MAGNESIUM FERRITE WITH Sn 4+ AND / OR Mo 6+ SUBSTITUTIONS AS SENSING ELEMENT FOR ACETONE AND ETHANOL

Transcription

1 MAGNESIUM FERRITE WITH Sn 4+ AND / OR Mo 6+ SUBSTITUTIONS AS SENSING ELEMENT FOR ACETONE AND ETHANOL C. DOROFTEI, E. REZLESCU, N. REZLESCU, P. D. POPA Institute of Technical Physics, Bd. D. Mangeron 47, Iasi, Romania. docorneliu@yahoo.com Received September 26, 2005 In this paper we investigated the Mg ferrite with Sn 4+ and / or Mo 6+ substitutions as sensing element for two reducing gases, acetone and ethanol. The nominal compositions of the investigated samples are: MgFe 2 O 4, Mg 0.9 Sn 0.1 Fe 2 O 4, MgFe 1.98 Mo 0.02 O 4 and Mg 0.9 Sn 0.1 Fe 1.98 Mo 0.02 O 4. The samples were prepared by coprecipitation self combustion starting from metal nitrates. The structural properties were investigated by SEM and X-ray diffraction. Both Sn 4+ and Mo 6+ ions induce structural changes regarding grain sizes and porosity. The finest granulation, of about 100 nm, was observed in the Sn containing sample Mg 0.9 Sn 0.1 Fe 2 O 4. The sensitivity measurements evidenced that the gas sensitivity depends on the ferrite composition, working temperature and test gas composition. Key words: ferrite, preparation, sensor. INTRODUCTION The detection mechanism of these devices is based on the property of changing the conductivity of the sensing material in the presence of a determinate gas [1]. As an effect of an oxidizing reaction of the reducing gas with oxygen from the sensor surface, the formation of the oxygen electrons enhances the conductance of the sensing material. Transition metal oxides appear to be the best candidate for reducing gas sensors [2]. In this paper, we investigated the Mg-based ferrite as sensing element for two reducing gases, acetone and ethanol. A great advantage of this ferrite is its high porosity which is an important requirement for a gas sensor. A porous structure favors the gas molecules to enter easier into the material and so the resistance modulation by gas adsorption in the material will be larger. Another advantage is high resistivity which can decrease so much when the working temperature is elevated. We attempted to improve the gas sensitivity of MgFe 2 O 4 Paper presented at the National Conference of Physics, September, 2005, Bucharest, Romania. Rom. Journ. Phys., Vol. 51, Nos. 5 6, P , Bucharest, 2006

2 632 C. Doroftei et al. 2 ferrite by using substitutions and by changing the preparation method. The influence of two ions with different sizes and valences (Sn 4+ and Mo 6+ ) which partially substitute Mg and/or Fe in MgFe 2 O 4 ferrite on the microstructure, electronic transport properties and sensitivity to acetone and ethanol was studied. It is known that the SnO 2 is an n-type semiconductor and favors the oxidation of reducing gases [3, 4]. The preparation procedure of the samples was a combination of sol-gel technique with selfcombustion [5, 6]. This procedure offers the advantage to produce ultra-fine, homogeneous and reproducible ferrite powders using aqueous solutions of constituent ions (metal nitrates) salts, in comparison with conventional ceramic technology. The nominal composition of the investigated samples is Mg 1 x Sn x Fe 2 y Mo y O 4 (x = 0 and 0.1 and y = 0 and 0.02). EXPERIMENTAL Four compositions were prepared: AG1 MgFe 2 O 4 ; AG2 Mg 0.9 Sn 0.1 Fe 2 O 4 ; AG3 MgFe 1.98 Mo 0.02 O 4 ; AG4 Mg 0.9 Sn 0.1 Fe 1.98 Mo 0.02 O 4. To increase the specific area exposed to gases, we have applied a new procedure for preparing precursor nanopowders which is a combination of sol-gel technique with selfcombustion [6]. This new method has a number of advantages including homogeneous mixing (on the atomic scale), good stoichiometric control (no filtering step is required) and it is suitable for the production of active powders using short processing times. By using selfcombustion, an external heat source is absent, the thermal energy is given out from the fast exothermic reaction and the product obtained is a finely divided powder. As starting materials were used metal nitrates, ammonium hydroxide and polyvinyl alcohol. The powders of metal nitrates were weighed in the desired proportions and dissolved with small amounts of water. An aqueous solution of polyvinyl alcohol was added to make a colloidal solution. Then, small amounts of NH 4 OH solution (25% concentration) were dropped into the colloidal solution to adjust PH to about 8. A sol of metal hydroxides was formed. By mixing with a magnetic stirrer for 5 to 10 min, we obtained the viscous gel. This was dried at 120 C for 12 h and then was ignited in a corner. A combustion wave spontaneously propagates through the dried gel and converts the hydroxides into metal oxides and the synthesis between oxides begins to form spinel structure. Due to rapid heating and cooling by the passage of a combustion wave from the point of ignition throughout the solid compact, a powder containing very fine crystallites were obtained. The reactions which take place can be schematized as follows:

3 3 Magnesium ferrite with Sn 4+ and/or Mo 6+ substitutions 633 2Fe(NO 3 ) 3 + Mg(NO 3 ) 2 + 8NH 4 OH 2Fe(OH) 3 + Mg(OH) 2 + 8NH 4 NO 3 (1) 2Fe(OH) 3 + Mg(OH) 2 Fe 2 O 3 + MgO + 4H 2 O (2) Fe 2 O 3 + MgO MgFe 2 O 4. (3) The as-prepared powder was compacted in a disk shape, by uniaxial pressing at a pressure of 200 MPa. The compacts were heated at 1100 C for 4 h, in a furnace. After slow cooling in the furnace, the weight and dimensions of the disks were measured to determine sintered density, d, and porosity, p, by the formula: p = 1 d (4) dx where d x is the X-ray density. The phase composition of the sintered pellets was identified by the standard X-ray diffraction (XRD), using CuKα radiation. The microstructure was examined by the scanning electron microscopy (SEM). The average grain size was determined by the linear-intercept technique from micrographs of the fracture surface of the samples. The specific surface area was calculated using the following equation [7]: A = s (5) v d where s and v are the particle surface and volume, respectively and d is the bulk density. For electric measurements, the disks of about 1.8 mm thickness and 17 mm in diameter were silvered on a face, as in Fig. 1. Electrical resistance was measured by a two-point method, with an ohmmeter. For gas sensing measurements, the sensor element was provided with a heater. The assemble heater-sensor element was introduced in a glass enclosure. The test gases were injected into the glass enclosure with a syringe through the inlet. The measurements were performed in the temperature range from 136 C to 420 C, to avoid the effect of the water vapors [8]. A cromel-alumel thermocouple located in close proximity of the sensor element was used to measure the working temperature. As reducing gases, ethanol (C 2 H 5 OH) and acetone (CH 3 COCH 3 ) were used. When the sensor element (ferrite) was introduced into the measuring chamber, its resistance was measured at fixed operating temperatures in the interval 136 C and 420 C both in air and in the presence of the reducing gases (saturated vapors). As the MgFe 2 O 4 is a n-type semiconductor, the resistance in a reducing gas atmosphere, R g, becomes smaller than the resistance in air, R a. The sensitivity, S, was calculated with the relation R Ra Rg S = Δ =, (6) R R a a

4 634 C. Doroftei et al. 4 Fig. 1. Design of ferrite sensor with silver electrodes. where R a is the sensor resistance in the air and R g is the sensor resistance in the presence of test gas at a given temperature. Taking into account the thermal inertia of ferrite ceramics, all measurements were carried out under the ferrite thermal stabilization conditions. After each exposure to gas, at a certain temperature, before the beginning of a new measurement, the sensor element was submitted to a heat treatment at 500 C for 2 minutes, in order to be activated, namely to form the initial structure and its thermodynamically stabilize it. Also, after each change in gas composition, the sensor element was stabilized before the beginning a new experiment. The measuring procedure, described above, was repeated for each sample exposed to the two test gases, acetone and ethanol, after one day, one week, one month and two months respectively and reproducible results we obtained for a long time. RESULT AND DISCUSSIONS Fig. 2 shows the X ray diffractograms of samples. It is evident that only pure ferrite and Mo substituted ferrite are monophasic. The XRD lines belonging to the spinel structure are presented with Miller indices. In contrast to Mo ions, the Sn ion induces a foreign phase. This new phase has not yet been identified. Judging from the peak height, it seems that a large amount of foreign phase is present in the samples containing Sn (AG2 and AG4 compounds). The presence of the foreign phase can suggest a solubility limit of Sn 4+ ions in the spinel lattice due to its radius (0.083 nm [9]. It is known that the

5 5 Magnesium ferrite with Sn 4+ and/or Mo 6+ substitutions 635 Fig. 2. XRD patterns for studied samples: AG1 MgFe 2 O 4 ; AG 2 Mg 0.9 Sn 0.1 Fe 2 O 4 ; AG3 MgFe 1.98 Mo 0.02 O 4 ; AG 4 Mg 0.9 Sn 0.1 Fe 1.98 Mo 0.02 O 4. degree of replacement of the host cations by the other ions in the host lattice depends on the cationic radius of the substituents [10]. The values of lattice constant for cubic spinel structure of the investigated samples are listed in Table 1 (column 3). The slight changes in the lattice constants of the substitutional solid solutions should be attributed to the presence of Sn 4+ and Mo 6+ ions on the substitutional positions in the host ferrite lattice. When a cation whose radius and valence are different from those of the host cations is introduced into the spinel lattice, the ionic size mismatch between the host and dopant cations creates a strain in the lattice which can affect the unit cell size [10, 11]. However, even for high doping level, such as AG-2 and AG-4 samples, the lattice constant does not differ so much from that of the undoped ferrite. This fact reveals a good formed crystalline structure of spinel type. Scanning electron micrographs on the fracture surfaces of the four compositions are shown in Fig. 3. Each composition is characterized by a typical porous structure and small rounded grains. It is evident that the structure is affected by the substitutions. One observes a decrease in the grain size and an increase in porosity by using Sn and Mo. The Sn substituted ferrite (AG-2) exhibits the finest and uniform granulation (Fig. 3b). In the undoped sample on can observe the presence of macro-agglomerations formed from small Sample symbol Cationic substituent Table 1 Structural data for ferrites sintered at 1100 C Lattice constant [nm] Average grain size D m [nm] Porosity [%] Specific surface area S [m 2 /g] AG1 No AG2 Sn AG3 Mo AG4 Sn and Mo

6 636 C. Doroftei et al. 6 a) b) c) d) Fig. 3. SEM micrographs for the studied ferrites: a) AG1 - MgFe 2 O 4 ; b) AG2 - Mg 0.9 Sn 0.1 Fe 2 O 4 ; c) AG3 - MgFe 1.98 Mo 0.02 O 4 ; d) AG4 - Mg 0.9 Sn 0.1 Fe 1.98 Mo 0.02 O 4

7 7 Magnesium ferrite with Sn 4+ and/or Mo 6+ substitutions 637 crystallites (500 nm) without inside pores. Many large and small pores are present in whole material. Mo ions also affect the morphology of Mg ferrite (Fig. 3c), but in a smaller measure than Sn ions. AG3 and AG4 compounds have very similar morphology (Fig. 3c, 3d). The SnO 2 incorporation lead to radical changes in microstructure (Fig. 3b) which consist in the followings: i) a significant decrease in the crystallite size, from 500 nm to about 100 nm; ii) the formation of grain bridges around the large pores and interconnected pores in the form of capillary tubes between the grain chains; iii) the agglomerations of grains are absent. It is possible that the presence of the foreign phase inhibits the grain growth and agglomeration process. The structural characteristics are summarized in Table 1. One can see that the sample doped only Sn (AG2) has the highest porosity, of 51%, and specific area, of about 24 m 2 /g. In porous ferrites, reducing gas molecules will react not only on the surface of the material, but also with ferrite grains inside the material. Also, a higher specific surface implies a much more active surface in front of gases and, thus, the gas sensitivity is larger. Low surface area of pure ferrite (AG1) can be due to the presence of large amounts of dense aggregates (hard agglomerates), as can see in Fig. 3a. The observed changes in particle size and surface area suggest that the incorporation of Sn and Mo in solid solution occurs during selfcombustion preparation which enables a better homogeneity in the powders and, hence, a more controlled microstructure. The electrical resistivity measurements at room temperature (Table 2) shows that the investigated substitutions influence the conduction process in the Mg ferrite. The Sn substituted ferrite (AG2) has the highest resistivity, of about 10 9 Ω cm. The more porous structure of this sample (AG2) (Fig. 3b) impedes the motion of charge carriers that will lead to an increase of the resistivity. Moreover, the tetravalent ions are known to localize the Fe 2+ ions if they are present on the same sublattice and reduce Fe 2+ Fe 3+ hopping conduction [12]. Presence of Sn 4+ ions on B sublattice is the most effective in this regard because Fe 2+ ion has a preference for this site. Really, the measurements show that the ρ values increase with about two orders of magnitude as Sn 4+ partially substituted Mg in MgFe 2 O 4 ferrite. Table 2 Electrical resistivity ρ at room temperature and activation energy E a Sample composition Resistivity ρ [Ω cm] Activation energy E a [ev] MgFe 2 O Mg 0.9 Sn 0.1 Fe 2 O MgMo 0.02 Fe 1.98 O Mg 0.9 Sn 0.1 Mo 0.02 Fe 1.98 O

8 638 C. Doroftei et al. 8 Another possible explanation may be the marked decrease of the grain size as observed in SEM photograph (Fig. 3b) accompanied by an increase of the grain boundary surface which normally accounts for high electrical resistance of a polycrystalline material. Also, the formation of foreign phase which acts as a blocking material for electron transport can reduce the conductance. On doping with Mo 6+ in ferrite, the creation of the cation vacancies becomes predominant to maintain electroneutrality and, in consequence, the electrical resistivity ρ will decrease as a result of dissociation of oxygen in the vicinity of cation vacancies [13]. In regard to gas sensing properties, the sensitivity of the electric resistance to acetone and ethanol saturated vapors in air was investigated. The gas sensitivity is strongly related to the working temperature, material composition, mean particle size and porosity. The results of the gas sensing measurements are summarized in Figs. 4 and 5. The measurements were carried out over 136 C. Below this temperature, the samples are insensitive to test gases. Some samples such as AG1 and AG3 are insensitive to test gases below 240 C. Moreover, over 136 C, the influence of the water vapors is minimized. From Figs. 4 and 5 one can conclude that the gas sensitivity is thermally activated and depends on gas type and ferrite composition. As expected, the gas sensitivity increases with increasing the operating temperature and reached a maximum value at an optimum temperature of about 380 C, except for AG3 and AG4 samples. Fig. 4. Sensitivity versus operating temperature characteristics of the studied ferrites: AG1 MgFe 2 O 4 ; AG2 Mg 0.9 Sn 0.1 Fe 2 O 4 ; AG3 MgFe 1.98 Mo 0.02 O 4 ; AG4 Mg 0.9 Sn 0.1 Fe 1.98 Mo 0.02 O 4, in acetone gas. Fig. 5. Sensitivity versus operating temperature characteristics of the studied ferrites: AG1 MgFe 2 O 4 ; AG2 Mg 0.9 Sn 0.1 Fe 2 O 4 ; AG3 MgFe 1.98 Mo 0.02 O 4 ; AG4 Mg 0.9 Sn 0.1 Fe 1.98 Mo 0.02 O 4, in ethanol gas.

9 9 Magnesium ferrite with Sn 4+ and/or Mo 6+ substitutions 639 Molybdenum incorporation in MgFe 2 O 4 decreases the optimum operating temperature for acetone from 380 C to 340 C in AG3 and AG4 compounds. An interesting observation is that the sensitivity to acetone and ethanol are less for Mo doped elements in comparison with Sn doped ferrite (AG2). Also, it is evident that the sensitivity to acetone is higher than that to ethanol for all the samples. The best sensitivity, 0.82, was obtained for Sn substituted ferrite (compound AG2) to acetone vapors and these results can be correlated with the structure analysis. We think that it is reasonable to believe that the smaller grain size is, the higher the surface area and the sensitivity to gases are. Also, the porous structure promotes the increase of the sensitivity. These results suggest that the role of tin in MgFe 2 O 4 ferrite is to facilitate the oxidation of reducing gases. Due to the oxidizing reaction, in the semiconducting ferrites appear the oxygen vacancies (point defects) which change the electrical conductivity (the free electron concentration increases). If the sensor material is porous, the gas will easily penetrate into the internal part of the sintered material resulting in a large change in the resistance (i.e. a large sensitivity). This may be referred to as a structure effect. Another important factor for every gas sensor is the response and recovery time, when the sensor is exposed to and then removed from a reducing gas. The response characteristics to acetone and ethanol vapors are given in Fig. 6. One can see that Sn incorporated ferrite (AG2) exhibits very short response time to acetone. The time taken by this sensor element at the optimized temperature of 380 C to reach about 90% of the maximum sensitivity was found to be 3 minutes and the time taken by the sensor to come back once the acetone gas was removed is found to be longer, of about 6 minutes. Further work is required to short the response time. The faster response to acetone of the Sn doped sample should be explained by the high porosity and nanogranular structure (see Fig. 3b). Fig. 6. Response and recovery curves to acetone and ethanol vapours for studied ferrites: AG1 MgFe 2 O 4 ; AG2 Mg 0.9 Sn 0.1 Fe 2 O 4 ; AG3 MgFe 1.98 Mo 0.02 O 4 ; AG4 Mg 0.9 Sn 0.1 Fe 1.98 Mo 0.02 O 4.

10 640 C. Doroftei et al. 10 CONCLUSIONS The influence of Sn 4+ and/or Mo 6+ ions on the electrical properties and gas sensitivity of Mg ferrite, MgFe 2 O 4, prepared by autocombustion was studied. As reducing test gases were used acetone and ethanol. The structure of the samples was investigated by XRD and SEM. It was evidenced the presence of a foreign phase in the Sn containing ferrite. Both Sn 4+ and Mo 6+ ions induce structural changes regarding grain sizes and porosity. The finest granulation, of about 100 nm, and the highest porosity were observed in the Sn containing sample, Mg 0.9 Sn 0.1 Fe 2 O 4. The sensitivity measurements to acetone and ethanol vapors evidenced that the gas sensitivity largely depends on the ferrite composition, working temperature and gas composition. It was established, by a number of experiments, that the ferrite composition Mg 0.9 Sn 0.1 Fe 2 O 4 shows the best sensitivity (0.83) to acetone at optimum working temperature of 380 C and a response time, of about 3 minutes. The sensitivity to ethanol is lower for all samples. These preliminary results have shown that the promoting effects of Sn ions in Mg ferrite on reducing gas sensitivity can be related with the changes in porosity and grain size (specific surface area). But further investigations are necessary to clarify the exact role of Sn 4+ ion which is responsible for improving the sensitivity of Sn substituted Mg ferrite particularly to acetone and to test the long-term stability to acetone of such a sensor. REFERENCES 1. T. Seiyama, A. Kato and M. Nagatoni, Analytical Chemistry 34, 1052 (1962). 2. H. Meixner, J. Gerblinger and M. Fleischer, Sensors and Actuators B 23, 119 (1995). 3. A. R. Phani, S. Manorama, and V. J. Rao, Appl. Phys. Lett. 66, 3489 (1995). 4. O. V. Safonova, G. Delabouglise, B. Chenevier, A. M. Gaskov, and M. Labeau, Materials Science and Engineering C 21, 105 (2002). 5. P. D. Popa, and N. Rezlescu, Rom. Rep. in Phys. 52, 769 (2000). 6. P. D. Popa, Study of substitutions and additives influence on some ferrites prepared by autocombustion method, Ph. D. Thesis, Alexandru Ioan Cuza University, Iasi, C. Laberty, P. Alphonse, J. J. Demai, C. Sarada, and A. Rousset, Materials Research Bulletin 32, 249 (1997). 8. C. H. Shin, D. S. Lee, S. I. Hwang, M. B. Lee, J. S. Huh, and D. D. Lee, Sensors and Actuators B 81, 176 (2002). 9. J. E. Huheey, E. A. Keiter, and R. J. Keiter, Chemistry. Principles of structure and reactivity, Harper Collins, College Publishers, D. J. Kim, J. Am. Ceram. Soc. 72, 1415 (1989). 11. P. Ingel, and D. Lewis, J. Am. Ceram. Soc. 69, 325 (1986). 12. J. S. Baijal, Ch. Prakash, and P. Kishan, Adv. In Ceramics 15, 187 (1985). 13. Y. Yamamoto, A. Makino, and T. Nikaidou, J. Phys. IV France 7, C1-121 (1997).