Gel synthesis and crystallization of potassium tetragermanate glass powders

Gel synthesis and crystallization of potassium tetragermanate glass powders

Journal of Non-Crystalline Solids 249 (1999) 17±22 Gel synthesis and crystallization of potassium tetragermanate ...

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Journal of Non-Crystalline Solids 249 (1999) 17±22

Gel synthesis and crystallization of potassium tetragermanate glass powders M. Catauro, G. Laudisio, A. Marotta


Department of Materials and Production Engineering, University Federico II, Piazzale Tecchio, 80125 Naples, Italy Received 20 August 1998; received in revised form 31 March 1999

Abstract Potassium tetragermanate gel (K2 O 4GeO2 ) was synthesized by hydrolytic polycondensation of germanium ethoxide with potassium ethoxide in alcoholic medium. The crystallization behavior of the gel, examined by di€erential thermal analysis and X-ray di€raction, is reported and discussed. Potassium tetragermanate gel crystallizes in two steps. At about 593°C, metastable K4 Ge9 O20 crystals are formed as the major crystalline phase with a small amount of K2 Ge4 O9 crystals. At higher temperatures, metastable K4 Ge9 O20 are converted to the thermodynamically stable K2 Ge4 O9 phase. These results are compared with those of conventionally prepared glass of the same composition. Ó 1999 Elsevier Science B.V. All rights reserved.

1. Introduction The sol±gel method for making inorganic glass has been intensively studied in recent years [1]. Interest in this process has been stimulated, in part, by the low preparation temperature. The preparation involves hydrolysis and polycondensation of organometallic compounds. A gel forms which is dried to a porous particulate material. Proper thermal treatments are, therefore, required to convert the gel into the glass. The ionic size and the ionic charge of Ge4‡ are very similar to that of Si4‡ and, therefore, the chemistry of silicates and germanates somewhat resemble each other. However, the ionic radius of  is very close to the cation/anion Ge4‡ (0.53 A) radius ratio limit that separates tetrahedral and

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octahedral coordination and, therefore, Ge4‡ can assume both fourfold and sixfold coordination. Investigations of the binary alkali germanate gel preparation and their crystallization behavior are of interest for elucidating the nature of the crystallizing phases and for guiding research on glass ceramic materials. The work reported is part of a more general study with the objective of determining the suitability and advantages of gels as starting materials for the preparation of glass ceramics. In previous papers, lithium tetragermanate and sodium tetragermanate gel glasses were synthesized by hydrolytic polycondensation of germanium ethoxide in alcohol with lithium hydroxide monohydrate [2] and sodium ethoxide [3], respectively. The lithium tetragermanate gel was found to crystallize in two steps. In the primary transformation, at about 560°C, microcrystallites of Li2 Ge4 O9 and GeO2 are crystallized in an amorphous matrix. In the second transformation, at about 600°C, Li2 Ge4 O9

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M. Catauro et al. / Journal of Non-Crystalline Solids 249 (1999) 17±22

and Li2 Ge7 O15 crystals are formed. Sodium tetragermanate gel is also crystallized in two steps. Metastable Na2 Ge4 O9 crystals were formed initially and were then converted at a higher temperature into stable Na4 Ge9 O20 crystals. In this work, a potassium tetragermanate gel (K2 O 4GeO2 ) was prepared and its crystallization behavior studied with the aid of X-ray di€raction and di€erential thermal analysis. This composition was chosen because the crystallization of a potassium tetragermanate oxide glass had previously been studied [4] and the results obtained for the gel could be compared with those obtained for the conventional glass. 2. Experimental procedure A potassium tetragermanate gel was prepared using Ge(C2 H5 O)4 (TEOG) and K(C2 H5 O) (KOEt) analytical grade reagents as starting materials. Water-free ethanol (EtOH), obtained by the distillation of commercial anhydrous ethanol with metallic sodium was used, since the TEOG is very water sensitive. Bidistilled water was used for the hydrolysis reaction. The alcoholic solutions were prepared in a dry box at room temperature. A ¯ow chart indicating the preparation procedure and the compositions employed is given in Fig. 1. The alcoholic solution of TEOG was mixed at 0°C with a water±alcohol solution of potassium ethoxide. Under these conditions, complete gelation occurred at room temperature in two days. The gel obtained appeared opaque and homogeneous. The gelled system was held for one day longer at room temperature before drying. The gel was fully dried in air at 50°C for one day. After these treatments, an amorphous powder was obtained. The potassium content in the dried gel was veri®ed using atomic absorption analysis. Samples of dried gel were placed in a platinum holder which was introduced into an electric oven at 1200°C in air. After 3 h, at this temperature, the melted gel was air quenched, weighed, dissolved in concentrated HF and diluted in bidistilled water. The nature and temperatures of the various reactions that occur during the heating of the dried gel were determined using simultaneous thermo-

Fig. 1. Flow chart of gel synthesis.

gravimetric (TGA) and di€erential thermal analysis (DTA). A 20 mg powder sample of the dried gel was subjected to a TGA/DTA run in N2 at a heating rate of 10°C/min from room temperature to 850°C. Powdered Al2 O3 was used as the reference material. The amorphous nature of the dried gel and identi®cation of the phases crystallizing during the DTA runs were determined by X-ray di€raction (XRD). Powders of each sample were scanned from 2h ˆ 5° to 60° using CuKa radiation. Non-isothermal crystallization of dried gel was examined by DTA. Powder samples of 40 mg of the dried gel were subjected to DTA runs in He at heating rates of 2, 5, 10, 20°C/min from room temperature to 800°C. A high temperature thermoanalyzer was used with powdered Al2 O3 as the reference material. Fourier transform infrared (FTIR) transmittance spectra were recorded in the 400±1200 cmÿ1 region using a system equipped with a DTGS KBr (deuterated triglycine sulfate with potassium bromide windows) detector. A spectral resolution of

M. Catauro et al. / Journal of Non-Crystalline Solids 249 (1999) 17±22

2 cmÿ1 was chosen. Each test sample was mixed with KBr (1 wt% of gel) in an agate mortar, and then was pressed into 200 mg pellets of 13 mm diameter. The spectrum for each sample represents an average of 20 scans, which were normalized to the spectrum of the blank KBr pellet. The FTIR spectra have been analyzed by a Mattson software package (FIRST Macros). 3. Results Chemical analysis of the melted gel showed that good composition control was provided by the sol±gel process (Table 1). The analyzed content and the theoretical value are in fairly good agreement. Fig. 2 shows the TGA and DTA curves of the dried gel. A large endothermic peak from room temperature to about 150°C, appears on the DTA curve, with a maximum at about 90°C, and a simultaneous weight loss occurs in the TGA curve. The weight loss was 12.5%. These e€ects were due to evaporation of the water and alcohol physically trapped in the gel from open pores. No appreciable e€ects were observed on TGA and DTA curves


in the range of 250±500°C. That is due to the absence of organic substances which can be produced in sol±gel processing [1±6]. The DTA curve of the gel exhibits a slope change that may be attributed to the glass transition. In this work, the in¯ection point of the DTA curve was taken as the glass transition temperature (Tg ˆ 556 ‹ 2°C). A high and sharp exothermic peak appears, just above the Tg , on the DTA curve at the temperature of 593°C. To convert the gel into glass, the dried gel samples were held for 3 h at 450°C. The FTIR transmittance spectra obtained from the gel-glass and the oxide-glass shown in Fig. 3 exhibit broad bands as expected for the glassy system. In both glasses, the highest absorption band lies at about 800 cmÿ1 .

Table 1 Chemical analysis of melted gel Constituent

Analyzed, wt%

Theoretical, wt%

K2 O GeO2

18.06 81.94

18.38 81.62

Fig. 2. TGA and DTA curves of the dry gel recorded in N2 at 10°C/min.

Fig. 3. FTIR transmittance spectra of: (a) oxide glass; (b) gel glass and (c) GeO2 glass.


M. Catauro et al. / Journal of Non-Crystalline Solids 249 (1999) 17±22

±Ge±OEt ‡ HO±Ge ) ±Ge±O±Ge± ‡ HOEt

…dealcoholation†; …2†

±Ge±OH ‡ HO±Ge ) ±Ge±O±Ge ‡ H±OH



±KOEt ‡ Ge±OH ) Ge±Oÿ ‡ K‡ ‡ C2 H5 ±OH: …4†

Fig. 4. Powder XRD patterns for: (a) a sample of gel dried for 3 h at 450°C; (b) a sample of dried gel heated in DTA furnace up to 593°C and (c) a sample of the dried gel after a DTA run from room temperature to 800°C.

Fig. 4 shows the X-ray di€raction patterns of samples of the dried gel treated under di€erent conditions: (a) dried gel held 3 h at 450°C; (b) dried gel heated in the DSC furnace up the temperature of the exo-peak (593°C) and (c) dried gel after a DSC run carried out from room temperature to 800°C. Trace (a) has broad peaks characteristic of the amorphous state of the gel. The di€ractogram (b) exhibits sharp lines. The major crystalline phase was found to be K4 Ge9 O20 . In addition to the peaks due to K4 Ge9 O20 , three very small peaks (marked in Fig. 4) corresponding to the strongest re¯ections for K2 Ge4 O9 were also detected on this pattern. No re¯ections of K4 Ge9 O20 were found on the di€ractogram (c), all peaks could be assigned to K2 Ge4 O9 .

4. Discussion Gelation is the result of hydrolysis and condensation reactions according to the following equations: ±Ge±OEt ‡ H±OH ) ±Ge±OH ‡ H±OEt



At room temperature the hydrolysis reaction (1) is much faster than the condensation reaction (Eqs. (2) and (3)) so that the number of Ge±O±Ge bridges formed are insucient to give gelation and precipitation of hydrated germanium oxide±alkoxide aggregate. Mixing at a lower temperature (0°C) allows: (a) control of the hydrolytic reactivity of TEOG so that soluble polymeric intermediates are obtained, which then undergo further polymerization to form a gel; and (b) the introduction of the modi®er cation, K‡ , into the germanate network according to Eq. (4). The above reactions can be catalyzed by acids or bases. The reaction mechanisms are not known in detail, however, it is generally accepted that they proceed through a second order nucleophilic substitution [5]. The infrared spectra of a number of compositions in the alkali germanate systems show that in each system the highest frequency band at 878 cmÿ1 , due to Ge±O±Ge bond stretching, shifts to lower frequencies. The shift is related to the change in the coordination number of Ge from 4 to 6. The higher the alkali concentration the greater is the shift [7]. The infrared spectra of the potassium tetragermanate gel glass and oxide glass exhibit the same shift. These results suggest a similar structure in the gel and oxide glass. In Fig. 5 the DTA curve of a sample of the gel glass powders is compared with those of samples of bulk and powdered glass prepared by the conventional method. All the curves exhibit a slope change followed by a sharp exothermic peak nearly at the same temperature. There are two types of crystallization which take place in a glass, based on the surface and bulk nucleation. The number of nuclei, N, is the sum of surface nuclei

M. Catauro et al. / Journal of Non-Crystalline Solids 249 (1999) 17±22


The structure of K4 Ge9 O20 contains chains of GeO4 tetrahedra connected by Ge4 O6 groups which consist of edge-shared GeO6 octahedra [8]. The structure of K2 Ge4 O9 contains isolated GeO6 octahedra by Ge3 O9 rings consisting of three GeO4 tetrahedra to form a three dimensional network [7]. The XRD results indicate a devitri®cation mechanism in two steps. In the primary transformation, metastable K4 Ge9 O20 crystals are formed, which are then converted at higher temperatures into thermodynamically stable K2 Ge4 O9 crystals. If more than one crystal is precipitated from a glass the structure of the metastable phase can be expected to be more like the original glass than the stable one. The coalescence of GeO6 octahedra by shearing edges with Ge4 O16 groups must already be present in the investigated glass. The activation energy, E, for crystallization was evaluated from the DTA curves using the following equation [11]: lnb ˆ ÿ…E=R†…1=Tp † ‡ const:

Fig. 5. DTA curves recorded in He at 10°C/min of: (a) gel glass; (b) bulk oxide glass and (c) powder oxide glass.

proportional to the speci®c surface area of the sample and bulk nuclei formed during the heat treatments of the samples. The higher the value of N, the lower the temperature, Tp , of the DTA crystallization peak [9]. The shape of the crystallization peak is strongly a€ected by the crystallization mechanism [10], to surface and bulk crystallization corresponding to broad and sharp peaks, respectively. It can be observed, in Fig. 5, that the DTA crystallization peaks of powdered (high speci®c surface area) samples of gel glass and oxide glass have the same sharp shape and occur at nearly the same temperature as the DTA crystallization peak of the bulk (low speci®c surface area) samples of the oxide glass. Taking into account the great increase of the number of surface nuclei, due to the high speci®c surface area of gel and oxide glass powders, this suggests a dominant bulk crystallization in both gel and oxide glasses.

This equation is based on the shift of the temperature, Tp , of the DTA crystallization peak as DTA heating rate, b, is changed. Multiple DTA runs were recorded in He at di€erent heating rates on the gel and oxide glasses. Plots of lnb against 1/Tp , shown in Fig. 6 gave straight lines in both cases. The value of the activation energy of crystallization was calculated from their slopes. Taking into account an experimental error of about ‹10%, the value E ˆ 585 kJ molÿ1 for the gel glass is very

Fig. 6. Plot of lnb against 1/Tp : (n) gel glass; (d) oxide glass.


M. Catauro et al. / Journal of Non-Crystalline Solids 249 (1999) 17±22

close to that of E ˆ 558 kJ molÿ1 for the oxide glass.

5. Conclusions The gel preparation involves hydrolysis and polycondensation of germanium ethoxide with potassium ethoxide. The gel thus prepared is an amorphous solid containing residues that are lost on heating. Processing temperatures are well below the glass transition temperature of the glass being formed and therefore, during the heat treatment required for the gel into glass conversion, the gel is kinetically stable to crystallization. This suggests the possibility of converting the porous particulate gel-glass into a monolithic dense glass by sintering or hot pressing techniques without devitri®cation. The potassium tetragermanate gel heated at constant heating rate during a DTA run, crystallizes as the oxide glass of the same composition, in two steps. Metastable K4 Ge9 O20 crystals are formed

initially and are then converted at higher temperature into stable K2 Ge4 O9 crystals. References [1] C.J. Brinker, G.W. Scherer, Sol±Gel Science, Academic Press, New York, 1990. [2] A. Aronne, M. Catauro, P. Pernice, A. Marotta, J. NonCryst. Solids 152 (1993) 137. [3] A. Aronne, M. Catauro, P. Pernice, A. Marotta, Phys. Chem. Glasses 35 (1994) 160. [4] G. Laudisio, M. Catauro, J. Mater. Sci. Lett. 16 (1997) 1309. [5] C. Sanchez, J. Livage, M. Henry, F. Babonneau, J. NonCryst. Solids 100 (1988) 65. [6] A. Osaka, M. Yuasa, Y. Miura, K. Takahashi, J. NonCryst. Solids 100 (1988) 409. [7] M.K. Murthy, Aguayo, Phys. Chem. Glasses 5 (1964) 144. [8] H. Verweij, J.H.J.M. Buster, J. Non-Cryst. Solids 34 (1979) 81. [9] A. Marotta, A. Buri, F. Branda, J. Mater. Sci. 16 (1981) 341. [10] A. Marotta, A. Buri, F. Branda, Thermochim. Acta. 40 (1980) 1397. [11] T. Ozawa, Polymer 12 (1971) 150.