two broad bands and attributed to vibrations of alkali cations in two distinct network environments. Cation effective charges were calculated from the integrated absorption of the bands due to cation vibrations, and were found to increase with alkali ionic radius.

The devitrification of lithium metaborate: polymorphism and glass formation

Abstract The ambient pressure devitrification of glassy lithium metaborate has been studied by Raman and infrared spectroscopies. Of the three different polymorphs obtained, two are identified as α- and γ-LiBO2, while a third one, β′-LiBO2, is thought to consist of alternating borate triangles and tetrahedra respecting the metaborate stoichiometry (BO2-. The network of glassy LiBO2 prepared by melt quenching consists mainly of local arrangements similar to those encountered in all three polymorphs in accordance with the ideas of Krogh-Moe. It is argued that the glass structure exhibits the memory of structural transformations occuring in the vicinity of Tg, and may be described by models implying the existence of structural ‘granularity’.

On the structure of palygorskite by mid- and near-infrared spectroscopy

Abstract The OH-structural characteristics of an iron-rich palygorskite from Western Macedonia, Greece (Gr-1) and an aluminous palygorskite from Florida (PFl-1) were examined by combined Fourier-transform near-infrared reflectance (NIR) and mid-infrared attenuated total reflectance (ATR) spectroscopy. Analyses of samples heated from ambient to 130 °C allowed for the development of a self-consistent set of band assignments for the structural and surface OH and H2O species of both the hydrated and dehydrated forms. The inner octahedral sites of both samples are largely accounted for by dioctahedral AlAlOH, AlFe3+OH, and Fe3+Fe3+OH pairs. Band intensities for these pairs are consistent with variations in the concentration of octahedral Fe and Al in the two samples. In addition, both samples display a trace trioctahedral signature in NIR, which may be related to local trioctahedral domains, or the presence of sepiolite in trace amounts, or as intergrowths. A surface H2O species typical of the hydrated phase was identified via its NIR combination mode at 5317 cm-1. The desorption of this species by heating revealed distinct silanol groups with overtone and combination modes at 7255 and 4575 cm-1, respectively. Mg-coordinated and zeolitic H2O species are strongly coupled in the hydrated phase and give rise to NIR combination modes at 5190 and 5240 cm-1. The removal of zeolitic H2O causes the blue shift of the three dioctahedral OH overtones by ca. 20 cm-1 and the rearrangement of the coordinated H2O manifested by the growth of sharp combination modes at ca. 5215 and 5120 cm-1.

Vibrational investigation of lithium metaborate-metaaluminate glasses and crystals

Abstract Lithium metaborate-metaaluminate glasses xLiAlO2·(1−x)LiBO2, (0≤ x ≤0.42), and crystals (Li3B2AlO6, Li2BAlO4) have been prepared and studied by infrared and Raman spectroscopies. The two crystalline compounds have similar networks made of AlO4− and BO2O− units (O, O: bridging and terminal oxygen atoms, respectively) in arrangements which maximize the number of BOAl linkages. Both compounds decompose at high temperatures into α-LiBO2 and γ-LiAlO2. Increasing x in glass induces the isomerization of BO4− tetrahedra to BO2O− triangles, while aluminum forms AlO4− units. This structural scheme is sufficient to account for the minimum in the glass transition temperature observed for x ≈ 0.25. There is no vibrational evidence for the presence of AlO5 or AlO6 polyhedra in glass, although their presence in small amounts cannot be excluded. Glasses and crystals of the same stoichiometry (x = 0.33) are differentiated by an excess of kinetically arrested BOB and AlOAl linkages in the former.

Alkali sites in glass

In this paper we study the nature of sites occupied by alkali metal ions in oxide and organic glasses by probing the localised metal-site vibrations which are active in the far-infrared. Analysis of the far-infrared profiles of the glasses considered in this investigation indicates the presence of one or two distributions of cation hosting sites. It is argued that the energetics of these sites are determined by both the bonding requirements of the metal cation and the versatility of the anionic glass network to fulfil these requirements. This is demonstrated by comparing earlier data on alkali borates and more recent results on alkali germanates and ionic organic glasses, as well as by studying the effect of the method of glass formation (fast melt quenching, slow cooling, sol–gel) on the far-infrared absorption envelope. The alkali metal sites inferred from far-infrared have been compared to those probed by ultraviolet spectroscopy using Tl+ and Pb2+ ions electrolysed into borate glasses. The results suggest that mobile alkali ions are mainly those occupying sites of high basicity.

Octahedral cation distribution in palygorskite

Abstract The OH speciation of 18 palygorskite samples from various localities were evaluated by near infrared spectroscopy (NIR) and compared to the corresponding octahedral composition derived from independent, single-particle analytical electron microscopy (AEM). NIR gives evidence for dioctahedral-like (AlAlOH, AlFe3+OH, Fe3+Fe3+OH) and trioctahedral-like (Mg3OH) species. Therefore, palygorskite can be approximated by the formula yMg5 Si8O20(OH)2·(1 - y)[xMg2Fe2·(1 - x)Mg2Al2] Si8O20(OH)2, where x is the Fe content of the dioctahedral component, and y is the trioctahedral fraction. The values of x estimated from the NIR data are in excellent agreement with the Fe/(VIAl + Fe) ratio from AEM (R2 = 0.98, σ = 0.03), thus suggesting that all octahedral Al and Fe in palygorskite participate in M2M2OH (dioctahedral-like) arrangements. Furthermore, y values from AEM can be compared to NIR (R2 = 0.90 and σ = 0.05) after calibrating the relative intensity of the Mg3OH vs. (Al,Fe)2OH overtone bands using AEM data. The agreement between the spectroscopic and analytical data are excellent. The data show that Fe3+ for Al substitution varies continuously in the analyzed samples over a broad range (0 < x < 0.7), suggesting that fully ferric dioctahedral palygorskites (x = 1) may exist. On the other hand, the observed upper trioctahedral limit of y = 0.50 calls for the detailed structural comparison of Mg-rich palygorskite with sepiolite.

Use of NIR for structural characterization of urea–formaldehyde resins

Abstract In this paper, the effect of pH and temperature on the structure of urea–formaldehyde resins was studied. GPC, NMR and Raman measurements were performed to elucidate the structural characteristics of the resin systems. Fourier Transform Near Infrared (FT-NIR) spectroscopy via optical fibers was used to monitor the reaction progress in situ. It was found that the reactions of urea and formaldehyde at different temperatures and pH values result in resins with different structures and properties: Resins produced at high temperatures and acidic pH values exhibit higher degrees of condensation, presumably because of the development of more cross-linked structures.