J. Cent. South Univ. Technol. (2010) 17: 1211-1215

DOI: 10.1007/s11771-010-0621-2

Microwave absorbing characteristics and temperature increasing behavior of basic cobalt carbonate in microwave field

LIU Bing-guo(刘秉国)^{1, 2}, PENG Jin-hui(彭金辉)^{1, 2}, ZHANG Li-bo(张利波)^{1, 2}, SRINIVASAKANNAN C³,

HUANG Min(黄铭)⁴, ZHANG Ze-biao(张泽彪)^{1, 2}, GUO Sheng-hui(郭胜惠)^{1, 2}

1. Faculty of Materials and Metallurgical Engineering, Kunming University of Science and Technology,

Kunming 650093, China;

2. Key Laboratory of Unconventional Metallurgy, Ministry of Education,

Kunming University of Science and Technology, Kunming 650093, China;

3. School of Engineering, Monash University, Sunway Campus, 46150, Petaling Jaya, Selangor, Malaysia;

4. School of Information Science and Engineering, Yunnan University, Kunming 650091, China

? Central South University Press and Springer-Verlag Berlin Heidelberg 2010

Abstract: The microwave absorbing characteristics of basic cobalt carbonate, cobalt oxide (Co₃O₄), and the mixture of basic cobalt carbonate and cobalt oxide were investigated by means of microwave cavity perturbation, their temperature increasing curves were measured, and their ability to absorb microwave energy was also assessed based on the temperature increasing behavior of the material exposed to microwave field. Analyses of spectrum attenuation and relative frequency shift show that basic cobalt carbonate has weak capability to absorb microwave energy, while cobalt oxide has very strong capability to absorb microwave energy. It is feasible to thermally decompose basic cobalt carbonate though addition of small amount of cobalt oxide in microwave fields. The capability to absorb microwave energy of sample increases with an increase in mixing ratio of Co₃O₄.

Key words: basic cobalt carbonate; cobalt oxide (Co₃O₄); microwave; temperature increasing behavior; absorption

1 Introduction

Cobalt oxide (Co₃O₄) is a technologically important material with a variety of applications such as lithium ion batteries [1-2], gas sensors [3], magnetic materials [4], and catalysts [5]. Various techniques were developed for the preparation of Co₃O₄ powders, including thermal decomposition method [6-7], sol-gel process [8], hydrothermal method [9-10], and solvothermal process [11-12]. Among these methods, hydrothermal methods are found to be inappropriate because of high temperature and pressure, which is economically unviable [13]. Sol-gel process is utilized to produce high quality product, but it is time-consuming and expensive [14]. Co₃O₄ is currently manufactured commercially using conventional thermal decomposition of basic cobalt carbonate due to the process simplicity. However, the major drawbacks of conventional thermal decomposition process are high energy consumption and long duration of the process. So, it is very important to develop new technologies that are simple and economical for the preparation of Co₃O₄ powders.

Microwave is a form of electromagnetic energy with associated electric and magnetic fields. Microwave processing is an alternative heating method that has the potential to overcome conduction problems associated with conventional heating. Compared with conventional thermal decomposition, microwave heating has a number of advantages, such as short heating time, high heating efficiency and good control of the heating processes [15]. In addition, microwave heating has the potential to heat the materials directly at high heating rates compared to conventional heating [16].

The objective of this work is to study microwave absorbing characteristics of basic cobalt carbonate and the mixture of basic cobalt carbonate-Co₃O_4. In addition, the temperature increasing behavior of basic cobalt carbonate and the mixture of basic cobalt carbonate- Co₃O₄ was also assessed.

2 Experimental

2.1 Materials

Co₃O₄ powders were prepared by thermal decomposition of basic cobalt carbonate from Shanghai Sinopharm Chemical Reagent Co., Ltd., China, at a temperature of 500-873 K under atmospheric pressure. Basic cobalt carbonate-Co₃O₄ mixtures with mass fraction of Co₃O₄ being 5%, 10%, 15%, 20%, 25%, 30%, 35% and 100% were prepared and ground in a stainless steel ball mill for 1 h.

2.2 Quantitative methods

2.2.1 Measuring equipment of microwave absorbing characteristics

The microwave absorption characteristics of samples were quantified based on the measurement of resonant frequency and the output voltage of the resonant sensor [17] under loading and unloading conditions. The schematic diagram of the measurement principle utilized is shown in Fig.1.

Fig.1 Schematic diagram of measurement principle for measuring capability to absorb microwave energy

The main components of the equipment include sweeping signal, detector and digital signal processing (DSP) resonant sensor [18], interface circuit, and computer. All components are integrated to the Windows XP operating system, and the controlling software is programmed using Visual Basic 6. The microwave signals are transmitted into the microwave sensor, whose output signals are picked up by the linear detector, which are fed into the low pass filter. The output signal of the low pass filter is amplified and converted by the A/D converter [19].

2.2.2 Equipment for microwave heating

Microwave reactor employed in the present study was made by Key Laboratory of Unconventional Metallurgy, Ministry of Education, Kunming University of Science and Technology, which had the ability to alter the power intensity from 0 to 3 kW at a frequency of 2.40 GHz. The system was equipped with a water-cooled condenser. Temperature was controlled with a temperature controller that changed the microwave power to match the set temperature. The crucible that hold the sample material was made of ceramic materials and the temperature was measured using a K-type thermocouple. A schematic diagram of the microwave- heating system is shown in Fig.2.

2.2.3 Principle of measurement

The principle of measurement is that microwave is coupled with the microwave resonator [20]. The presence of a small piece of dielectric sample in the resonant

Fig.2 Schematic diagram of experimental equipment of microwave heating

cavity will cause a shift of resonant frequency and a decrease of the quality factor of the cavity. The dielectric constant and loss tangent of the specimen can be calculated from the changes of frequency and quality factor [21-23]. The perturbations technique is applied to smaller quantities of sample. The following equations are derived from the fundamental principles [24].

               (1)

               (2)

      (3)

where Δω=ω-ω₀,, D₀,  and B₀ are the fields in the interior of the sample, E, D₁ and B₁ are the fields loaded with the sample; V_c and V_e are the volumes of the cavity and the sample, respectively; Q₀ and ω₀ are the quality factor and resonance frequency of cavity in the unperturbed condition; Q and ω are the corresponding parameters of the cavity loaded with the sample, respectively;  and  are the real and the imaginary parts of the complex permittivity of the sample, respectively; and W is the storage energy. The ability to absorb microwave energy was assessed based on the output voltage and resonator frequency of microwave sensor with sample or without sample in the cavity.

Materials with microwave absorbing properties can be effectively heated depending on the dielectric loss factor of the materials. In addition, the microwave absorbing characteristic of a material is a function of resonant frequency [25]. Hence, the variation of resonant frequency with the output voltage was captured under the conditions of with and without samples in the cavity.  Experiments were carried out at different proportions of basic cobalt carbonate-Co₃O₄ in the cavity, to estimate the capability of the samples to absorb the microwave. The attenuation and frequency shift correspond to the change in peak position of the output signal of microwave resonant and the resonant frequencies for the samples, with and without samples in the cavity [26].

3 Results and discussion

3.1 Measurement results of basic cobalt carbonate and relative materials

The microwave spectra of output voltage versus frequency, showing the capability of the sample to absorb microwave energy, are presented in Fig.3. The microwave absorbing characteristics of basic cobalt carbonate and its mixture are calculated using data in Fig.3 and listed in Table 1. The capability to absorb microwave energy of sample has been computed by program according to microwave spectra [27-28].

Fig.3 Microwave spectra of basic cobalt carbonate (0% Co₃O₄) and relative materials

Table 1 Microwave absorbing characteristics of basic cobalt carbonate mixed with different mass fractions of Co₃O₄

According to Table 1, the plots of voltage attenuation and frequency shift versus mass fraction of Co₃O₄ are shown in Figs.4-5, respectively.

It can be seen from Figs.4-5 that the attenuation reduces with the increase in mass fraction of Co₃O₄, while the relative frequency shift increases with the increase in mass fraction of Co₃O₄. This indicates that the mass fraction of Co₃O₄ has a significant effect on the microwave absorbing properties of basic cobalt carbonate. The reason is the interaction of microwave with the materials depends on both the dielectric and the magnetic properties of the medium, which are described in terms of the complex permittivity (ε) and the complex permeability (μ), respectively. For a material, which does not have significant magnetic properties, only the complex permittivity needs to be considered and can be defined as follows, ε=ε′-jε″ (ε′ is the real component of the complex permittivity, and ε″ is the imaginary part of the complex permittivity) [29]. The real and the imaginary part of the complex permittivity are used to express the dielectric response of materials in an applied microwave field. The real component of the complex permittivity indicates the capability of material to store microwave energy; and the imaginary part of the complex permittivity represents the capability of material to dissipate the stored energy into heat. In most cases, ε′ and ε″ are strong functions of the microwave frequency. According to Eqs.(1) and (2), the imaginary part of the complex permittivity (ε″) of the sample is in proportion to the capability to absorb microwave energy but in inverse proportion to changes in the attenuation of microwave sensors. The real component of the complex permittivity (ε′) of the sample is proportional to the relative frequency shift. The imaginary part of the complex permittivity (ε″) increases with an increase in mass fraction of Co₃O₄, and the order of increase is as follows: ＜＜＜＜＜＜＜＜ [27-28]. The shift of microwave spectra indicates the increase in ability to absorb microwave energy, with 100% Co₃O₄ showing the maximum absorption capacity. Since the basic cobalt carbonate is a relatively poor microwave absorber compared with cobalt oxide, the addition of cobalt oxide should enhance the interaction of the basic cobalt carbonate with microwave [30].

Fig.4 Voltage attenuation versus mass fraction of Co₃O₄

Fig.5 Frequency shift versus mass fraction of Co₃O₄

3.3 Microwave heating behavior of basic cobalt carbonate and relative materials

In order to assess the microwave heating behavior of different samples, 10 g sample was put in the crucibles at the center of the microwave chamber. The tip of the thermocouple was placed at the center of the sample for every test. The sample in turn was heated in microwave fields. The effect of the microwave irradiation time on temperature increasing behavior at microwave power of 800 W is presented in Fig.6. It shows that during the microwave irradiation process the sample temperature increases rapidly with an increase in the mass fraction of Co₃O₄. As discussed previously, the addition of a material which has good microwave absorbing capacity, has significant effects on the microwave absorbing characteristics of the poor absorber. It can be seen that the addition of small amount of Co₃O₄ results in a significant increase in the sample temperature and a maximum temperature of about 375 K is achieved in 110 s, with a mass fraction of Co₃O₄ being 20%. This is in a good agreement with the results of microwave absorbing characteristics estimated using microwave cavity perturbations. This should be attributed the fact that, when electric field intensity (E), ω and V are fixed the imaginary of the complex permittivity (ε″) directly determines the extent of the heat source strength P (P=ωε″E²V). According to Ref.[7] and TG/DTG analysis, the decomposition of basic cobalt carbonate can be divided into two stages. StageⅠ begins at 303 K and stops at 493 K. Which is attributed to the dehydration of basic cobalt carbonate; and stageⅡ begins at 493 K and stops at 625 K, which is attributed to the dehydration of basic cobalt carbonate and the decomposition of residues. This indicates that basic cobalt carbonate is not decomposed after 120 s in microwave field.

3.4 Calcination experiment

A basic cobalt carbonate-Co₃O₄ mixture containing 20% Co₃O₄ was chosen to study the phase development of the calcination processes. XRD pattern is shown in Fig.7. The final solid product calcined for 10 min at microwave power of 800 W is subjected to XRD analysis and the result is shown in Fig.8. The results show that cobalt oxide is the only solid product identified, with diffraction pattern satisfactorily matching Co₃O_4.  No other decomposition products or reaction intermediates are identified in the XRD pattern. It is feasible to prepare Co₃O₄ by calcination under microwave fields from basic cobalt carbonate through the addition of small amount of cobalt oxide.

Fig.6 Effect of microwave irradiation time on increasing temperature behavior of basic cobalt carbonate and relative materials in microwave field at 800 W

Fig.7 XRD pattern of basic cobalt carbonate-Co₃O₄ mixture containing 20% Co₃O₄

Fig.8 XRD pattern of final solid product for decomposition of basic cobalt carbonate-Co₃O₄ mixture containing 20% Co₃O₄

4 Conclusions

(1) Basic cobalt carbonate has weak capability to absorb microwave energy, while cobalt oxide has very strong capability to absorb the microwave energy. The capability to absorb microwave energy of the mixture sample increases with an increase in mass fraction of Co₃O_4. This is in good agreement with the results of temperature increasing behavior of the sample.

(2) The microwave absorbing characteristics of basic cobalt carbonate and relative materials are measured by microwave cavity perturbation technique. Mass fraction of 20% Co₃O₄ is found to be the optimum condition to prepare Co₃O₄ from basic cobalt carbonate.

(3) It is feasible to thermally decompose basic cobalt carbonates though the addition of small amount cobalt oxide in microwave field.

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(Edited by CHEN Wei-ping)

Corresponding author: PENG Jin-hui, PhD, Professor; Tel: +86-871-5192076; E-mail: jhpeng@kmust.edu.cn