Abstract
Biomorphic charcoal/TiO2 composites (C/TiO2) from moso bamboo templates were produced for absorbing microwave. Subsequently, the characteristics of the C/TiO2 were investigated by scanning electron microscopy, thermogravimetric analysis, and vector network analysis. The results showed that the biomorphic microstructure of the moso bamboo charcoal was duplicated in the C/TiO2. Thus, the density of the C/TiO2 sintered at 1200 °C was lower and approximately 0.916 ± 0.003 g/cm3. Moreover, the ignition, the maximum combustion, and the burnout temperatures of the C/TiO2 sintered at 600 °C were ~320 °C, ~530 °C, and ~585 °C, respectively. Additionally, with the rising of the temperature sintering C/TiO2, the microwave absorbency of the C/TiO2 was improved over high frequency zones. Furthermore, the average imaginary-part values of the permittivity of the C/TiO2 sintered at 600 °C and 1200 °C notably increased by 11.16-fold. In addition, the peak of microwave reflection loss of the samples (2.0 mm thickness) from the C/TiO2 powder (wt. 20%) sintered at 1200 °C and the paraffin wax (wt. 80%) was observed as -18.0 dB at 17.4 GHz. Therefore, the C/TiO2 sintered at higher temperatures exhibited lower geometrical density, better thermostability, and favorable microwave absorptive properties.
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Synthesis of Biomorphic Charcoal/TiO2 Composites from Moso Bamboo Templates for Absorbing Microwave
Liangcun Qian,*,a Shuyun Yang,b Weining Hong,a Peirong Chen,a and Xiaolin Yao a
Biomorphic charcoal/TiO2 composites (C/TiO2) from moso bamboo templates were produced for absorbing microwave. Subsequently, the characteristics of the C/TiO2 were investigated by scanning electron microscopy, thermogravimetric analysis, and vector network analysis. The results showed that the biomorphic microstructure of the moso bamboo charcoal was duplicated in the C/TiO2. Thus, the density of the C/TiO2 sintered at 1200 °C was lower and approximately 0.916 ± 0.003 g/cm3. Moreover, the ignition, the maximum combustion, and the burnout temperatures of the C/TiO2 sintered at 600 °C were ~320 °C, ~530 °C, and ~585 °C, respectively. Additionally, with the rising of the temperature sintering C/TiO2, the microwave absorbency of the C/TiO2 was improved over high frequency zones. Furthermore, the average imaginary-part values of the permittivity of the C/TiO2 sintered at 600 °C and 1200 °C notably increased by 11.16-fold. In addition, the peak of microwave reflection loss of the samples (2.0 mm thickness) from the C/TiO2 powder (wt. 20%) sintered at 1200 °C and the paraffin wax (wt. 80%) was observed as -18.0 dB at 17.4 GHz. Therefore, the C/TiO2 sintered at higher temperatures exhibited lower geometrical density, better thermostability, and favorable microwave absorptive properties.
Keywords: Biomorphic; Charcoal/TiO2 composites; Moso bamboo; Thermostability; Microwave-absorbing properties
Contact information: a: School of Science and b: School of Resources and Environment, Anhui Agricultural University, Hefei, 230036, P. R. China;*Correspondence author: qianliangc@126.com
INTRODUCTION
With the use of the various communication tools, including high power microwave generators, our daily environment has become filled with microwave interference, microwave pollution, and radar (Petrov and Gagulin 2001; Chen et al. 2007; Zou et al. 2010). Thus, microwave absorbing materials have been widely researched, e.g. the dielectric/magnetic materials, metal materials, conjugated polymers (CPs), CNTs, graphene, and their various combinations (composites/blends/hybrids) with the outstanding absorbing properties (Dang et al. 2014; Saini 2015; Verma et al. 2015a). However, due to their higher densities, the dielectric/magnetic and metal materials have been excluded from usage in light-weight microwave absorbers (Kim et al. 2004; Wang et al. 2012). Although the CPs with lower densities can be used in light-weight microwave absorbers, difficulties have been encountered in applying to higher temperature circumstance owing to their uncertain thermostability. CNTs and graphene with the characteristics of low density, high electrical conductivity, and high thermostability can be regarded as suitable candidates, but they have disadvantages with lower dielectric constants (Verma et al. 2015b). According of the principle of electromagnetic interference shielding, electric field energy can be stored in a combination of the dielectric materials and is proportional to the relative dielectric constant of the materials under the stable electric field (Abbas et al. 2006; Saini and Arora 2013; Dang et al. 2014). Moreover, under an alternating electromagnetic field, the dielectric is polarized through the rotation of electric moment and displacement of the electron cloud. Furthermore, electric field energy is converted into heat energy by fast moving charge dissipation (Phang et al.2008; Dang et al. 2014). Therefore, the various composites of CNTs and graphene structure with dielectric materials have become a focus for the development of absorbing microwave materials (Chung 2012; Saini 2015; Verma et al. 2015a).
Bamboo charcoal sintered at high temperature exhibits characteristics of low density and a structure similar to that of CNTs and graphene (Jiang et al. 2004), which makes it an attractive material in microwave absorbers. Also, its dielectric constant is lower. However, titanium dioxide(TiO2) has a larger static dielectric constant (Xia et al. 2013). Thus, TiO2 was anticipated to increase the dielectric constant of the composite. Additionally, the use of natural wood and charcoal templates for inorganic materials is an effective strategy to obtain controllable materials with biomorphic microstructure (Qian et al. 2015; Li et al. 2016; Qian et al. 2016). Further, owing to the porous microstructure of bamboo, it is accessible to gaseous or aqueous infiltration and suitable for producing porous biomorphic composites (Li et al. 2014; Chen et al. 2015). Therefore, the charcoal/TiO2 composite from moso bamboo templates can be prepared and become an alternative candidate for microwave absorbing materials.
In this work, the infiltration behavior of butyl titanate sol was investigated with moso bamboo templates. Vacuum/positive pressure technology was used to initiate sol-gel infiltration into the pores of moso bamboo. Subsequently, the C/TiO2 was sintered from the templates of the moso bamboo in a N2 atmosphere. The biomorphic structures, crystalline phase, sol impregnation effect, thermostability, and microwave absorbing properties of the C/TiO2 were characterized by scanning electron microscopy (SEM), thermogravimetric analysis (TGA), X-ray diffraction (XRD), and vector network analysis (VNA), respectively. Additionally, the density, thermostability, and absorbing properties of the C/TiO2 were compared with other microwave absorbent materials.
EXPERIMENTAL
Materials and Reagents
Chemical-grade butyl titanate (Ti(OC4H9)4) and analytical-grade acetic acid (CH3COOH), and anhydrous ethanol (CH3CH2OH) were purchased from the Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. All reagents were used without further purification. The purity of nitrogen (N2) was 99.999%.
Specimens of moso bamboo were harvested from the Forest Farm of She County, Anhui Province, China. Native moso bamboo stems were from plants more than or equal to 5 yrs old, and the specimens were cut into chips with a radial thickness of 6.02 ± 0.01 mm, a tangential width of 15.32 ± 0.02 mm, and an average axial length of 10.03 ± 0.02 mm (excluding the exterior part and interior part of moso bamboo along the radial direction). All chips were heated in a microwave oven at 1200 W for 10 min with water, and then the water was replaced. The above heating process was repeated 10 times. Finally, all samples were dried at 90 °C for 24 h.
Composites Preparation
Ti(OC4H9)4, CH3COOH, and CH3CH2OH (volume ratio of 68:12:50) were stirred for 2 h at room temperature to obtain the TiO2 sol. The dried samples were immersed in TiO2 sol for 7 days and subsequently subjected to infiltration in an adjustable air pressure dipping can at a vacuum pressure of 1.0 kPa for 30 min and a positive air pressure of 2.0 MPa for 30 min. Finally, all samples were removed from the sol. The sol of moso bamboo templates absorbed water from air for 12 h, resulting in the formation of Ti(OC4H9)4 gelatin, CH3COOH, and CH3CH2OH. All samples were dried at 130 °C for 2 h so that the TiO2 gel moved into cell cavities. To produce the C/TiO2, all samples were pyrolyzed in N2 (GLS-1700X).
A furnace containing a vacuum tube (GLS-1700X, Kexing Materials Co., Ltd., China) was used for sintering the samples. The temperature program was set as follows: 20 °C to 400 °C at a heating rate of 1 °C/min; 400 °C to the maximum temperature of 600 °C, 700 °C, 800 °C, 950 °C, and 1200 °C in N2, respectively, at a heating rate of 2 °C/min; the maximum temperature was maintained for 1 h; and the maximum temperature to room temperature cooling rate of -2 °C/min. After the sintering process, the samples were pyrolyzed at various temperatures to produce the C/TiO2.
Manufacture Microwave Absorbers
The scattering parameters and properties of microwave absorbers are relevant to the absorbing microwave component, percent of absorbing units, and the thickness of the absorbers (Petrov and Gagulin 2001; Saini 2015). Referring to the manufacturing methods of microwave absorbers in the correlative literatures (Kim et al. 2005; Wang et al. 2015), after the above C/TiO2 were ground into a powder, the fine powder was sifted through a sieve with an average pore diameter of 100 μm. The C/TiO2 powder and paraffin wax (mass ratio of 2:8) were homogeneously mixed and stirred at 60 °C for 2 h, and the specimens were manufactured into microwave absorbers of toroidal shape with a thickness of 2.0 mm and an inner diameter of 3.0 mm.
Characterization
The anatomic structures of the biomorphic charcoal and the C/TiO2 from the moso bamboo templates were observed under a scanning electron microscope (S-4800, Hitachi, Tokyo, Japan). The phase structure of TiO2 after sintering was identified using an X-ray diffractometer (XD-3, Persee, Beijing, China), operating with Cu Kα radiation (λ = 1.540563 Å), a scan rate of 1°/min, an accelerating voltage of 36 KV, an applied current of 20 mA, and a diffraction angle (2θ) ranging from 10° to 70°.
A thermogravimetric analyzer (TG-209, NETZSCH, Selb, Germany) was used to measure the thermostability of the C/TiO2, and it was operated with a purged air flow rate of 20 mL/min. The C/TiO2 powder (10.0 mg) was loaded into an alumina crucible on the thermogravimetric analyzer and heated at 10 °C/min from 40 °C to 700 °C. The corresponding mass remnants (TiO2) and thermostability of the C/TiO2 were analyzed using thermogravimetric-differential thermogravimetric (TG-DTG) profiles. The slope of the TG profile equaled the speed of mass loss (MS). The DTG profiles also expressed the rate of MS. According to Eq. 1, the value of Twas defined as the temperature when the differential of the DTG profile equaled zero, i.e., the inflection point for temperature in the DTG profiles. Therefore, T1, T2, and T3 represented the ignition temperature at the first inflection point of the DTG profile, the burnout temperature at the last inflection point of the DTG profile, and the temperature of the maximum combustion rate in the bottom inflection point of the DTG profile, respectively,
(1)
where M is the mass of the powder and t is the temperature of the powder in the crucible.
The scattering parameters (ɛ´, µ´, ɛʺ, and µʺ) of the toroidal-shaped absorbers were measured using a vector network analyzer (E5071C, Agilent, CA, USA) in the frequency range of 2 GHz to 18 GHz. The relative permeability (µr) and permittivity (ɛr) values were calculated using Eq.2 and 3 (Petrov and Gagulin 2001; Wang et al. 2012),
(2)
(3)
where ɛ´ and µ´are the real-part values of the relative permittivity (ɛr) and permeability (µr); ɛʺand µʺ are the imaginary-part values of the ɛr and µr, respectively. According to the above electromagnetic parameters and the transmission line theory (Wang et al. 2012), the input impedance (Zin) of the absorber interface can be calculated as follows,
(4)
where f is the frequency, d is the thickness of the absorber, c is the velocity of the light, and Z0 is the impedance of air. The reflection loss (RL, dB) of the microwave at the absorber surface is given by Eq. 5 (Saini 2015; Verma et al. 2015a):
(5)
Density Measurements
The mass of the samples (M) was measured using a balance with the precision of 0.001 g. The sample volume (V) was measured using vernier calipers with the precision of 0.01 mm. Densities (ρ) were calculated using Eq. 6:
(6)
RESULTS AND DISCUSSION
Microstructure Analysis
Figures 1a and 1b show the SEM images of moso bamboo charcoal sintered at 800 °C in N2. The different sizes and distributions of the pores were visualized from the formation. The first pipe pores originated from the big vessels in Fig. 1a, with pore diameters of approximately 150 μm in the transverse section. The second and third pipe pores originated from the smaller pipes and parenchymas, with pore sizes of 5 μm to 10 μm parallel to the fibers and 10 μm to 30 μm in the transverse section (the top-right of Fig. 1a). The fourth pipe pores originated from the fibers with pore diameters of 3 μm to 8 μm in the axial section (the left of Fig. 1b). The pits in the cell walls (approximately 1 μm in diameter) were visualized in the axial section (the right of Fig. 1b). Because the pits were smaller and less numerous, sol embolisms were destroyed in these pipes during the vacuum/positive pressure process of sol infiltration. Thus, sol was impregnated completely and homogenously on the moso bamboo templates using this method. Figure 1c shows the SEM images of the C/TiO2 from the moso bamboo templates sintered at 600 °C in N2. For the C/TiO2 composites, the cell cavity of the big vessels, small pipes, and parenchymas were incompletely filled by TiO2 crystal grains. Additionally, all of the pipe pores maintained the microstructure of moso bamboo charcoal.
Fig. 1. SEM of moso bamboo charcoal and charcoal/TiO2 composites: (a) transverse section (100X magnification); small pipes and parenchymas of the transverse section (500X magnification on the top-right figure); (b) fibers of the axial section on the right (500X magnification); and parenchymas of the axial section on the left (1500X magnification); and (c) the charcoal/TiO2 composites of transverse section (100X magnification) and small pipe (500X magnification on the top-right figure)
Density Measurements
Table 1 shows the geometrical densities of the moso bamboo charcoals and the C/TiO2 sintered at various temperatures. The maximum density of the moso bamboo charcoal and C/TiO2 were 0.614 ± 0.003 and 0.916 ± 0.003 g/cm3, respectively. Moreover, the densities increased with the sintering temperature. However, the density of rutile TiO2 is 4.26g/cm3 (Xia et al. 2013), and the density of ordinary metal and magnetic materials is greater than 1.00 g/cm3 (Kim et al. 2004). Because of the hollow and porous microstructure of charcoal, the moso bamboo charcoal and the C/TiO2 were lower in density.
Table 1. Density of Moso Bamboo Charcoal and Charcoal/TiO2 Composites Sintered at Various Temperatures
TiO2-Phase Formation
The TiO2-phase formations of the C/TiO2 were evaluated by XRD analysis during the heat treatment by pyrolyzing charcoal under a N2 atmosphere between 700 °C and 1200 °C (Fig. 2). The diffraction peaks at 25.2°, 37.8°, 48.1°, and 55.2° were assigned to the diffractions of the (101), (004), (200), and (211) planes of anatase TiO2, respectively (JCPDS number 21-1272) (Yang et al. 2009). The diffraction peaks at 27.5°, 36.0°, 41.5°, and 54.0° were assigned to the diffractions of the (110), (101), (200), and (211) planes of rutile TiO2, respectively (JCPDS number 21-1276) (Xia et al. 2013). The phase transformation shift from anatase to rutile was similar to that of poplar templates. As the temperature increased, the diffraction peaks of TiO2became steeper. Thus, the crystalline structure of TiO2 was more perfect (Qian et al. 2015). After the TiO2 phase transformation shifted from anatase to rutile, the static dielectric constant of TiO2increased from 48 to 180 (Phang et al. 2008). Consequently, the rutile phase of the C/TiO2, sintered at a higher temperature, was anticipated to absorb more microwaves. Further, their dielectric constants and RL with the microwave frequency were measured and discovered.
Fig. 2. X-ray diffraction patterns of moso bamboo charcoal/TiO2 composites
Microwave Absorbing Properties
Generally, there are dielectric and magnetic absorbing units in microwave absorbing materials (Abbas et al. 2006; Chen et al. 2007). The microwave absorbent properties of materials are expressed by the complex permittivity and complex magnetic permeability (Wang et al. 2012). The real-part values (ɛ´ and µ´) of complex permittivity and magnetic permeability are related to the storage capacity of electrical and magnetic energy in the materials, respectively; however, the imaginary-part values (ɛʺ and µʺ) are related to the dissipation (or loss) capacity of electrical and magnetic energy of the materials, respectively (Abbas et al. 2006; Zou et al. 2010; Saini 2015; Zhang et al. 2015). In addition, the loss tangent of the dielectric/magnetic can be expressed as tanδE = ɛʺ/ɛ´ and tanδM =µʺ/µ´ respectively. Hence, a microwave absorbing material is expected to constantly increase its imaginary part and decrease its real part in the electromagnetic parameters.
The actual power level (0.25 mW) of the incident microwave radiation was used for the measurements. Figures 3a and 3b show the curves of the relative complex dielectric constants, ɛ´and ɛʺ, of the microwave absorbents manufactured by using moso bamboo C/TiO2, sintered at 600 °C (600 C/TiO2), 950 °C (950 C/TiO2), and 1200 °C (1200 C/TiO2), respectively.
Fig. 3. The relative complex permittivity: (a) real parts (ɛ´) and (b) imaginary parts (ɛʺ)
Both ɛ´ and ɛʺ of 600 C/TiO2 maintained constant values with increasing microwave frequency; their average values were 2.642 and 0.081, respectively. The average values of ɛ´ and ɛʺ of 950 C/TiO2 were 5.438 and 0.666; likewise the average values of 1200 C/TiO2 were 8.600 and 0.904, respectively. As the sintering temperature increased, ɛ´ and ɛʺ notably increased. When the sintering temperature increased from 600 °C to 1200 °C, the average real-part values increased from 2.642 to 8.600 (3.26-fold amplification). However, the average imaginary-part values increased from 0.081 to 0.904 (11.16-fold amplification). Therefore, as the sintering temperature increased, both the storage and the dissipation (or loss) capacity of electrical energy in the materials were improved. Nevertheless, the dissipation (or loss) capacity was enhanced more rapidly than the storage capacity of electrical energy, which was in favor of the improvement on tanδE and decreasing the RL of the materials (Saini 2015). The values of ɛ´ in 950 C/TiO2decreased rapidly from 12.72 GHz to 13.68 GHz, and the ɛ´ of 1200 C/TiO2 decreased rapidly from 10.80 GHz to 11.44 GHz. Thereafter, the values of ɛ´ were maintained. In contrast, the ɛʺ of 950 C/TiO2 and 1200 C/TiO2 increased rapidly to the peak value of 13.36 GHz and 10.96 GHz, respectively. The results showed that a higher sintering temperature improved both the storage and dissipation capability of the electric field energy of the C/TiO2. Previous reports showed that a higher calcination temperature improves the graphitization degree and the conductivity of bamboo charcoal, which may be an important reason for the loss of electric field energy in the C/TiO2 (Jiang et al. 2004).
Figures 4a and 4b show the µ´ and µʺ values of 600C/TiO2, 950C/TiO2, and 1200C/TiO2 from 2 GHz to 18 GHz.
Fig. 4. The relative complex permeability: (a) real parts (µ´) and (b) imaginary parts (µʺ)
The average real-part values of µ´ for 600 C/TiO2, 950 C/TiO2, and 1200 C/TiO2 were 1.095, 1.055, and 1.076, respectively. Likewise, the average imaginary-part values of µʺ for 600 C/TiO2, 950 C/TiO2, and 1200 C/TiO2 were 0.045, 0.011, and 0.062, respectively. With the change in calcination temperature, the average values of µ´ were approximately equal; the average µʺdecreased initially and then increased slightly. Consequently, the sintering temperature almost had less effect on the storage capacity of the magnetic energy of C/TiO2. However, the values of µʺfluctuated and decreased slightly from 2 GHz to 18 GHz. Thus, the sintering temperature decreased slightly the magnetic loss capacity. As the measurement frequency increased, the storage capabilities of the magnetic energy in the C/TiO2 slightly increased above 9.5 GHz, but the dissipation capabilities of magnetic energy slightly declined. The results showed that the calcination temperature minimally affected the storage and dissipation capabilities of the magnetic energy in the C/TiO2. The above results and phenomena were caused owing to the very weak magnetic properties of the C/TiO2 (Xia et al. 2013). Therefore, the absorbing microwave of the C/TiO2 mainly was focused on the dielectric properties.
An important factor in absorbency is an outstanding RL value from the surface of absorbent (Liu et al. 2007; Wang et al. 2012; Li et al. 2015). Therefore, RL values were calculated at a given frequency, percent of the C/TiO2, and absorbent thickness using Eq. 5. Figure 5 shows the RL values of 600 C/TiO2, 950 C/TiO2, and 1200 C/TiO2 from 2 GHz to 18 GHz. The RL values of 600 C/TiO2 were equal to 0 dB; however, the RL values of 950 C/TiO2 were started with 9 GHz and terminated at 17 GHz. Furthermore, the best RL value was -9.64 dB at 15.6 GHz. 1200 C/TiO2 (from 9 GHz to 18 GHz) notably exhibited the stronger performance of absorbing microwave. Moreover, the RL peak value for 1200 C/TiO2 was -18.0 dB at 16.8 GHz. The absorbency performance of 1200C/TiO2 was equal to that of hybrid MoS2 and reduced graphene oxide, as previously reported (Wang et al. 2015). With increasing calcination temperature, the RLpeak of the C/TiO2 shifted to the high frequency region. Additionally, the RL amplitude of C/TiO2 was enlarged. This change should be attributed to the development of the graphitization degree in the bamboo charcoal and the crystallization degree of the TiO2.
Fig. 5. The reflection loss of microwaves related to the frequency values
Fig. 6. The TG-DTG curves of the bamboo charcoal/TiO2 composites sintered at 600 °C in N2
Thermogravimetric and Differential Thermogravimetric Analyses
The TG-DTG curves expressed the thermostability of the heated C/TiO2 (Mahr et al. 2012). Figure 6 shows the TG-DTG curves of 600 C/TiO2 below 700 °C in an air atmosphere. The 600 C/TiO2 composite (below 340 °C) was stable in an air atmosphere. The corresponding remnant (TiO2) was estimated at approximately 31.46%. The ignition temperature (T1), burnout temperature (T2), and temperature of the maximum combustion rate (T3) were determined by using Eq. 2. The DTG curves revealed that the T1, T2, and T3 of 600 C/TiO2 were approximately 320 °C, 530 °C, and 585 °C, respectively (Fig. 6). The DTG curve appeared to shift from 380 °C to 400 °C, corresponding to a faster combustion rate at 390 °C. The above phenomenon was attributed to the tight packing of TiO2 around the charcoal on the surface of C/TiO2, hindering oxygenolysis. Moreover, the ignition temperature of the 600 C/TiO2 composite was higher than that of the poplar C/TiO2 sintered at 400 °C (Qian et al. 2016). Thus, the TG-DTG results showed that the C/TiO2 were better thermostability in an air circumstance.
CONCLUSIONS
The hollow biomorphic C/TiO2 from templates of moso bamboo charcoal exhibited the characteristics of lower density, better thermostability, and outstanding properties of absorbing microwave. Their maximum density was only 0.916 ± 0.003 g/cm3 in the 1200 C/TiO2, and the ignition temperature of 600 C/TiO2 had reached for 320 °C. When the thickness of the absorbent (wt. 20% of the C/TiO2 power) was only of 2.0 mm, the absorbing amplitude of 1200 C/TiO2 was -18 dB at 16.8 GHz. As the calcination temperature increased, the storage and dissipation capability of the electric field energy were improved; favorable RL values of the C/TiO2 shifted to the high frequency region and the RL amplitude was increased.
ACKNOWLEDGEMENTS
This work was financially supported by the National Key Technology R&D Program in the 12thFive-Year Plan of China (No. 2014BAK09B03), the Cultured Subject Program of Anhui Agricultural University in China (No. 2014XKPY-36), the Talents Program of Anhui Agricultural University in China (No. wd2015-09), and the Higher Educational Natural Science Foundation of Anhui Province in China (No. KJ2016A230).
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Article submitted: April 16, 2016; Peer review completed: May 29, 2016; Revised version received and accepted: June 19, 2016; Published: July 11, 2016.
DOI: 10.15376/biores.11.3.7078-7090