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Fe2O3
Materials Research Bulletin 46 (2011) 81–86
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Crystallization and microstructure of glasses in the system Na2O/MnO/SiO2/Fe2O3 Ruzha Harizanova a,*, Gu¨nter Vo¨lksch b, Christian Ru¨ssel b a b
Physics Dept., University of Chemical Technology and Metallurgy, 8 Kl. Ohridski Blvd., 1756 Sofia, Bulgaria Otto-Schott-Institut, Universita¨t Jena, Fraunhoferstr. 6, 07743 Jena, Germany
A R T I C L E I N F O
A B S T R A C T
Article history: Received 24 May 2010 Received in revised form 4 August 2010 Accepted 24 September 2010 Available online 7 October 2010
Glasses with the compositions (100 x)(0.16Na2O/0.10MnO/0.74SiO2)/xFe2O3, (x = 5–30) and 16Na2O/ 10MnO/(74 y)SiO2/yFe2O3 (y = 5–30) were studied using X-ray diffraction and scanning electron microscopy. The effect of the chemical composition and the thermal history on the phase formation and the resulting microstructure was investigated. During cooling, the precipitation of ferrimagnetic solid solutions Fe3O4/Mn3O4 was observed. These crystals show dendritic or platelet shape, whereby the platelets are ferromagnetic and the dendrites – mainly paramagnetic. The tendency towards crystallization can be suppressed by increasing the Na2O-concentration. In contrast to glasses without manganese oxide, the precipitation of hematite is not observed. Therefore, the addition of reducing agents is not required, in order to crystallize large volume concentrations of the ferrimagnetic phase. ß 2010 Elsevier Ltd. All rights reserved.
Keywords: A. Glasses C. Atomic force microscopy C. Electron microscopy D. Microstructure D. Magnetic properties
1. Introduction Conventional glass-forming systems with high concentrations of transition metal oxides show interesting electrical and magnetic properties [1–12], but usually exhibit a high tendency towards crystallization. However, spontaneous crystallization during cooling often leads to obtaining of materials whose properties are difficult to control. Hence, using suitable cooling rates often results in the formation of partially crystalline products which find application, especially if iron oxides are present, for the fabrication of pigments and in combination with appropriate other oxides – as a part of building materials and refractories [13]. Many of these materials exhibit semiconducting electrical properties [6–12,14–18] and, depending on the transition metals used, are also ferromagnetic [17]. In glasses transition metals frequently occur in different oxidation states, e.g. iron as Fe2+ and Fe3+ [19–25]. This enables electron hopping and therefore the redox ratio strongly affects the electric conductivity. The electrical properties of the synthesized materials depend not only on the iron oxide concentration, but also on the composition of the glass matrix in which the iron ions are incorporated. Thus, two possibilities to control the electrical properties of transition metal containing oxide glasses were reported in the literature – first, the ratio alkali/transition metal oxides is varied and second, the ratio glass-former/transition metal
is changed [9]. Also, the effect of a second transition metal was studied by some authors [7,10]. The crystallization has also a large effect on the properties and especially, on electrical conductivity, in case conductive phases are precipitated. Hence, the effect of crystallization and the control of the resulting phases and microstructures are essential for the electrical properties. Magnetite, Fe3O4 and hausmannite, Mn3O4 [26] are spinel phases, which may possess strongly defect structures, form solid solutions or even build superlattices. These phases may have strongly distorted lattices and also non-stoichiometric structures can be formed (at room temperature hausmannite is reported to possess a distorted cubic lattice – c/a 1.16 [27], while magnetite is cubic). At the same time, the investigation of the physical properties in the case of simultaneous crystallization of magnetite and hausmannite is facilitated by the fact that the first is ferrimagnetic at room temperature, while the second is paramagnetic [26]. Some spinel phases possess interesting applications in sensor technology, medicine and rheology, [28,29] and are materials suitable for use at radio- and microwave frequencies. Some of these spinel phases can be crystallized from oxide glasses containing large amounts of transition metal oxides. In this paper, a study on glasses and crystallized products in the system Na2O/ MnO/SiO2/Fe2O3 is reported. The microstructures resulting from different thermal histories as well as the dependency of the microstructure on the chemical composition are studied. 2. Experimental
* Corresponding author. Tel.: +359 2 8163449, fax: +359 2 8685488. E-mail addresses: [email protected], [email protected] (R. Harizanova). 0025-5408/$ – see front matter ß 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2010.09.036
Reagent grade raw materials were used (Na2CO3, MnCO3, SiO2, Fe2O3 or FeC2O42H2O) for the preparation of the glasses. After
R. Harizanova et al. / Materials Research Bulletin 46 (2011) 81–86
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Table 1 Chemical compositions (synthesis), dilatometrically obtained glass-transition temperatures and pycnometrically determined densities of the samples studied. Tg, 8C
Density, g/cm3
5.0 10.0 15.0 20.0 25.0 30.0
540 501 501 501 506 – 514
2.646 0.003 2.773 0.006 2.906 0.003 2.970 0.001 3.175 0.002 3.2057 0.0007 3.164 0.001
70.3 66.6 62.9 59.2 55.5
5.0a 10.0a 15.0a 20.0a 25.0a
491 495 490 470 500
2.6294 0.0005 2.800 0.001 2.897 0.003 2.982 0.006 3.0448 0.0008
10 10 10 10 10 10
69 64 59 54 49 44
5 10 15 20 25 30
491 488 494 484 500 587
2.674 0.002 2.806 0.002 2.955 0.001 3.1000 0.0008 3.1803 0.0008 3.305 0.001
10 10 10 10 10
69 64 59 54 49
5a 10a 15a 20a 25a
510 520 495 495 490
2.786 0.002 2.826 0.002 2.9062 0.0007 3.012 0.001 3.0641 0.0009
Sample series
Sample name
Composition, mol% Na2O
MnO
SiO2
Fe2O3
Series 1a
A B C D E F-quenched F0
15.2 14.4 13.6 12.8 12.0 11.2
9.5 9.0 8.5 8.0 7.5 7.0
70.3 66.6 62.9 59.2 55.5 51.8
Series 1b
G H I J K
15.2 14.4 13.6 12.8 12.0
9.5 9.0 8.5 8.0 7.5
Series 2a
L M N O P Q
16 16 16 16 16 16
Series 2b
R S T U V
16 16 16 16 16
a
Sample prepared from FeC2O42H2O as a raw material.
mixing the powders, the batches (100 g) were homogenized and melted in covered SiO2-crucibles using an MoSi2-furnace and melting temperatures in the range from 1400 to 1450 8C (kept for 1.5 h in air). Some of the melts were quenched on a Cu-block (cooling rate 300 K/min, about 30 g melt) and some were cast into a pre-heated graphite mould (cooling rate 200 K/min, 80–90 g melt). The cast glasses were transferred to a muffle furnace and kept at 480 8C (around Tg) for 10 min, see Table 1. Then, the furnace was switched off and the samples were allowed to cool. The chemical compositions (synthesis) of the samples are also summarized in Table 1. The phase compositions were analyzed by X-ray diffraction (XRD: Siemens, D 5000), using CuKa-radiation, 2u values in the range from 108 to 608. The glass-transition temperature and the softening point were determined by dilatometry (Netzsch: DIL 402 PC). The densities were determined using a helium pycnometer (Micromeritics, AccuPyc 1330 V3.00). Samples for the investigation with the scanning electron microscope (SEM: Zeiss, DSM 940A) were cut, polished and coated with carbon. Secondary (SE), as well as backscattered (BSE), electrons were used for imaging. Energy dispersive x-ray analyses (EDX) were done using a Link: eXL10 spectrometer and they did not show any deviations (within the limits of error) from those calculated from the batch composition. Additionally, the microstructure was studied using an atomic force microscope (AFM: Zeiss Ultraobjektiv) with a magnetic tip. Within the system Na2O/MnO/SiO2/Fe2O3 two sub-systems were investigated: series 1a: the [Fe2O3] concentration was varied, while the ratio of all other components was kept constant. For this purpose, glasses in the sub-system (100 x)(0.16Na2O/0.10MnO/0.74SiO2)/ xFe2O3 with x = 5, 10, 15, 20 and 30 were melted. series 2a: the [Fe2O3] concentration was increased at the expense of the SiO2 concentration. The concentrations of all other components were kept constant. Here, glasses with the compositions 16Na2O/10MnO/(74 y)SiO2/yFe2O3 with y = 5, 10, 15, 20, 25 and 30 were studied.
Additionally, the above compositions with up to x = 25 (series 1b) and y = 25 (series 2b), were also melted using FeC2O42H2O (instead of Fe2O3) as raw material. Here glasses with a larger [FeO]/ [Fe2O3] ratio are obtained. These samples in the following are denoted as ‘‘reduced’’, while samples melted from Fe2O3 as raw material are designated as ‘‘oxidized’’. The cooling rate is 200 K/ min for all melts cast into graphite mould and only for sample F, which is quenched between copper blocks, the cooling rate is 300 K/min, see Table 1. 3. Results The pycnometrically determined densities of all synthesized samples are listed in Table 1. For all investigated sub-systems, both for oxidized and reduced compositions the density increases with increasing iron oxide concentration. The densities of the reduced samples in the composition series 1 were smaller than those of the oxidized samples. For series 2, only the densities of the reduced samples with iron oxide concentrations 15 mol% were smaller than those of the oxidized samples. Fig. 1 presents XRD-patterns of the annealed samples A to F0 and the quenched between Cu blocks sample F (series 1a), where the iron oxide concentration was varied, while the ratios of all other components were kept constant. The samples prepared from Fe2O3 as raw material were amorphous up to an Fe2O3-concentration of 15 mol%, while for larger iron oxide concentrations crystallization occurs and distinct lines are observed. The positions of these lines are attributable to jacobsite (JCPDS-Nr. 10-319) as well as to magnetite (JCPDS-Nr. 86-1352). By comparison, the lines of the quenched sample F are shifted to slightly larger 2u values. As also shown in Fig. 1 (graph 8), the lines due to magnetite show a somewhat stronger shift to larger 2u values. In Fig. 2, the XRD-patterns of samples L to Q (series 2) are shown. In these samples the Fe2O3 concentration is increased at the expense of SiO2 while all other concentrations are kept constant. In analogy to series 1a, samples with up to an Fe2O3 concentration of 15 mol% are amorphous, while larger iron
[()TD$FIG]
[()TD$FIG]
R. Harizanova et al. / Materials Research Bulletin 46 (2011) 81–86
Fig. 1. XRD-patterns (CuKa-radiation) of the samples A–F from the sub-system (100 x)(0.16Na2O/0.10MnO/0.74SiO2)/xFe2O3, respectively with x = 5 (1 – Sample A), x = 10 (2 – Sample B), x = 15 (3 – Sample C), x = 20 (4 – Sample D), x = 25 (5 – Sample E), x = 30 (6 – Sample F0 ), x = 30 quenched (7 – Sample F) and microcrystalline magnetite (8). The lines designate the peaks of the MnFe2O4phase.
concentrations led to sharp lines in the XRD-patterns. Also, in the reduced samples (series 1b and 2b) Fe2O3 concentrations 20 mol% led to crystallization. In these sample series, the same lines as in Fig. 1 for curve 8 were observed. For all reduced compositions for [Fe2O3] 20 mol% crystallization occurs, the phase composition of the formed crystals being the same as for the oxidized samples. Fig. 3 presents an SEM-micrograph of sample C with the composition 13.6Na2O/8.5MnO/62.9SiO2/15.0Fe2O3. The XRD-pattern of this sample does not show any distinct lines and hence the sample appears to be amorphous. In the micrograph, spheres of light appearance, some hundred nm average size, are observed. Figs. 4–6 show SEM micrographs of samples with larger iron concentrations. In Fig. 4, a micrograph of the crystallized sample O with the composition 16Na2O/10MnO/54SiO2/20Fe2O3 is shown.
[()TD$FIG]
Fig. 2. XRD-patterns (CuKa-radiation) of the samples L–Q from the sub-system 16Na2O/10MnO/(74 y)SiO2/yFe2O3, respectively with y = 5 (1 – Sample L), y = 10 (2 – Sample M), y = 15 (3 – Sample N), y = 20 (4 – Sample O), y = 25 (5 – Sample P) and y = 30 (6 – Sample Q).
83
Fig. 3. SEM (BSE) micrograph of the C-coated polished surface of sample C with composition 13.6Na2O/8.5MnO/62.9SiO2/15.0Fe2O3. The formation of nano-sized crystals is observed.
Here, large interpenetrating dendritic needles are observed. These dendrites possess a size of some ten micrometers. The dendritic structures grow radially from a centre. Fig. 5 shows a SEMmicrograph of sample U (series 2a), which is a reduced sample with the composition 16Na2O/10MnO/54SiO2/20Fe2O3d. Here, also globular dendritic structures are observed. In comparison to the oxidized samples, the crystals are notably smaller and possess a size of around 5–8 mm. Under the preparation conditions supplied, the dendrites are isolated from each other and are embedded in the amorphous matrix. The SEM-micrograph shown in Fig. 6 is attributed to the sample K which is a reduced sample (series 1a) with the composition 12.0Na2O/7.5MnO/55.5SiO2/25.0Fe2O3d. Here, two types of crystals with different morphologies are observed: thick platelets and
[()TD$FIG]
Fig. 4. SEM (BSE) micrograph of the C-coated polished surface of sample O with the composition 16Na2O/10MnO/54SiO2/20Fe2O3.
[()TD$FIG]84
[()TD$FIG]
R. Harizanova et al. / Materials Research Bulletin 46 (2011) 81–86
Fig. 5. SEM (BSE) micrograph of the C-coated polished surface of sample U with the composition 16Na2O/10MnO/54SiO2/20Fe2O3d.
Fig. 6. SEM (BSE) micrograph of the C-coated polished surface of sample K with the composition 12.0Na2O/7.5MnO/55.5SiO2/25.0Fe2O3d.
[()TD$FIG]
Fig. 7. AFM images in field mode of sample E with the composition 12.0Na2O/7.5MnO/55.5SiO2/25.0Fe2O3: (a) image of a platelet with domain structure; (b) image of needlelike paramagnetic crystals; (c) image of needle-like paramagnetic crystals, obtained after positioning the sample in external permanent magnetic field.
[()TD$FIG]
R. Harizanova et al. / Materials Research Bulletin 46 (2011) 81–86
Fig. 8. SEM (BSE) micrograph of the C-coated polished surface of the quenched sample F with composition 11.2Na2O/7.0MnO/51.8SiO2/30.0Fe2O3.
needle-like dendrites. The microstructure of the partially crystallized samples from the two studied sub-systems for both reduced and oxidized compositions are similar. The performed EDX analyses, showed that the platelets contain both manganese and iron with a ratio [Mn]/[Fe] of approximately 1/10 for series 1 and 1/ 7 for series 2. However, an exact determination of the elemental composition of the dendritic crystals using EDX point analyses was impossible, due to their relatively small thickness. Since the occurring crystals were magnetic, additional microstructural investigations of some samples with similar composition, microstructure and morphology were carried out using an atomic force microscope with a magnetic tip. The samples were cut, so that two plane-parallel surfaces were obtained and these were finely polished. In Fig. 7a, an image recorded from sample E (composition 12.0Na2O/7.5MnO/55.5SiO2/25.0Fe2O3) in field mode is shown. The examined platelet crystal consists of smaller crystals which are magnetic with distinct domain structure, as concluded from the dark and bright areas present in the AFM micrograph. Fig. 7b shows again an AFM micrograph of sample E in field mode. That part of the sample contains needle-like thin crystals, which are not ferromagnetic but obviously, paramagnetic – no change in the image contrast for these crystals is observed in field mode. Then, the same sample was positioned on a permanent magnet and image in field mode from other part of the sample surface was recorded. The result is presented in Fig. 7c – here again dark and bright regions are observed. Hence, the dendritic crystals also exhibit the presence of magnetic dipoles, which can only be observed when the thin needle-like crystals are exposed to stronger external magnetic field. Fig. 8 presents an SEM-micrograph of the quenched sample F (series 1a) which has the composition 11.2Na2O/7.0MnO/51.8SiO2/ 30.0Fe2O3. In this sample, very thin dendrites are observed. The thin dendritic structures are due to the applied high cooling rate and, according to the XRD patterns are composed mainly of magnetite. 4. Discussion X-ray diffraction patterns show that in the sub-system (100 x)(0.16Na2O/0.10MnO/0.74SiO2)/xFe2O3 (series 1a) with increasing the iron concentration the peaks are continuously
85
shifted to larger 2u values, see Fig. 1. At an iron concentration of 5 mol%, the lattice constant is 8.4699 A˚, and close to that of jacobsite (8.4970 A˚). At an iron concentration of 30 mol%, in the quenched sample, the lattice constant is 8.4187 A˚ and hence closer to that of magnetite (8.3960 A˚). The exact separation of the jacobsite and magnetite phases is difficult due to the formation of solid solutions (Fe, Mn)3O4 and possible distortions of the corresponding crystalline lattices, as reported in Refs. [26,27] and since the manganese oxide, Mn3O4 (JCPDF 13-162) lines are also very close situated to those of magnetite. Additionally, the lattice constants of magnetite (a = 8.3960 A˚) and jacobsite (a = 8.4970 A˚) are very similar and hence the exact composition of the solid solutions can hardly be determined from the XRDpatterns. The microstructure of the synthesized samples was studied by scanning electron microscopy (SEM). The SEM micrographs show that the morphology of the formed crystals is similar for all crystallized samples, no matter to which sub-system they belong (see Figs. 4–8). As already stated in Section 3, two morphologically different types of crystals are formed: interpenetrating dendritic needles or a combination of needles and platelets (see Figs. 4–6 and 8). Similar observations have already been reported by other authors in the case of complex oxide glasses prepared from a goethite waste and with the starting composition CaO/ZnO/PbO/ Fe2O3/Al2O3/SiO2 [13]. The samples described in [13] contained up to 39.6 wt% iron oxide. They were prepared using conventional melting techniques and showed strong crystallization tendency on cooling, the same as observed in our case. The main crystalline phases formed and reported by Romero and Rincon [13] are magnetite and zinc-ferrite with the same morphological shape as in our case – mainly dendrites. However, in the case of machinable ferrimagnetic glass-ceramics containing between 2.4 and 13.6 mol% Fe-oxides, for example, different nucleation and crystal growth processes were described. There, first phase separation into two phases occurs: a droplet-like phase enriched in Mg, Al, K, Si, Fe3+ and Fe2+ and matrix phase, containing SiO2, as described for the system MgO/Al2O3/SiO2/K2O/FeO/Fe2O3/F in [30], are formed. Further, in the reported system, depending on the Fe-concentration, which was comparable with the concentrations of Fe in our work, the slow cooling of the melt resulted in formation either of magnetite or of complex Mg, Al, Fe-containing spinel phase. Then, after additional thermal treatment of the unmixed glass the crystal growth of the ferrimagnetic crystals was promoted. In our work we do not observe the initial stage of phase separation for all the Feoxide concentrations utilized, but only the precipitation of ferrimagnetic crystalline phase for Fe-oxide concentrations equal or larger than 15 mol%. In the present case, (see sample F in Fig. 8), the samples which were faster cooled, contain smaller crystals and platelet-like structures are not observed, even if their Fe2O3 concentration is larger. Reduced samples, i.e. those melted from FeC2O4 as a raw material possess smaller crystals in comparison to oxidized samples with the same thermal history and the same iron oxide concentration. Actually for the reduced samples crystallization occurs for Fe2O3 20 mol% (see Figs. 4 and 5). In the case of the sub-system (100 x)(0.16Na2O/0.10MnO/ 0.74SiO2)/xFe2O3, it could be assumed that due to the higher acidity of the glass-matrix, the number of non-bridging oxygen is smaller and consequently, the number of places for the iron ions to uniformly incorporate in the glass network is insufficient. Thus, the iron ions are most probably not randomly distributed in the glass but show a tendency for clustering. The latter has already been observed for silicate glasses solely doped with Fe-ions [20–24,31]. If Fe3+ is incorporated in the glass matrix in tetrahedral coordination, i.e. as FeO4, the formally negative charge must be
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R. Harizanova et al. / Materials Research Bulletin 46 (2011) 81–86
compensated preferably by Na+ [32]. With decreasing Na+concentration, the stabilisation of FeO4-tetrahedra decreases and clustering occurs. As a result, for the sample with 15 mol% Fe2O3 from the sub-system (100 x)(0.16Na2O/0.10MnO/ 0.74SiO2)/xFe2O3 crystallization occurs, as seen in Fig. 3, while the corresponding sample from the sub-system16Na2O/10MnO/ (74 y)SiO2/yFe2O3 with the same iron concentration is amorphous. At high temperatures, Fe2+, Fe3+ and the physically dissolved oxygen are in equilibrium. 4Fe2þ þ O2 @ 4Fe3þ þ 2O2
(1)
This equilibrium is shifted to the left, i.e. to the reduced state with increasing temperature [23–25,32–34]. Usually, the raw materials, i.e. oxidizing or reducing components of the batch are decisive and the furnace atmosphere has only a minor effect on the Fe2+/Fe3+-ratio. In the case of glasses similar to those in this paper, however, solely doped with iron (without manganese), the additional crystallization of hematite is possible. The reason is that despite the high melting temperature, the amount of ferric ions (Fe3+) is considerable [21–25,32–34] and hence, the formation of hematite in case crystallization occurs could not be excluded. This is due to the thermodynamics of the equilibrium, according to Eq. (1). At an oxygen activity of the melt in the range from 0.21 (air) to 1 bar (oxygen), only 10–25% of the total iron occurs as Fe2+ at the melting temperature supplied. The only possibility to obtain larger Fe2+/ Fe3+-ratio is the addition of reducing agents to the batch, since for a total crystallization of Fe3O4, an Fe2+/Fe3+-ratio of 1:2 is required. Similar redox equilibrium is also formed for manganese containing glasses: 4Mn
2þ
3þ
þ O2 @ 4Mn
2
(2)
þ2O
3+
Here, by contrast to Eq. (1), the oxidized species Mn occurs only in trace quantities ([Mn2+]/[Mn3+] > 20), [25,33,34]. The addition of both manganese and iron oxide enables the crystallization of a solid solution of the ferrimagnetic (Fe,Mn)3O4 phase or more precisely, of a phase from the type (Fe2+, Mn2+)(Fe3+, Mn3+)2O4, in which the respective quantities of di- and trivalent ions are incorporated. The addition of reducing agents is not required. This is quite essential, because the effect of reducing agents, such as carbon or Fe(COO)2 on the Fe2+/Fe3+-ratio of a melt depends on many experimental parameters, such as quantity of melt, geometry of the crucible, heating rate, etc. and hence is difficult to control. The additional microstructural investigations carried by means of AFM on sample E without magnetic field and in field mode revealed the presence of two types of magnetic crystals. Those are ferromagnetic platelets (Fig. 7a) with distinct domain structure and needle-like dendrites which are not ferromagnetic (Fig. 7b), but after positioning the sample on a permanent magnet also show presence of magnetic dipoles in field mode (Fig. 7c), i.e. the needles are paramagnetic. The results from SEM and AFM measurements suggest that the platelets contain both Fe and Mn and the phase composition of the corresponding crystals is obviously of the type (Mn, Fe)2+(Mn, Fe)23+O4, i.e. defect ferrimagnetic spinel phase with, according to AFM results, ferromagnetic properties. Taking into account the proximity of the lines of magnetite, hausmannite and jacobsite phases in the x-ray diffractograms, it could be supposed, that the needles consist of pure hausmannite, which is also a spinel phase but is paramagnetic at room temperature [26], while magnetite and jacobsite are ferromagnetic. However the ratio Mn/ Fe for the investigated sample is about 0.3 (see Table 1) and
consequently, the formation of crystals containing solely Mn without Fe participation is very unlikely. Then, the needles are most probably a solid solution of the same type, as in the case of platelets, but no domain structure is formed due to their smaller thickness (about 1–2 mm as seen in Fig. 7). Since the (Mn, Fe)2+(Mn, Fe)23+O4 solid solutions have a high electronic conductivity, the electrons are not localized and the Fe2+/Fe3+- as well as the Mn2+/Mn3+-ratios cannot be determined. By contrast to glasses solely containing iron, in the samples containing additionally manganese, the precipitation of hematite was never observed. 5. Conclusions The microstructures of the prepared samples in the system Na2O/MnO/SiO2/Fe2O3 strongly depend on the iron oxide concentration and crystallization occurs for [Fe2O3] 15 mol% for the oxidized and [Fe2O3] 20 mol% for the reduced compositions. Also, a dependency of the microstructure on the chemical composition and more precisely, on the alkali concentration is observed – the lower the sodium oxide concentration, the higher the tendencies towards crystallization and clustering. The addition of manganese oxide to the composition leads to the formation solely of spinel phases, additional crystalline phases are not observed. The addition of reducing agents to the glass batch or the use of reducing atmosphere is not required in order to avoid the formation of hematite and to maximize the crystallization of the magnetic phase. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34]
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Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Crystallization and microstructure of glasses in the system Na2O/MnO/SiO2/Fe2O3 Ruzha Harizanova a,*, Gu¨nter Vo¨lksch b, Christian Ru¨ssel b a b
Physics Dept., University of Chemical Technology and Metallurgy, 8 Kl. Ohridski Blvd., 1756 Sofia, Bulgaria Otto-Schott-Institut, Universita¨t Jena, Fraunhoferstr. 6, 07743 Jena, Germany
A R T I C L E I N F O
A B S T R A C T
Article history: Received 24 May 2010 Received in revised form 4 August 2010 Accepted 24 September 2010 Available online 7 October 2010
Glasses with the compositions (100 x)(0.16Na2O/0.10MnO/0.74SiO2)/xFe2O3, (x = 5–30) and 16Na2O/ 10MnO/(74 y)SiO2/yFe2O3 (y = 5–30) were studied using X-ray diffraction and scanning electron microscopy. The effect of the chemical composition and the thermal history on the phase formation and the resulting microstructure was investigated. During cooling, the precipitation of ferrimagnetic solid solutions Fe3O4/Mn3O4 was observed. These crystals show dendritic or platelet shape, whereby the platelets are ferromagnetic and the dendrites – mainly paramagnetic. The tendency towards crystallization can be suppressed by increasing the Na2O-concentration. In contrast to glasses without manganese oxide, the precipitation of hematite is not observed. Therefore, the addition of reducing agents is not required, in order to crystallize large volume concentrations of the ferrimagnetic phase. ß 2010 Elsevier Ltd. All rights reserved.
Keywords: A. Glasses C. Atomic force microscopy C. Electron microscopy D. Microstructure D. Magnetic properties
1. Introduction Conventional glass-forming systems with high concentrations of transition metal oxides show interesting electrical and magnetic properties [1–12], but usually exhibit a high tendency towards crystallization. However, spontaneous crystallization during cooling often leads to obtaining of materials whose properties are difficult to control. Hence, using suitable cooling rates often results in the formation of partially crystalline products which find application, especially if iron oxides are present, for the fabrication of pigments and in combination with appropriate other oxides – as a part of building materials and refractories [13]. Many of these materials exhibit semiconducting electrical properties [6–12,14–18] and, depending on the transition metals used, are also ferromagnetic [17]. In glasses transition metals frequently occur in different oxidation states, e.g. iron as Fe2+ and Fe3+ [19–25]. This enables electron hopping and therefore the redox ratio strongly affects the electric conductivity. The electrical properties of the synthesized materials depend not only on the iron oxide concentration, but also on the composition of the glass matrix in which the iron ions are incorporated. Thus, two possibilities to control the electrical properties of transition metal containing oxide glasses were reported in the literature – first, the ratio alkali/transition metal oxides is varied and second, the ratio glass-former/transition metal
is changed [9]. Also, the effect of a second transition metal was studied by some authors [7,10]. The crystallization has also a large effect on the properties and especially, on electrical conductivity, in case conductive phases are precipitated. Hence, the effect of crystallization and the control of the resulting phases and microstructures are essential for the electrical properties. Magnetite, Fe3O4 and hausmannite, Mn3O4 [26] are spinel phases, which may possess strongly defect structures, form solid solutions or even build superlattices. These phases may have strongly distorted lattices and also non-stoichiometric structures can be formed (at room temperature hausmannite is reported to possess a distorted cubic lattice – c/a 1.16 [27], while magnetite is cubic). At the same time, the investigation of the physical properties in the case of simultaneous crystallization of magnetite and hausmannite is facilitated by the fact that the first is ferrimagnetic at room temperature, while the second is paramagnetic [26]. Some spinel phases possess interesting applications in sensor technology, medicine and rheology, [28,29] and are materials suitable for use at radio- and microwave frequencies. Some of these spinel phases can be crystallized from oxide glasses containing large amounts of transition metal oxides. In this paper, a study on glasses and crystallized products in the system Na2O/ MnO/SiO2/Fe2O3 is reported. The microstructures resulting from different thermal histories as well as the dependency of the microstructure on the chemical composition are studied. 2. Experimental
* Corresponding author. Tel.: +359 2 8163449, fax: +359 2 8685488. E-mail addresses: [email protected], [email protected] (R. Harizanova). 0025-5408/$ – see front matter ß 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2010.09.036
Reagent grade raw materials were used (Na2CO3, MnCO3, SiO2, Fe2O3 or FeC2O42H2O) for the preparation of the glasses. After
R. Harizanova et al. / Materials Research Bulletin 46 (2011) 81–86
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Table 1 Chemical compositions (synthesis), dilatometrically obtained glass-transition temperatures and pycnometrically determined densities of the samples studied. Tg, 8C
Density, g/cm3
5.0 10.0 15.0 20.0 25.0 30.0
540 501 501 501 506 – 514
2.646 0.003 2.773 0.006 2.906 0.003 2.970 0.001 3.175 0.002 3.2057 0.0007 3.164 0.001
70.3 66.6 62.9 59.2 55.5
5.0a 10.0a 15.0a 20.0a 25.0a
491 495 490 470 500
2.6294 0.0005 2.800 0.001 2.897 0.003 2.982 0.006 3.0448 0.0008
10 10 10 10 10 10
69 64 59 54 49 44
5 10 15 20 25 30
491 488 494 484 500 587
2.674 0.002 2.806 0.002 2.955 0.001 3.1000 0.0008 3.1803 0.0008 3.305 0.001
10 10 10 10 10
69 64 59 54 49
5a 10a 15a 20a 25a
510 520 495 495 490
2.786 0.002 2.826 0.002 2.9062 0.0007 3.012 0.001 3.0641 0.0009
Sample series
Sample name
Composition, mol% Na2O
MnO
SiO2
Fe2O3
Series 1a
A B C D E F-quenched F0
15.2 14.4 13.6 12.8 12.0 11.2
9.5 9.0 8.5 8.0 7.5 7.0
70.3 66.6 62.9 59.2 55.5 51.8
Series 1b
G H I J K
15.2 14.4 13.6 12.8 12.0
9.5 9.0 8.5 8.0 7.5
Series 2a
L M N O P Q
16 16 16 16 16 16
Series 2b
R S T U V
16 16 16 16 16
a
Sample prepared from FeC2O42H2O as a raw material.
mixing the powders, the batches (100 g) were homogenized and melted in covered SiO2-crucibles using an MoSi2-furnace and melting temperatures in the range from 1400 to 1450 8C (kept for 1.5 h in air). Some of the melts were quenched on a Cu-block (cooling rate 300 K/min, about 30 g melt) and some were cast into a pre-heated graphite mould (cooling rate 200 K/min, 80–90 g melt). The cast glasses were transferred to a muffle furnace and kept at 480 8C (around Tg) for 10 min, see Table 1. Then, the furnace was switched off and the samples were allowed to cool. The chemical compositions (synthesis) of the samples are also summarized in Table 1. The phase compositions were analyzed by X-ray diffraction (XRD: Siemens, D 5000), using CuKa-radiation, 2u values in the range from 108 to 608. The glass-transition temperature and the softening point were determined by dilatometry (Netzsch: DIL 402 PC). The densities were determined using a helium pycnometer (Micromeritics, AccuPyc 1330 V3.00). Samples for the investigation with the scanning electron microscope (SEM: Zeiss, DSM 940A) were cut, polished and coated with carbon. Secondary (SE), as well as backscattered (BSE), electrons were used for imaging. Energy dispersive x-ray analyses (EDX) were done using a Link: eXL10 spectrometer and they did not show any deviations (within the limits of error) from those calculated from the batch composition. Additionally, the microstructure was studied using an atomic force microscope (AFM: Zeiss Ultraobjektiv) with a magnetic tip. Within the system Na2O/MnO/SiO2/Fe2O3 two sub-systems were investigated: series 1a: the [Fe2O3] concentration was varied, while the ratio of all other components was kept constant. For this purpose, glasses in the sub-system (100 x)(0.16Na2O/0.10MnO/0.74SiO2)/ xFe2O3 with x = 5, 10, 15, 20 and 30 were melted. series 2a: the [Fe2O3] concentration was increased at the expense of the SiO2 concentration. The concentrations of all other components were kept constant. Here, glasses with the compositions 16Na2O/10MnO/(74 y)SiO2/yFe2O3 with y = 5, 10, 15, 20, 25 and 30 were studied.
Additionally, the above compositions with up to x = 25 (series 1b) and y = 25 (series 2b), were also melted using FeC2O42H2O (instead of Fe2O3) as raw material. Here glasses with a larger [FeO]/ [Fe2O3] ratio are obtained. These samples in the following are denoted as ‘‘reduced’’, while samples melted from Fe2O3 as raw material are designated as ‘‘oxidized’’. The cooling rate is 200 K/ min for all melts cast into graphite mould and only for sample F, which is quenched between copper blocks, the cooling rate is 300 K/min, see Table 1. 3. Results The pycnometrically determined densities of all synthesized samples are listed in Table 1. For all investigated sub-systems, both for oxidized and reduced compositions the density increases with increasing iron oxide concentration. The densities of the reduced samples in the composition series 1 were smaller than those of the oxidized samples. For series 2, only the densities of the reduced samples with iron oxide concentrations 15 mol% were smaller than those of the oxidized samples. Fig. 1 presents XRD-patterns of the annealed samples A to F0 and the quenched between Cu blocks sample F (series 1a), where the iron oxide concentration was varied, while the ratios of all other components were kept constant. The samples prepared from Fe2O3 as raw material were amorphous up to an Fe2O3-concentration of 15 mol%, while for larger iron oxide concentrations crystallization occurs and distinct lines are observed. The positions of these lines are attributable to jacobsite (JCPDS-Nr. 10-319) as well as to magnetite (JCPDS-Nr. 86-1352). By comparison, the lines of the quenched sample F are shifted to slightly larger 2u values. As also shown in Fig. 1 (graph 8), the lines due to magnetite show a somewhat stronger shift to larger 2u values. In Fig. 2, the XRD-patterns of samples L to Q (series 2) are shown. In these samples the Fe2O3 concentration is increased at the expense of SiO2 while all other concentrations are kept constant. In analogy to series 1a, samples with up to an Fe2O3 concentration of 15 mol% are amorphous, while larger iron
[()TD$FIG]
[()TD$FIG]
R. Harizanova et al. / Materials Research Bulletin 46 (2011) 81–86
Fig. 1. XRD-patterns (CuKa-radiation) of the samples A–F from the sub-system (100 x)(0.16Na2O/0.10MnO/0.74SiO2)/xFe2O3, respectively with x = 5 (1 – Sample A), x = 10 (2 – Sample B), x = 15 (3 – Sample C), x = 20 (4 – Sample D), x = 25 (5 – Sample E), x = 30 (6 – Sample F0 ), x = 30 quenched (7 – Sample F) and microcrystalline magnetite (8). The lines designate the peaks of the MnFe2O4phase.
concentrations led to sharp lines in the XRD-patterns. Also, in the reduced samples (series 1b and 2b) Fe2O3 concentrations 20 mol% led to crystallization. In these sample series, the same lines as in Fig. 1 for curve 8 were observed. For all reduced compositions for [Fe2O3] 20 mol% crystallization occurs, the phase composition of the formed crystals being the same as for the oxidized samples. Fig. 3 presents an SEM-micrograph of sample C with the composition 13.6Na2O/8.5MnO/62.9SiO2/15.0Fe2O3. The XRD-pattern of this sample does not show any distinct lines and hence the sample appears to be amorphous. In the micrograph, spheres of light appearance, some hundred nm average size, are observed. Figs. 4–6 show SEM micrographs of samples with larger iron concentrations. In Fig. 4, a micrograph of the crystallized sample O with the composition 16Na2O/10MnO/54SiO2/20Fe2O3 is shown.
[()TD$FIG]
Fig. 2. XRD-patterns (CuKa-radiation) of the samples L–Q from the sub-system 16Na2O/10MnO/(74 y)SiO2/yFe2O3, respectively with y = 5 (1 – Sample L), y = 10 (2 – Sample M), y = 15 (3 – Sample N), y = 20 (4 – Sample O), y = 25 (5 – Sample P) and y = 30 (6 – Sample Q).
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Fig. 3. SEM (BSE) micrograph of the C-coated polished surface of sample C with composition 13.6Na2O/8.5MnO/62.9SiO2/15.0Fe2O3. The formation of nano-sized crystals is observed.
Here, large interpenetrating dendritic needles are observed. These dendrites possess a size of some ten micrometers. The dendritic structures grow radially from a centre. Fig. 5 shows a SEMmicrograph of sample U (series 2a), which is a reduced sample with the composition 16Na2O/10MnO/54SiO2/20Fe2O3d. Here, also globular dendritic structures are observed. In comparison to the oxidized samples, the crystals are notably smaller and possess a size of around 5–8 mm. Under the preparation conditions supplied, the dendrites are isolated from each other and are embedded in the amorphous matrix. The SEM-micrograph shown in Fig. 6 is attributed to the sample K which is a reduced sample (series 1a) with the composition 12.0Na2O/7.5MnO/55.5SiO2/25.0Fe2O3d. Here, two types of crystals with different morphologies are observed: thick platelets and
[()TD$FIG]
Fig. 4. SEM (BSE) micrograph of the C-coated polished surface of sample O with the composition 16Na2O/10MnO/54SiO2/20Fe2O3.
[()TD$FIG]84
[()TD$FIG]
R. Harizanova et al. / Materials Research Bulletin 46 (2011) 81–86
Fig. 5. SEM (BSE) micrograph of the C-coated polished surface of sample U with the composition 16Na2O/10MnO/54SiO2/20Fe2O3d.
Fig. 6. SEM (BSE) micrograph of the C-coated polished surface of sample K with the composition 12.0Na2O/7.5MnO/55.5SiO2/25.0Fe2O3d.
[()TD$FIG]
Fig. 7. AFM images in field mode of sample E with the composition 12.0Na2O/7.5MnO/55.5SiO2/25.0Fe2O3: (a) image of a platelet with domain structure; (b) image of needlelike paramagnetic crystals; (c) image of needle-like paramagnetic crystals, obtained after positioning the sample in external permanent magnetic field.
[()TD$FIG]
R. Harizanova et al. / Materials Research Bulletin 46 (2011) 81–86
Fig. 8. SEM (BSE) micrograph of the C-coated polished surface of the quenched sample F with composition 11.2Na2O/7.0MnO/51.8SiO2/30.0Fe2O3.
needle-like dendrites. The microstructure of the partially crystallized samples from the two studied sub-systems for both reduced and oxidized compositions are similar. The performed EDX analyses, showed that the platelets contain both manganese and iron with a ratio [Mn]/[Fe] of approximately 1/10 for series 1 and 1/ 7 for series 2. However, an exact determination of the elemental composition of the dendritic crystals using EDX point analyses was impossible, due to their relatively small thickness. Since the occurring crystals were magnetic, additional microstructural investigations of some samples with similar composition, microstructure and morphology were carried out using an atomic force microscope with a magnetic tip. The samples were cut, so that two plane-parallel surfaces were obtained and these were finely polished. In Fig. 7a, an image recorded from sample E (composition 12.0Na2O/7.5MnO/55.5SiO2/25.0Fe2O3) in field mode is shown. The examined platelet crystal consists of smaller crystals which are magnetic with distinct domain structure, as concluded from the dark and bright areas present in the AFM micrograph. Fig. 7b shows again an AFM micrograph of sample E in field mode. That part of the sample contains needle-like thin crystals, which are not ferromagnetic but obviously, paramagnetic – no change in the image contrast for these crystals is observed in field mode. Then, the same sample was positioned on a permanent magnet and image in field mode from other part of the sample surface was recorded. The result is presented in Fig. 7c – here again dark and bright regions are observed. Hence, the dendritic crystals also exhibit the presence of magnetic dipoles, which can only be observed when the thin needle-like crystals are exposed to stronger external magnetic field. Fig. 8 presents an SEM-micrograph of the quenched sample F (series 1a) which has the composition 11.2Na2O/7.0MnO/51.8SiO2/ 30.0Fe2O3. In this sample, very thin dendrites are observed. The thin dendritic structures are due to the applied high cooling rate and, according to the XRD patterns are composed mainly of magnetite. 4. Discussion X-ray diffraction patterns show that in the sub-system (100 x)(0.16Na2O/0.10MnO/0.74SiO2)/xFe2O3 (series 1a) with increasing the iron concentration the peaks are continuously
85
shifted to larger 2u values, see Fig. 1. At an iron concentration of 5 mol%, the lattice constant is 8.4699 A˚, and close to that of jacobsite (8.4970 A˚). At an iron concentration of 30 mol%, in the quenched sample, the lattice constant is 8.4187 A˚ and hence closer to that of magnetite (8.3960 A˚). The exact separation of the jacobsite and magnetite phases is difficult due to the formation of solid solutions (Fe, Mn)3O4 and possible distortions of the corresponding crystalline lattices, as reported in Refs. [26,27] and since the manganese oxide, Mn3O4 (JCPDF 13-162) lines are also very close situated to those of magnetite. Additionally, the lattice constants of magnetite (a = 8.3960 A˚) and jacobsite (a = 8.4970 A˚) are very similar and hence the exact composition of the solid solutions can hardly be determined from the XRDpatterns. The microstructure of the synthesized samples was studied by scanning electron microscopy (SEM). The SEM micrographs show that the morphology of the formed crystals is similar for all crystallized samples, no matter to which sub-system they belong (see Figs. 4–8). As already stated in Section 3, two morphologically different types of crystals are formed: interpenetrating dendritic needles or a combination of needles and platelets (see Figs. 4–6 and 8). Similar observations have already been reported by other authors in the case of complex oxide glasses prepared from a goethite waste and with the starting composition CaO/ZnO/PbO/ Fe2O3/Al2O3/SiO2 [13]. The samples described in [13] contained up to 39.6 wt% iron oxide. They were prepared using conventional melting techniques and showed strong crystallization tendency on cooling, the same as observed in our case. The main crystalline phases formed and reported by Romero and Rincon [13] are magnetite and zinc-ferrite with the same morphological shape as in our case – mainly dendrites. However, in the case of machinable ferrimagnetic glass-ceramics containing between 2.4 and 13.6 mol% Fe-oxides, for example, different nucleation and crystal growth processes were described. There, first phase separation into two phases occurs: a droplet-like phase enriched in Mg, Al, K, Si, Fe3+ and Fe2+ and matrix phase, containing SiO2, as described for the system MgO/Al2O3/SiO2/K2O/FeO/Fe2O3/F in [30], are formed. Further, in the reported system, depending on the Fe-concentration, which was comparable with the concentrations of Fe in our work, the slow cooling of the melt resulted in formation either of magnetite or of complex Mg, Al, Fe-containing spinel phase. Then, after additional thermal treatment of the unmixed glass the crystal growth of the ferrimagnetic crystals was promoted. In our work we do not observe the initial stage of phase separation for all the Feoxide concentrations utilized, but only the precipitation of ferrimagnetic crystalline phase for Fe-oxide concentrations equal or larger than 15 mol%. In the present case, (see sample F in Fig. 8), the samples which were faster cooled, contain smaller crystals and platelet-like structures are not observed, even if their Fe2O3 concentration is larger. Reduced samples, i.e. those melted from FeC2O4 as a raw material possess smaller crystals in comparison to oxidized samples with the same thermal history and the same iron oxide concentration. Actually for the reduced samples crystallization occurs for Fe2O3 20 mol% (see Figs. 4 and 5). In the case of the sub-system (100 x)(0.16Na2O/0.10MnO/ 0.74SiO2)/xFe2O3, it could be assumed that due to the higher acidity of the glass-matrix, the number of non-bridging oxygen is smaller and consequently, the number of places for the iron ions to uniformly incorporate in the glass network is insufficient. Thus, the iron ions are most probably not randomly distributed in the glass but show a tendency for clustering. The latter has already been observed for silicate glasses solely doped with Fe-ions [20–24,31]. If Fe3+ is incorporated in the glass matrix in tetrahedral coordination, i.e. as FeO4, the formally negative charge must be
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compensated preferably by Na+ [32]. With decreasing Na+concentration, the stabilisation of FeO4-tetrahedra decreases and clustering occurs. As a result, for the sample with 15 mol% Fe2O3 from the sub-system (100 x)(0.16Na2O/0.10MnO/ 0.74SiO2)/xFe2O3 crystallization occurs, as seen in Fig. 3, while the corresponding sample from the sub-system16Na2O/10MnO/ (74 y)SiO2/yFe2O3 with the same iron concentration is amorphous. At high temperatures, Fe2+, Fe3+ and the physically dissolved oxygen are in equilibrium. 4Fe2þ þ O2 @ 4Fe3þ þ 2O2
(1)
This equilibrium is shifted to the left, i.e. to the reduced state with increasing temperature [23–25,32–34]. Usually, the raw materials, i.e. oxidizing or reducing components of the batch are decisive and the furnace atmosphere has only a minor effect on the Fe2+/Fe3+-ratio. In the case of glasses similar to those in this paper, however, solely doped with iron (without manganese), the additional crystallization of hematite is possible. The reason is that despite the high melting temperature, the amount of ferric ions (Fe3+) is considerable [21–25,32–34] and hence, the formation of hematite in case crystallization occurs could not be excluded. This is due to the thermodynamics of the equilibrium, according to Eq. (1). At an oxygen activity of the melt in the range from 0.21 (air) to 1 bar (oxygen), only 10–25% of the total iron occurs as Fe2+ at the melting temperature supplied. The only possibility to obtain larger Fe2+/ Fe3+-ratio is the addition of reducing agents to the batch, since for a total crystallization of Fe3O4, an Fe2+/Fe3+-ratio of 1:2 is required. Similar redox equilibrium is also formed for manganese containing glasses: 4Mn
2þ
3þ
þ O2 @ 4Mn
2
(2)
þ2O
3+
Here, by contrast to Eq. (1), the oxidized species Mn occurs only in trace quantities ([Mn2+]/[Mn3+] > 20), [25,33,34]. The addition of both manganese and iron oxide enables the crystallization of a solid solution of the ferrimagnetic (Fe,Mn)3O4 phase or more precisely, of a phase from the type (Fe2+, Mn2+)(Fe3+, Mn3+)2O4, in which the respective quantities of di- and trivalent ions are incorporated. The addition of reducing agents is not required. This is quite essential, because the effect of reducing agents, such as carbon or Fe(COO)2 on the Fe2+/Fe3+-ratio of a melt depends on many experimental parameters, such as quantity of melt, geometry of the crucible, heating rate, etc. and hence is difficult to control. The additional microstructural investigations carried by means of AFM on sample E without magnetic field and in field mode revealed the presence of two types of magnetic crystals. Those are ferromagnetic platelets (Fig. 7a) with distinct domain structure and needle-like dendrites which are not ferromagnetic (Fig. 7b), but after positioning the sample on a permanent magnet also show presence of magnetic dipoles in field mode (Fig. 7c), i.e. the needles are paramagnetic. The results from SEM and AFM measurements suggest that the platelets contain both Fe and Mn and the phase composition of the corresponding crystals is obviously of the type (Mn, Fe)2+(Mn, Fe)23+O4, i.e. defect ferrimagnetic spinel phase with, according to AFM results, ferromagnetic properties. Taking into account the proximity of the lines of magnetite, hausmannite and jacobsite phases in the x-ray diffractograms, it could be supposed, that the needles consist of pure hausmannite, which is also a spinel phase but is paramagnetic at room temperature [26], while magnetite and jacobsite are ferromagnetic. However the ratio Mn/ Fe for the investigated sample is about 0.3 (see Table 1) and
consequently, the formation of crystals containing solely Mn without Fe participation is very unlikely. Then, the needles are most probably a solid solution of the same type, as in the case of platelets, but no domain structure is formed due to their smaller thickness (about 1–2 mm as seen in Fig. 7). Since the (Mn, Fe)2+(Mn, Fe)23+O4 solid solutions have a high electronic conductivity, the electrons are not localized and the Fe2+/Fe3+- as well as the Mn2+/Mn3+-ratios cannot be determined. By contrast to glasses solely containing iron, in the samples containing additionally manganese, the precipitation of hematite was never observed. 5. Conclusions The microstructures of the prepared samples in the system Na2O/MnO/SiO2/Fe2O3 strongly depend on the iron oxide concentration and crystallization occurs for [Fe2O3] 15 mol% for the oxidized and [Fe2O3] 20 mol% for the reduced compositions. Also, a dependency of the microstructure on the chemical composition and more precisely, on the alkali concentration is observed – the lower the sodium oxide concentration, the higher the tendencies towards crystallization and clustering. The addition of manganese oxide to the composition leads to the formation solely of spinel phases, additional crystalline phases are not observed. The addition of reducing agents to the glass batch or the use of reducing atmosphere is not required in order to avoid the formation of hematite and to maximize the crystallization of the magnetic phase. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34]
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