Facile Synthesis of Highly Ordered Mesoporous and Well Crystalline

Article pubs.acs.org/cm

Facile Synthesis of Highly Ordered Mesoporous and Well Crystalline TiO2: Impact of Different Gas Atmosphere and Calcination Temperatures on Structural Properties Lars Robben,*,†,∇ Adel A. Ismail,‡,§,∥ Sven Jare Lohmeier,⊥ Armin Feldhoff,# Detlef W. Bahnemann,∥ and Josef-Christian Buhl† †

Leibniz University Hannover, Institute for Mineralogy, Callinstrasse 3, 30167 Hannover, Germany Advanced Materials Department, Central Metallurgical R&D Institute (CMRDI), P.O. Box 87, Helwan, Cairo 11421, Egypt § Centre for Advanced Materials and Nanoengineering (CAMNE), Narjan University, P.O. Box 1988, Narjan 11001, Saudi Arabia ∥ Leibniz University Hannover, Institute for Technical Chemistry, Callinstrasse 3, 30167 Hannover, Germany ⊥ Leibniz University Hannover, Institute for Inorganic Chemistry, Callinstrasse 9, 30167 Hannover, Germany # Leibniz University Hannover, Institute of Physical Chemistry and Electrochemistry, Callinstrasse 3a, 30167 Hannover, Germany ‡

S Supporting Information *

ABSTRACT: Ordered mesoporous TiO2 nanoparticles with amorphous pore walls were synthesized by a facile one pot template synthesis. The samples were heat treated at different temperatures under oxidizing (air), inert (N2), and reducing (N2/H2) conditions before removing the template under air atmosphere at 350 °C. The use of N2/H2 or N2 atmospheres can effectively reduce the crystallite size by 2 nm compared to the calcination in air, leading to a better ordering of the mesoporous system than in the case of the calcination under air. The phase analysis of the powder diffraction data shows that the anatase content remains constant with temperature and is fed by an amorphous precursor and consumed by rutile when it is formed.

KEYWORDS: ordered mesoporous material, TiO2, phase relationships, anatase, rutile

1. INTRODUCTION Since the first description of an ordered mesoporous molecular sieve by Kresge et al.1 ordered mesoporous materials (OMMs) have attracted a lot of research efforts due to their potentials for a broad range of technical applications like water and air purification.2−5 The research in the recent years has shown that the synthesis of a broad variety of pore systems is possible by often very simple synthesis pathways; also the range of pore wall materials has been extended from SiO2 to other metal oxides (e.g., Ti, Fe, Al).5−7 A typical simple synthesis pathway for OMMs is a template synthesis (Liquid Crystal Templating8) in which the type of template defines the pore arrangement and size. Here different approaches like evaporation induced self-assembly (EISA) or synthesis with regularly packed monodispersive polymethyl methacrylate (PMMA) microspheres9 are feasible. For the EISA approach important factors described in the literature are the hydrolysis ratio, type of the solvent, concentration of the surfactant, and the titanium source.10 After synthesis the template has to be removed from the sample to make the pores accessible. This can be achieved by thermal treatment (calcination) or solvents.11 In most cases OMM-TiO2 materials are © 2012 American Chemical Society

heat treated, during this step the interplay between pore wall material and the pore system is very important to achieve the desired properties of the compounds. The most important factor is the choice of the calcination temperature, which determines the final degree of order of the OMM and must be chosen in such a way that it is high enough to remove the template completely but low enough so that crystallization processes in the pore wall material are not destroying the pores. Several approaches to overcome these limitations have been proposed (e.g., the strengthening of the pores by in situ formed carbon12 or approaches like the ″brick and mortar″ strategy13). With regard to the applicability of ordered mesoporous materials (OMM) especially TiO2 based OMMs are promising materials for technical applications due to the photocatalytic properties of the anatase TiO2 modification.2 It was shown that the nanostructuring of TiO2 is able to enhance the photocatalytic efficiency significantly compared to commercially available products (e.g., Degussa P2514,15). Received: October 25, 2011 Revised: March 2, 2012 Published: March 6, 2012 1268

dx.doi.org/10.1021/cm203203b | Chem. Mater. 2012, 24, 1268−1275

Chemistry of Materials

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6.7 6.4 6.4 6.0 6.15 6.884 6.654 1581 3.36 100

4767 3.26 103

3930 3.17 78

3.60 102

300 0 0

3.46 107

1682

3.08 79

5360

13 12.9 12.5 12.2 12.2 12.0 11.7 13.2 12.5 12.4 12.3 12.1 12.1 4605 0 0 0 3.14 3.85 4.39 5.84 84 105 117 117

232.30 143.70 157.50 227.50 181.70 167.90 212.00 184.80 170.90 0.3635 0.2533 0.2624 0.3448 0.2958 0.3009 0.3015 0.2869 0.2690 6.324 5.861 5.337 5.863 5.853 6.300 6.350 5.416 5.446 0 0 0 0 0 0 4.3 5.0 6.8 7.7 10.3 12.2 0 0 0 0 9.3 7.7 1.7 2.1 0 0 1.3 1.9 2.2 2.5 2.5 2.9 2.7 2.0 1.6 1.9 1.8 1.8 1.8 2.9 7.0 9.6 10.2 16.1 0 6.9 6.3 7.3 7.5 8.0 8.4 8.8 5.5 6.8 6.0 6.6 6.7 7.3 0 0 0 0 0 0 16 21 33 38 52 45 0 0 0 0 5 37 71 24 0 0 100 67 53 46 37 30 21 31 87 68 58 56 48 30 29 76 100 100 0 33 31 33 30 32 27 24 13 32 42 44 47 33 + + + + + + + 350 450 500 600 350 350 450 450 500 500 600 600 350 350 450 450 500 500 N2/H2

The block copolymer surfactants EO106-PO70EO106 (F-127, EO = −CH2CH2O-,PO = −CH2(CH3)CHO−), MW 12600 g/mol) from Sigma, tetrabutyl orthotitanate TBOT), Ti(OC(CH3)3)4, HCl,

N2

Table 1

2. EXPERIMENTAL SECTION

gas

air T (°C) treat-ment

anatase content (wt.-%)

APM content (wt.-%)

rutile content (wt.-%)

LvolIB LvolIB Anatase (nm) APM (nm)

LvolIB Rutile (nm)

pore size (nm)

total PV (cc/g)

pore area (m2/g)

n

radius of gyration (nm)

Bragg intensity (counts)

a (nm)

The processes taking place in the OMMs during calcination are well-known qualitatively. The higher the calcination temperature the higher is the crystallinity of the material and the lower the ordering and homogeneity of the pores. An approach to quantify these effects, and especially the X-ray scattering of the disordered pores, is given in a recent paper.16 The synthesis of OMM TiO2 materials with a defined pore wall material and well developed mesopores is not easily achievable due to the many factors influencing the crystallization of the TiO2 modifications. In the synthesis of bulk samples of TiO2 the phase relationships between brookite, anatase, and rutile are naturally influenced by temperature17 and pressure.18 The presence of different other materials in the sample (like metal cations or oxides19−21) is able to shift the transition temperature for the anatase-rutile phase transition drastically as well as the presence of different gas atmosphere during synthesis.22 The mechanisms for the aforementioned influences on the phase relationship are based mainly on defects in the TiO2 structure, either on the Ti-sites, by incorporation of other atoms, or in the form of oxygen vacancies, which play an important role if the synthesis or crystallization is carried out under reducing conditions.22 When synthesizing nanosized TiO2 particles the mean particle size is important for the phase relationships: rutile is the thermodynamically favorable phase in bulk materials, but as soon as the particle size is below 10− 15 nm anatase is more stable.23−25 Furthermore the presence of brookite is able to enhance the anatase - rutile phase transition in nanoparticles.26 As the size of the synthesized particles reaches the nanoscale the energies of the surfaces become more and more important in relation to the bulk energies and start to determine the materials’ properties.27−29 Ranade et al.30 give a short literature review about the possible transformations in the TiO2 system. Kirsch et al.31 have examined the crystallization behavior of anatase in mesoporous titania films and observed that in experiments carried out at different temperatures (400− 450 °C) the crystallization of anatase from the amorphous precursor starts rapidly and the crystallites are growing within a short time up to an upper limit; at 450 °C they observed that the crystallite size achieved 90% of its final value after 26 min. However the complex interdependencies in the crystallization processes of TiO2 in OMMs between phase stability and crystallite size (and thus the domination of surface energies) in the pore wall material make a simple prediction of the behavior of the system difficult. The key factor for a complete understanding of the synthetic processes in these materials and the ordering of the pore system is to understand the crystallization of the different TiO2 modifications and their equilibriums in nanosized structured particles. Our approach here is to use different gas atmospheres during the calcination to influence the crystallization of the TiO2 phases. This can have an impact on the mean crystallite size of the TiO2 phases and by this on the mesoporous system. We show that the application of different gas atmospheres during calcination is a simple method to achieve a desired composition of different TiO2 modifications. The processes taking place during crystallization are examined, and it is shown that by applying inert or reducing atmospheres the mesoporous system can retain a higher degree of order compared to calcination in air.

air

pore wall thickness (nm)

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Figure 1. Phase content in the samples as determined by Rietveld refinements of the powder patterns (The error of the calculation is approximately 5-wt.-%). C2H5OH, and CH3COOH were purchased from Sigma-Aldrich. The highly ordered TiO2 nanoparticles were synthesized through a simple one-step sol−gel process in the presence of F127 triblock copolymer as structure directing agent. The molar ratio of each reagent in the starting solution was fixed at TiO2/F127/C2H5OH/HCl/CH3COOH = 1:0.02:50:2:4 molar ratios. 1.6 g of F127, 2.3 mL of CH3COOH, and 0.74 mL of HCl were dissolved in 30 mL of ethanol and then added to 3.5 mL of TBOT. The mixture was stirred vigorously for 1 h and transferred into a Petri dish (diameter 125 mm). The ethanol was evaporated at 40 °C with a relative humidity 40% for 24 h, a transparent TiO2 nanocomposite was formed and was transferred into a 65 °C oven and aged for an additional 24 h. The as-made mesostructured hybrids were calcined under different gas atmospheres and temperatures according to Table 1. The heat treatment under N2 and N2/H2 (5% H2) atmospheres have been carried out in a Nabertherm tube furnace (R50/500/12) with temperature controller B170. Because the template was not removed under nonoxidizing atmospheres all these samples have been additionally heat treated under air at 350 °C. Heating rates were for all samples 1 °C/min with 4 h holding time at the target temperature and cooling rate of 2 °C/min. Small angle scattering data has been collected with a Bruker AXS D8 Advance diffractometer in transmission geometry. The machine is equipped with a Goebel mirror, fixed slits, and a secondary Ni-filter. Measurements were carried out with Cukα1,kα2 radiation from 0.15° to 5° 2Θ with a step width of 0.02° and 5 s measurement time per step. To ensure comparability between the measurements the samples were prepared on adhesive film, so that all samples contained an equal sample volume. The sample holder material was measured separately without sample material. This measurement was subtracted from the measurements of the samples giving just the scattering contribution of the sample material. Further data evaluation was carried out as described in a previous work16 for the evaluation of the contribution of statistically distributed pores to the scattering. X-ray powder diffraction data of the samples has been collected with a Bruker AXS D4 Endeavor diffractometer. The machine is equipped with fixed slits and a secondary Ni-Filter. Measurements were carried out with Cukα1,kα2 radiation from 20° to 80° 2Θ with a step width of 0.02° and 2 s measurement time per step. The obtained data were analyzed with the software TOPAS V4.2 (Bruker AXS). Here the Rietveld method was used;32 experimental settings were considered by the Fundamental Parameter Approach incorporated in the program. Structure data were

taken from ICSD database (anatase [9854], rutile [62679], and brookite [36410]). In the refinement one background parameter, the zero point error and the lattice parameters of the respective phases were refined. The crystallite sizes have been calculated as the integral breadth of the Lorentzian scattering volume by convolution of the reflections with Gauss and Lorentz based broadening functions. Nitrogen adsorption/desorption measurements were performed at −196 °C with an Autosorb-3 instrument (Quantachrome Instruments, Boynton Beach, Florida). The analysis station was equipped with highprecision pressure transducers to ensure a highly accurate determination of the adsorbed amount. During the equilibration time the sample cell was isolated to minimize the effective dead volume. Measurements were performed in the relative pressure range from 0.03 to 0.99. The samples were outgassed at 200 °C for 24 h. The classic relative pressure range (p/p0 = 0.05 to 0.30) was chosen to determine the specific BET surface area SBET. The desorption isotherms were used to calculate the pore size distributions with BJH (Barrett, Joyner, Halenda) method. Raman spectra were measured with a Bruker Optics IFS66v/s FTIR spectrometer with FRA-106 Raman attachment. For each spectrum 32 scans of the undiluted sample in backscattering geometry were measured in the range from 0 to 1000 cm−1 with a spectral resolution of 2 cm−1. Transmission electron microscopy (TEM) was conducted at 200 kV with a JEOL JEM-2100F-UHR field emission instrument equipped with a Gatan GIF 2001 energy filter and a 1k-CCD camera. Highresolution TEM (HRTEM) mode, selected area electron diffraction (SAED), and scanning TEM (STEM) mode were applied.

3. RESULTS AND DISCUSSION 3.1. Structural Development of the TiO2 Pore Wall Material. The starting material (as made) for all experiments is a mesoporous TiO2 material with amorphous pore walls. The XRD pattern is shown in the Supporting Information Figure 1 along with the powder patterns of the template material (F127) and the samples calcined in air. The powder pattern of the as made material shows only scattering of amorphous material and a Bragg-reflection of the template. The increasing crystalline quality of the pore wall material with increasing calcination temperature can be clearly seen by the increasing intensity of 1270



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Figure 2. Mean crystallite sizes determined as the Lorentzian scattering volume by the integral breadth of the broadening functions in the Rietveld refinements.

Figure 3. SAXS pattern of the samples temperatures (black: 350 °C; red: 450 °C; different gas atmospheres (circles:air; triangles down: N2/H2) after subsequent calcination at shifted vertically for the sake of clarity).

Figure 4. Development of the lattice parameter a of the mesopore’s hexagonal lattice. Here additional samples are considered which have been treated at temperatures 150 and 200 °C which are well below the temperature at which the template is removed but high enough to remove the ethanol of the samples.

treated at different green: 500 °C) with up: nitrogen; triangles 350 °C in air (graphs

material are in the same positions in the XRD measurements as the main reflections of brookite. This material, designated as amorphous precursor material (APM) from now on, shows only a low crystallite size of ≈2 nm whereas anatase is for low temperatures the minor component but with a much higher crystallite size. For temperatures over 500 °C, APM has completely transformed to anatase, and only these crystallites are growing to a size of ≈16 nm. The inert gas atmosphere (N2) is suppressing the development of anatase at 350 °C; only brookite is crystallizing with a similar low crystallite size of 2 nm as under oxidizing conditions, which remains constant with increasing temperature. In contrast to the experiments under air, rutile is crystallizing besides anatase for temperatures higher than 450 °C. The APM content declines constantly with temperature, whereas the

the main reflection of anatase at 25° 2Θ. XRD-patterns of samples heat treated under N2 and N2/H2 atmospheres are shown in Supporting Information Figures 2 and 3, respectively. A more detailed analysis of this data was carried out by Rietveld refinements, and the results will now be described with respect to the different phases (Figure 1) and their crystallite sizes (Figure 2). Under the influence of an oxidizing atmosphere (air) the first phases developing are an amorphous TiO2 material and anatase. The amorphous TiO2 material was included in the refinements by using the crystal structure data of brookite, because the main intensity contributions of the amorphous 1271



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Figure 5. Nitrogen adsorption/desorption isotherms measured at −196 °C and the pore size distributions calculated from desorption isotherms by using the BJH method.

anatase content remains fairly constant from 450 °C on. This implies a sequence of transition for the single phases. APM transforms to anatase, which itself transforms to rutile, so there is equilibrium between the phases leading to a constant anatase content fed by APM decomposition and consumed by rutile crystallization. The subsequent heating of the samples in air for template removal leads only to minor changes in the phase composition and crystallite sizes. The reductive N 2 /H2 atmosphere shows similar effects than the inert one. The crystallization of rutile is shifted to higher temperatures and it is emerging at 500 °C. APM is of constant low crystallinity and transforms to anatase with a considerably higher crystallite size. The subsequent heating in air is increasing the anatase content and for the sample containing rutile increasing its content by 30 wt.-%. These results can be summarized as follows: The first phase, which crystallizes out of the amorphous raw material, is APM, which shows independently from the temperature a low crystallite size. This APM transforms to anatase which itself forms rutile. The temperature at which the formation of rutile begins depends on the gas atmosphere and follows the sequence N2 (450 °C) < N2/H2 (500 °C) < Air (>600 °C). The results of the XRD phase analysis are confirmed by Raman spectra shown in Supporting Information Figure 4. The samples calcined under air show increasing anatase content with increasing temperature, whereas the samples calcined under N2 or N2/H2 atmosphere show mainly rutile in the spectra of the samples calcined at high temperature. 3.2. Structural Development of the Mesoporous System. The SAXS patterns of the samples after subsequent calcination in air are given in Figure 3. Numerical results of the fitting of the SAXS patterns according to the procedure described in a recent paper16 are given in Table 1. The samples calcined in air show only for 350 °C a Bragg-reflection; all higher calcination temperatures lead to a destruction of the mesoporous scattering. For the inert gas atmosphere the Bragg reflection is observable at 450 °C and as a small shoulder at

500 °C. In the reducing atmosphere the Bragg reflection gains intensity from 350 to 450 °C, which indicates that the template reacts with the TiO2 in the N2/H2- atmosphere at 350 °C so that it remains in at least a part of the pores even after calcination. The total scattering of such mesoporous compounds can be described by a combination of scattering contributions of ordered and disordered pores.16 The first contribution can be measured by the intensity of the Bragg-reflection and the second one by the exponential decay at the lowest scattering angles. The exponential statistical scattering becomes more intense with increasing temperature and follows the sequence oxidizing > inert > reducing atmosphere. Accordingly, the radius of gyration and n, describing the shape of the statistically scattering pores and their number respectively, are significantly smaller for the samples treated in the reducing and inert atmospheres. The most important structural parameter of the porous system is its lattice parameter calculated on the basis of the scattering angle of the main Bragg-reflection. The initial hexagonal lattice parameter a is 21.5 nm for the as-made sample. The development of the lattice parameter in dependence of the temperature of further heat treatment is shown in Figure 4. In this graph measurements of samples are considered which have been treated at 150 and 200 °C. These temperatures are sufficient to remove the ethanol from the samples but not high enough for complete template removal. Additionally, only the samples without subsequent calcination in air are considered. The mesoporous lattice is shrinking independently from the gas atmosphere exponentially with temperature to a value of 12 nm. Thus, if the atmosphere is oxidizing, inert or reducing has no influence on the lattice shrinkage. Nitrogen adsorption isotherms and the determined pore sizes are shown in Figure 5. All samples show typical reversible type IV adsorption isotherms. The samples calcined in N2 and H2/N2 atmospheres show similar behavior regarding the total pore volume which is slightly higher than the one of the 1272



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Figure 6. TEM micrographs (left: TEM bright-field (except g: STEM dark-field), middle: HRTEM, right: selected area electron diffraction (SAED)) of the samples treated at 450 °C (upper row: air, middle row: N2, lower row: N2/H2).

samples calcined in air. The pore diameters show narrow distribution of the pore sizes around 6 nm. One peculiarity can be observed in the measurements of the samples heat treated at 350 °C, which show a measurement artifact at approximately 3.9 nm (cavitation). The sample treated in the reducing atmosphere shows an additional maximum of the pore diameters at 4.5 nm. The SAXS measurements (Figure 3) of this sample show an increase in intensity, indicating an increase in the electron density difference from 350 to 450 °C calcination temperature. It has to be noted that these samples underwent the treatment under air to remove the template. That means that in the sample treated in the reducing atmosphere at 350 °C some reaction between the template and the TiO2 has taken place which modifies the pores. The true nature of this effect remains unclear, but it is conceivable that the template induces chemical reactions in the outer surface areas of the particles under the influence of the reducing atmosphere and that these reactions cannot proceed fully into the grains, thus leading to two different pore sizes, an inner and an outer one, after the final calcination step. 3.3. TEM. The samples treated at 450 °C under the different gas atmospheres have been further examined by transmission

Figure 7. HRTEM of TiO2 powder calcined at 450 °C (N2/H2 atmosphere) showing an interface between two crystallites (see arrow).

electron microscopy. Representative micrographs are shown in Figure 6. All three samples show porosities with an opening 1273



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goes along with two kinds of pores of different size of unclear origin. In the case of the smaller ones it may be a possible explanation that the template reacts with the TiO2 and that in this case it is not possible to burn it off completely in the following calcination. An increase of the temperature in the heat treatment to 450 °C inhibits this effect. The examinations carried out here showed that the phase relationships in the TiO2-system are complex and can be easily influenced by experimental parameters like the gas atmosphere, with regard to photocatalytical applications, which need a high anatase content and a well ordered pore system, the application of inert or reducing gas atmospheres is able to achieve a good compromise between anatase crystallinity and retained order of the pore system. Additionally it is clear from the results obtained here that the knowledge of bulk material behavior (that is temperature dependent phase relations) cannot be easily transferred on nanostructured materials. This goes along with studies on the influence of the surface energies in TiO2 nanoparticles (as described in the Introduction) and with the work of Kirsch et al.,31 who describe that the limiting factor for the crystal growth of anatase in nanostructured TiO2 is the temperature and not the time.

diameter of approximately 5 nm, which complies very well with the results obtained by N2-gas adsorption measurements. The porosities show a channel-like parallel ordering over small areas and the pore walls consist out of TiO2 particles with a size of 5 to 10 nm. The pore walls in the sample treated in N2/H2 atmosphere consist out of slightly smaller particles. The HRTEM micrographs and the SAED measurements show only crystalline material; the samples do not contain any amorphous TiO2. So it can be concluded that the crystallization processes, as observed by XRD measurements, are occurring uniformly in the OMM. The initial points for the crystallization of anatase or rutile are interfaces between crystallites as shown in Figure 7 (red arrow), showing a HRTEM picture from the sample calcined under N2/H2 atmosphere. Such interfaces have been considered by Penn and Banfield33 as energetically favorable sites for the crystallization of new phases.

4. SUMMARY AND CONCLUSIONS Here examinations on the influence of different gas atmospheres and temperatures during the calcination of ordered mesoporous TiO2 nanoparticles made by a facile one-pot synthesis were presented. Regarding the pore-wall material it was shown that crystallization of the TiO2 modifications can be influenced by an additional heat treatment step under N2 or N2/H2 gas atmospheres. By comparison of the temperature dependent phase relationships in the respective samples it is clear that the temperature of the rutile formation can be reduced to 450 °C (inert atmosphere). The growing behavior of the pore wall material is homogeneous over the samples; no evidence for a partitioning of the pore wall material in amorphous and crystalline areas could be found. The phase analysis of the powder diffraction data shows clearly that anatase content remains constant with temperature and is fed by APM and consumed by rutile when it is formed. These results along with the TEM observations makes it clear that the interfaces between crystallites are the energetically favorable place in these materials for the beginning of phase transition from APM to anatase to rutile. If the anatase-rutile transition temperature is reduced by different gas atmospheres, these interfaces and the overall surface energy effects of the nanosized crystallites are more and more important. In the oxidizing atmosphere the APM crystallite boundaries are the initial point for the crystallization of anatase. Due to the fact that the oxidizing conditions inhibit the development of rutile, the anatase can grow further until all the APM material is consumed. The inert N2 and the reducing N2/H2 atmospheres reduce the temperature at which rutile can develop. In these cases the here obtained results of the phase compositions of the samples lead to the following picture of the development of the different phases: At first anatase forms from the APM at its crystallite boundaries. When anatase crystallites are present, boundaries involving anatase are the initial points for the crystallization of rutile. The observations made for the phase relationships are indicating that without anatase a development of rutile would not take place. The use of N2/H2 or N2 atmospheres can effectively reduce the crystallite size by 2 nm compared to the calcination in air. This leads to a better ordering of the mesoporous system than in the case of the calcination under air, which is clearly shown by the presented SAXS measurements, where even at temperatures of 500 °C small Bragg intensities can be observed. In the case of N2/H2 the temperature at which the template is completely burned off is increased by at least 50 °C. This effect

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ASSOCIATED CONTENT

S Supporting Information *

X-ray diffraction, Raman spectroscopy, and TG/DTA. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address ∇

University of Bremen, Solid State Chemical Crystallography, Leobener Strasse/NW2, 28359 Bremen, Germany. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to thank C. Hü b sch, Institut fü r Werkstoffkunde, University of Hanover, for the TG/DTA measurements.

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REFERENCES

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