Solid-State Synthesis of Calcium Carbide by Using 2.45 GHz

Article pubs.acs.org/IECR

Solid-State Synthesis of Calcium Carbide by Using 2.45 GHz Microwave Reactor Rajalekshmi C. Pillai,† Edward M. Sabolsky,*,† Steven L. Rowan,† Ismail B. Celik,† and Stan Morrow‡ †

Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, West Virginia 26506, United States Hadron Technologies Inc, 4941 Allison Street #15, Arvada, Colorado 80002, United States

‡

ABSTRACT: A simple solid-state method has been described for the synthesis of CaC2 from CaO and graphite powders by using a 2.45 GHz microwave reactor. The reaction mechanism of CaC2 formation in the microwave environment is explained and compared with that of the conventional method. The whole microwave process completed within 180 min because of efficient coupling of microwaves with graphite powder. X-ray diffraction quantitative analysis results revealed 71.8% CaC2 formation at 1700 °C for 30 min of microwave exposure, whereas conventional heat treatment at 1700 °C for 30 min showed 14.1% CaC2 and 1750 °C for 60 min showed 62.8% CaC2. The X-ray photoelectron spectroscopy quantitative chemical state analysis based on C 1s spectra indicated 54.02% CaC2 in microwave heat-treated samples and 30.75% CaC2 in conventional heattreated samples. The preliminary experimental results suggest that synthesis of CaC2 by microwave reactor is fast because of rapid volumetric heating.

1. INTRODUCTION

The use of microwave energy as an alternative method to synthesize ceramic, polymer, and composite materials has been attracting the attention of many researchers because of its advantages over conventional methods; the advantages include rapid, volumetric, and selective heating.9 Microwave heating is fundamentally different from conventional heating. In conventional heating, the surface of the material is first heated by convection and radiation followed by the transfer of thermal energy to the interior of the material via conduction. In the case of microwave heating, the electromagnetic energy is directly converted to thermal energy throughout the volume of the material depending on the interaction between the electric field of the microwaves with the charged particles of the material.10 In the case of liquids, the electric field of the microwaves induces short-range orientation of the dipoles. The dipoles’ reorientation is unable to follow the frequency of the microwaves, resulting in a phase lag that gives rise to a polarization current which is in phase with the electric field of microwaves, causing the material to heat.10 In ionic solids, the conduction loss is due to ionic drift current. In semiconductors and metals, the conduction loss is due to electronic drift current which is in phase with the applied electric field.11 The ability of a material to interact with a microwave field is dependent on its dielectric loss tangent, tan δ = ε″/ε′.12 ε′ is the dielectric constant which determines how much incident microwave energy is absorbed within the materials, and ε″ is the dielectric loss factor which determines how much absorbed energy is the converted to heat. The angle δ denotes the phase lag between the applied electric field and the resulting polarization within the material. Materials like Al2O3, SiO2, and BN are poor absorbers of microwaves while SiC and C are good absorbers of

Calcium carbide (CaC2) is one of the chemical compounds used to prepare acetylene, a precursor material commonly used for the production of polyethylene (PE) and polyvinyl chloride (PVC).1 The other industrial applications of calcium carbide are desulfurization of iron in steel and production of calcium cyanamide.2 The industrial production of calcium carbide relies on the century-old electric arc furnace which involves the reaction between calcium oxide (CaO) and coke (C) at 2200 °C for 1−2 h.3 The high energy requirement, complicated reactor design, low production capability, and high cost of this technology for making acetylene motivate the industries to depend on petroleum-derived ethylene as the starting material for the PE, PVC, and other related products.4 Efforts have been made to develop new technologies for energy efficient and economical production of CaC2.5 Mu and Hard4 developed a rotary kiln-based process for preparing CaC2 by reducing CaO in the presence of carbon. The carbon sources used were subbituminous coal, bituminous coals, anthracite, and petroleum coke. It was observed that almost complete reduction of lime to CaC2 occurred at ∼1870 °C in less than 30 min. However, the quality of the carbide formed was low and suitable only for on-site generation of acetylene. El-Naas et al.6 synthesized CaC2 from micron-sized CaO and graphite powder by using a spout-fluid bed reactor with a dc plasma torch. The major limitation of this method was the inability to achieve conversions beyond 30% because of the melting and agglomeration of particles in the plasma jet zone. Li et al.3 produced CaC2 from pulverized coke and CaO at 1750 °C by using an autothermal method. They were also successful in producing CaC2 from fine biochars and CaO using the same method.7 Attempts were also made to synthesize CaC2 by using a modified domestic microwave oven from CaO and graphite powders.8 However, the authors failed to quantify the conversion of CaO and graphite to CaC2. © XXXX American Chemical Society

Received: July 31, 2015 Revised: October 11, 2015 Accepted: October 11, 2015

A

DOI: 10.1021/acs.iecr.5b02821 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

and composites, this method is in the early stages of development and mostly confined to laboratories.16−21 The objective of this research was to synthesize CaC2 from CaO and graphite by using 2.45 GHz microwave energy. Comparison studies were also performed by using a conventional alumina tube furnace to investigate the effectiveness of microwave heating. Scanning electron microscopy (SEM) techniques were used to compare the morphology of the formed CaC2 powder. The CaC2 yield was determined quantitatively by using X-ray diffraction (XRD) as well as Xray photoelectron spectroscopy (XPS) techniques.

microwaves at room temperature. It is also important to note that the dielectric properties of the materials are sensitive to temperature, frequency, phase transformations, and chemical reactions.13 The absorption spectrum of water clearly illustrates the effect of frequency on the dielectric properties. Water absorbs microwaves at room temperature because it has an absorption peak at 2.45 GHz. On the other hand, ice is not a good absorber at 2.45 GHz because of the shift of absorption peak at the kilohertz frequency range at room temperature.14 Most of the ceramic materials do not couple with microwaves at room temperature. However, their coupling can be improved by increasing the temperature, changing the frequency, or adding microwave-absorbing materials. Temperature can be increased by employing either an independent heat source such as an electric furnace or an external susceptor material that couples well with the microwaves at room temperature.15 Once the material is heated to its critical temperature, the dielectric loss increases and microwave absorption becomes sufficient to cause self-heating of the material. The microwave power absorbed per unit volume, P (W/m3), is expressed as11 P = σ |E2| = 2πfε0ε″|E2| = 2πfε0ε′tan δ|E2|

2. EXPERIMENTAL SECTION 2.1. Powder Preparation and Consolidation. Figure 1 shows the various processing steps involved in the preparation

(1)

where σ (S/m) is the conductivity, f (Hz) the frequency of microwaves, εo the permittivity of the free space (εo = 8.86 × 10−12 F/m), ε″ the dielectric loss factor, ε′ the dielectric constant, tan δ the loss tangent, and E (V/m) the magnitude of the internal electric field. The intensity of the electric field is attenuated when the microwaves start to penetrate inside the material. This is represented through a parameter known as penetration depth, Dp (m), defined as the depth at which the electric field drops to 1/e = 0.368 of its value at the surface of the material:11

Dp =

λ 0 ε′ 2πε″

(2) Figure 1. Flowchart showing various processing steps of CaC2 from CaO and graphite.

where λo (m) is the free space wavelength of the incident microwaves. In the case of good conductors like metals, the penetration depth of microwaves is very low and is expressed by a parameter known as skin depth, d (m):11

d=

1 πfμσ

of CaC2 from CaO and graphite. The experimental setup used to prepare CaC2 powder is shown in Figure 2. The conventional furnace consists of 36 in. long alumina tube. The heating elements used in the furnace were molybdenum disilicide. Microwave experiments were performed in an industrial microwave reactor (Hadron Technologies Inc.). It consists of 2.45 GHz, 6 kW magnetron microwave source, wave guides to transport the microwave energy from magnetron to multimode applicator, tuner to match output impedance of waveguide to input impedance of the cavity applicator in order to deliver maximum power, microwave transparent quartz window, optical radiation pyrometer to monitor the surface temperature of the sample, and rotary pump to evacuate air from the microwave applicator. To minimize the thermal gradient, the sample is enclosed in a microwave transparent thermal insulation box. The thermal insulation box typically consists of two layers of microwave transparent insulations. The inner insulation is pure alumina, and the outer insulation is 80 vol % alumina, 20 vol % silica fiber board. The two insulation boards are separated by a spacer. The starting materials used in the synthesis of CaC2 were CaO and graphite powder. Reagent grade calcium oxide (Alfa Aesar) powder was heat treated at 1000 °C for 5 h to remove any calcium hydroxide present as an impurity. The heat-treated calcium oxide and graphite (Alfa Aesar) powders in 1/3 mol ratio were ball milled in acetone

(3) −6

μ = μo permeability of free space (μo = 1.25663706 × 10 H/ m). For copper, σ = 5.8 × 107 S/m and skin depth is approximately 1 μm at 2.45 GHz.12 The concept of skin depth explains why metal particles of submicron size can be heated with microwaves even though bulk metals reflect the microwaves. For graphite, σ = 2 × 105 S/m and skin depth is approximately 23 μm at 2.45 GHz. The last two decades witnessed tremendous progress in the application of microwave energy as an alternative, faster, and simpler route for the synthesis of organic and inorganic compounds with high reaction rate and yield.9 The high reaction rate under microwave irradiation is the manifestation of thermal and kinetic effect caused by high reaction temperature resulting from the volumetric heating nature of microwaves.10 There were also reports of nonthermal interactions of microwaves with materials such as rotational or vibrational transition of specific molecules. However, claims of accelerated reaction due to nonthermal microwave effects are currently under debate. Although microwave heating has been successfully applied to synthesize many materials including metals, ceramics, polymers, B


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Figure 2. Experimental setup used to synthesis CaC2 from CaO and graphite powder. (a) Conventional tube furnace; (b) Industrial microwave reactor.

binding energy of 932.62 eV. All spectra were fitted with a Shirley background, and a Voigt function was employed to simulate spectral lines with Gaussian component set to 40% for spectra collected with the Kratos instrument, and set to 70% for the PHI and SSX instruments. Best fits were determined by minimization of the summed root-mean-square value of the fit. The XPS data analyses were performed to understand the changes in the valence state and binding energy of the constituent elements present in the samples.

using stabilized zirconia balls for 24 h. The well mixed powder slurry was passed through a 325 mesh sieve to separate the balls. The slurry was vacuum-dried for 24 h at 100 °C. The dried powders were pressed into pellets using a die-press. The pressed pellets were placed in a graphite crucible and heated in the microwave furnace at 1700 °C for 30 min in Ar atmosphere. Conventional heat treatments were performed in an alumina tube furnace at 1700 and 1750 °C for 30 and 60 min, respectively. The whole microwave heating cycle completed within 180 min, whereas the conventional heating cycle lasted up to 1200 min because of the slow heating and cooling rate limitations imposed by the alumina tube. 2.2. SEM, XRD, and XPS Characterization. An X’PERT PRO Panalytical X-ray diffractometer (Westborough, MA) was used to determine the phase of the prepared CaC2 powders using Cu Kα radiation. Data were collected from 10°−110° angles (2θ) with a step size of 0.02 increments at a rate of 1°/ min. Rietveld refined method was used for quantitative phase analysis. The morphology of the synthesized CaC2 powders was examined by scanning electron microscopy (JEOL 7600F; Peabody, MA). The X-ray photoelectron spectroscopy measurements were carried out using a Physical Electronics, PHI 5000 Versa Probe (XPS/UPS) spectrometer with a monochromatic Al Kα source operated at 300 W and a base pressure of 5 × 10−8 Torr. The X-ray source operated at 15 kV and 25 W. The samples used in this study were composite powders of calcium oxide (lime) and graphite (C), heat treated by using both conventional and microwave furnaces. The powder samples were placed on the sample holder using a double-sided conductive tape followed by 6 h evacuation prior to analyses. Sample height positions were set from O 1s signal at 529 eV following changing of lateral coordinates such that the measured signal from the sample powders were maximized. As a reference, the C 1s signal of the adventitious carbon was used, which was fixed at 284.6 eV. Survey spectra were collected by 1.0 eV steps at analyzer pass energy of 160 eV, and the high-resolution analysis of small spectrum regions by 0.05 eV steps and pass energy of 20 eV. The composition and chemical states were determined from the charge-corrected high-resolution scans with analyzer pass energy of 20 eV. The acquisition time of the sample was kept low to minimize any surface oxidation state changes under Xray irradiation. The pressure was maintained at 5 × 10−8 Torr, and an electron flood gun was used for charge neutralization. The work function of the instrument was calibrated to a binding energy of 83.96 eV for the Au 4f 7/2 line for metallic gold, and dispersion of the spectrometer was adjusted to a

3. RESULTS AND DISCUSSION 3.1. Microwave Heating Characteristics of CaO− Graphite Pellets. The chemical reaction for the formation of CaC2 from CaO and C can be written as22 CaO + 3C → CaC2 + CO (4) The reaction proceeds according to the following two-step mechanism CaO + C → Ca(g) + CO

(5)

Ca(g) + 2C → CaC2

(6)

Figure 3 shows a simple binary phase diagram composed of two components, CaO and CaC2, which has a eutectic point at the eutectic temperature of 2103 K and eutectic composition of 48% of CaC2.1 Above the liquids line (blue line) both CaO and CaC2 exist in liquid form and below the solidus line (red line) in solid form. The melting temperature of 100% CaO is 2887 K

Figure 3. CaO−CaC2 binary phase diagram.1 C


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Industrial & Engineering Chemistry Research and 100% CaC2 is 2440 K. Tagawa and Sugawara22 suggested either diffusion of carbon into calcium oxide or calcium oxide into carbon is the rate-controlling step. On the other hand, Müller23−25 proposed that ionic diffusion is the only means of carbon transport into the CaO for the reduction of CaO to Ca vapor. The as-formed calcium vapor reacts with either the diffused carbon at the surface of the oxide or with the free carbon to form calcium carbide. According to this model, surface reaction is the rate-controlling step for calcium carbide formation. However, no experimental evidence have been reported in the literature that supports these models. Although several methods have been attempted to synthesize CaC2 economically, no detailed study has been reported in the literature using microwave energy to synthesize CaC2. In general, the microwave absorption in nonmagnetic solid materials results from coupling of electric field with conduction electrons, mobile ions, or lattice phonons. In highly conducting materials such as metal particles, microwave heating takes place by induction losses caused by eddy currents.12 In the present work, microwave susceptibility of graphitic carbon has been exploited because CaO itself is not a good coupler of microwaves. It has been reported that the heating rate of graphite powder with a particle size less than 1 μm is 600 °C/ min.11 Although excellent microwave absorption behavior of graphite is reported by many authors, the mechanism for such high absorption is yet to be understood. Rao et al.11 claimed that microwave heating of graphite occurs through the activation of the weak interlayer bonds. The resonance condition for microwave absorption is met through the excitation of state-to-state rotational modes. De-excitation of the absorbed microwave energy is dissipated as heat via internal mode coupling. Heat generated within the graphite particles is transferred uniformly by oxide particles throughout the volume of the pellet. It has also been observed that the minimum amount of CaO and graphite powder required to produce sufficient heat is 10 g. Otherwise, the low thermal conductivity of CaO traps the heat at the C−CaO interface and reduces the heating rates. Skin depth is another important parameter which determines the depth of microwave penetration, and for graphite it was calculated as 23 μm at 2.45 GHz, which was more than the particle size of the graphite powder used in the present study. While preparing the powder, care has also been taken to avoid agglomeration which otherwise prevents the penetration of microwaves into samples. Fine powders were obtained by ball milling CaO and graphite powders in acetone. Figure 4 shows the microwave and conventional heating characteristics of CaO and C samples. The time−temperature profile of the microwave shows very high heating rates (144 °C/min) initially and then decreases to a lower value (12 °C/ min) above 1460 °C. Even though a constant continuous microwave power of 2 kW was delivered to the sample, a considerable reduction in heating rate above 1460 °C occurred because of the chemical changes taking place during microwave heating. Above 1460 °C, the formation of CaC2 starts, which consumes carbon. The decrease of carbon content in the sample reduces the heating rate because carbon is the major source of microwave absorbtion. The other possible explanation for the drop of heating rate may be the heat lost from the surface of the sample. Beyond 1450 °C, the heat loss is significant because of thermal emissivity. The attempts to increase the heating rate by increasing the microwave power beyond 2 kW failed because of plasma generation caused by the ionization of Ar gas by microwaves. One can avoid the plasma

Figure 4. Comparison of microwave and conventional heating cycles used to synthesize CaC2 from CaO and graphite.

generation by switching the operational frequency to 915 MHz instead of 2.45 GHz. The heat loss from the surface of the sample can be minimized through hybrid heating technology by combining microwave heating with radiant surface energy supplied by electrical resistance heating or conventional gas firing. Another interesting observation is the remarkable difference in heating patterns of conventional versus microwave techniques. In the case of microwave heating, the whole cycle completed within 180 min because of its volumetric heating and natural cooling, whereas in conventional heating, the thermal cycle lasts up to 1200 min because of 3 °C per min heating and cooling rate limitations imposed on the furnace by the alumina tube.13 3.2. XRD and SEM Characterization of CaC2 Samples. The XRD full pattern fitted results of microwave and conventionally synthesized samples are shown in Figure 5. The XRD patterns revealed CaC2, CaO, and C. The conventionally heated sample at 1700 °C for 30 min and 1750 °C for 60 min exhibited monoclinic phase of CaC2, whereas the microwave heated sample at 1700 °C for 30 min revealed monoclinic as well as tetragonal phase, indicating that the actual reaction temperature at the center of the sample is higher than the surface temperature measured by pyrometer. Experimentally, four different modifications of CaC2 are known. The common form, CaC2-I, crystallizes in the body-centered tetragonal crystal structure with space group I4/mmm, and the metastable forms CaC2 -II and CaC 2−III crystallize in monoclinic structures with space group C2/C and C2/m, respectively. Finally, the cubic CaC2-IV formed at very high temperatures.26 In the heterogeneous reaction of C and CaO to form CaC2, molecular level mixing does not occur and the reaction can be started only at a point of intimate contact of gains. Hence, the rate of reaction depends on how well the reactants are mixed and the uniform distribution of reaction sites throughout volume of the sample. The other parameters which greatly influence the reaction rate are temperature and porosity of the reaction production through which further material transport occurs to continue the reaction. In conventional heating, heat transfer occurs from the surface to interior of the sample by conduction, whereas in microwave heating temperature is generated throughout the volume of the material. However, the temperature of the surface is lower than that of the interior of the sample because of surface thermal D


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Figure 5. XRD full pattern fitted results of CaC2 powders synthesized through microwave and conventional route.

emissivity, particularly at higher temperatures. As a consequence, the temperature measured at the surface of the sample by the pyrometer is lower than the actual temperature. One can argue that the temperature measured at the surface of the microwave heat-treated samples were lower than the local temperature at the points of contacts of C and CaO interior of the pellet. However, there exist reports in the literature suggesting that CaC2 can be synthesized at a temperature as low as 1750 °C.3,6 Recently, Li et al.3 reported that CaC2 can be produced at 1750 °C using fine particle sizes less than 0.1 mm. They also investigated autothermal heating of biochars as an alternative process for CaC2 preparation. The authors used fine CaO and biochar and achieved more than 95% conversion of CaO at 1750 °C in less than 5 min. They further explained that CaC2 production is controlled not only by mass transfer of CaO toward the biochar phase but also eutectic mixtures at the CaO/char interphase. Moreover, smaller particle size and larger contact surface area enhanced the reaction rate. Recently, the Rietveld refined method has been used for quantitative XRD phase analysis of carbide materials synthesized by the microwave method.17 Rietveld quantitative phase analysis mainly involves calculating diffraction patterns of individual components of a mixture using a crystal structure

model. A diffraction pattern can be calculated as the sum of all the Gaussian peaks and then fitted to the measured profile by nonlinear least-squares refinement of the various parameters embedded in the Rietveld equation.27 Ahlers and Ruschewitz8 reported that reliable quantitative determination of phases present in the reaction between CaO and C to form CaC2 was difficult because the intensity of reflections from (002) planes of graphite was too high because of preferred orientation of these crystal planes. However, the software (PANalytical High Score Plus) used by the present authors has the option for refining the preferred orientation effects. Preferred orientation occurs because of the “spotty” nature of diffraction in a nonrandom specimen, and it causes serious intensity aberrations in a diffraction pattern. Severe preferred orientation even results in absence of diffraction peaks leading to wrong quantitative phase information. Materials like clay have a platy habit and orient perpendicular to the (001) plane, and fibrous materials like asbestos orient parallel to their elongation direction. The software used by the present authors to refine preferred orientation effects used the March−Dollase model suitable for Debye−Scherrer and Bragg−Brentano geometry. In this work, the Rietveld refinement technique was used to determine the mass composition for all phases formed during E


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Figure 6. SEM microstructure showing CaC2 powder formed through (a) microwave and (b) conventional route.

spectrum of electron counts versus kinetic energy of emitted photoelectrons. The positions and shape of the peaks in a spectrum provide information about the chemical element and chemical state of the elements present in the material. Figure 7a,b shows a survey, or wide-range scan that covers all the

the reaction between CaO and C to form CaC2. The conventional heat-treated samples at 1700 °C for 30 min show 14.1% CaC2 (monoclinic crystal structure), 55.4% unreacted graphite, and 30.5% unreacted CaO. The conventional heat-treated samples at 1750 °C for 60 min show 62.8% CaC2 (monoclinic crystal structure), 32.3% unreacted graphite, and 4.9% unreacted CaO; the microwave heat-treated samples at 1700 °C for 30 min show 71.8% CaC2 (47.6% of CaC2 was monoclinic structure and 24.2% was tetragonal structure), 24.5 % unreacted graphite, and 3.7% unreacted CaO. The XRD quantitative analysis results suggest that microwave heat treatment is more energy efficient than conventional heat treatment due its volumetric heating nature. It is also interesting to note that the Ca(OH)2 phase is completely absent in both conventional and microwave heat-treated samples which otherwise forms because of the reaction between CaC2 and moisture present in the atmosphere according to the following reaction: CaC2 + 2H 2O → C2H 2 + Ca(OH)2

(7)

In this work the formation of Ca(OH)2 was completely eliminated by heat treating CaO at 1000 °C before mixing with graphite. Care has also been taken to avoid the contact of moisture with CaC2 after removal of the sample from the furnace. In addition,the CaC2 sample was vacuum sealed with a mylar thin film to avoid any contact with atmospheric moisture while performing X-ray diffraction. SEM analysis was carried out to compare the particle sizes of microwave and conventional heat-treated CaC2 samples. Panels a and b of Figure 6 show the SEM micrographs of microwave (1700 °C for 30 min) and conventional (1750 °C for 60 min) heat-treated CaC2, respectively. The SEM image of microwave heat-treated sample at 1700 °C for 30 min reveals a small particle size compared to conventional heat-treated sample at 1750 °C for 60 min. Satapathy et al.28 synthesized submicron-sized SiC particles by direct solid-state reaction of Si and C at 1300 °C for 5 min in a 2.45 GHz microwave field. It is expected that the exposure of the sample to microwave radiation for short duration prevents the growth of particle size. It is worth investigating the effect of CaC2 particle size on acetylene formation because the high surface area of fine grain CaC2 may improve the rate of reaction with water. 3.3. X-ray Photoelectron Spectroscopy Characterization of CaC2 Powder. XPS is a very reliable nondestructive characterization tool used to collect information about chemical elements as well as chemical states of the constituent elements present in the material. XPS uses X-rays of a characteristic energy to excite electrons from orbitals in atoms. The photoelectrons emitted from the orbitals of the material are collected as a function of their kinetic energy. The result is a

Figure 7. Survey scan from the CaC2 powder showing Ca 2p, C 1s, O 1s, O 2s, and O KLL peaks.

excited levels by the X-ray energy being used. The major levels identified were Ca 2p, C 1s, O 1s, O 2s, and O KLL. Panels a and b of Figure 8 show high-resolution C 1s spectra collected from microwave and conventional heat-treated CaC2 powder samples, respectively. The C 1s spectra were resolved into four Gaussian components. The energies are charge referenced with respect to C 1s signal of the adventitious carbon at 284.6 eV. The major peak at 282.8 eV is attributed to CaC2.29 The distinct feature observed in the case of the microwave heattreated sample is the high intensity of the peak compared to conventional heat-treated sample, suggesting that more CaC2 has formed in the case of microwave heat treatment than conventional heat treatment. One can also notice broadening and shifting of the C 1s spectrum, indicating that calcium diffuses or reacts with carbon. The full width at half-maximum (FWHM) of the conventional heat-treated sample is 1.26 eV, and that of the microwave-synthesized sample is 2.31 eV. F


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Figure 8. C 1s spectra collected from CaC2 powders synthesized through microwave and conventional routes.

Figure 9. Ca 2p spectra collected from CaC2 powders synthesized through microwave and conventional routes.

FWHM value of 2.31 eV, which is higher than that of the FWHM of conventional synthesis. This may be attributed to the formation of new chemical bonds subsequently formed during the structural changes that occurred in the microwavesynthesized sample. Another plausible reason for the increase in FWHM may be the enhanced calcium carbide formation at the high-energy end of the C 1s spectrum at a binding energy of 282.8 eV, which is one of the significant energy loss features of carbon.31 The quantitative chemical state analysis based on C 1s spectra indicate 30.75% CaC2, 51.65% graphite, 7.42% CaO, and 10.18% CaCO3 in the case of conventional heat-treated sample, whereas microwave heat-treated samples show 54.02% CaC2,13.78% graphite, 3.66% CaO, and 28.54% CaCO3. The lower values of XPS compared to XRD results may be due to its surface sensitive probing nature. Panels a and b of Figure 9 show the Ca 2p spectra collected from microwave and conventional heat-treated CaC2 powders, respectively. Spin−orbit interactions split the Ca 2p states into Ca 2p3/2 and Ca 2p1/2 sub states with binding energies of 346.32 and 349.72 eV in the case of microwave heated sample and 345.63 and 349.13 eV in the case of conventional heated sample, which are in agreement with previously reported values.30,32 Even though calcium exists in Ca2+ state, the Ca 2p3/2 spectra give information regarding the various oxidized compounds such as CaO, Ca(OH)2, and CaCO3. The peaks at 346.32, 346.70, and 347.20 eV (microwave heated sample) and 345.63, 346.83, and 347.20 eV (conventional heated sample) represent CaO, Ca(OH)2, and CaCO3 respectively.30 Rainer et al.33 reported that interpretation of a shift in binding energy of an atom in a solid material is very challenging. The shift may be caused by changes in the Fermi energy level due to variations in defect concentration or may be caused by changes in electron

It is also important to note that more conversion of CaO to CaC2 occurred not only at a lower temperature but also in a short period of time. This observation is further supported by the high intensity peak at 284.69 for conventional heat-treated sample, which represents graphite. These findings of unreacted graphite in the CaC2 powder sample point to the fact that poor conversion of graphite to CaC2 has taken place in conventional heat treatment compared to microwave heat treatment where the intensity of the peak at 284.39 is found to be lowered and shifted by 30 eV. The other two features observed in the C 1s spectra are the peaks at 285.87 and 288.49 eV, which represent CaO and CaCO3, respectively.30 The high intensity of CaCO3 peak along with low intensity of CaO peak observed in the case of microwave heat-treated sample compared to conventional heat-treated sample further indicate that microwave heat treatment enhances the conversion of CaO and graphite. Andersson et al.30 studied the oxidation behavior of the polyphenylenevinylene (PPV)/calcium interface and reported that the oxidation mechanism of the polymer starts with the formation of calcium carbide bond between the polymer and calcium at a binding energy of 289.9 eV. Because the binding energy of calcium carbide is very close to the C−C bond, it is very difficult to identify the CaC2 peak in the XPS spectra, although some identifications of CaC2 can be derived from the broadening of the C 1s peaks in the XPS spectra.30 Järvinen et al.31 investigated the core level studies of calcite and reported formation of CaCO3 at a binding energy of 289 eV in calcite. The FWHMs of carbonate peaks are 1.26 eV in conventionally synthesized calcium carbide, indicating that a surface sensitive mechanism is prevailing in conventional synthesis, which agrees with other reported FWHMs ranging from 1.1 to 1.8 eV.32,33 In contrast, the microwave-synthesized samples exhibited a G


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Figure 10. O 1s spectra collected from CaC2 powders synthesized through microwave and conventional routes.

conduction mechanism. As shown in the figure, the surface is hotter than the interior, and consequently the diffusion of CaO starts at the surface and no reaction is initiated at the center even after 30 min of heat treatment at 1700 °C. On the other hand, reaction between CaO and C was initiated throughout the volume of the microwave heated sample. The XRD quantitative analysis revealed 14.1% CaC2 yield in the case of conventional heat treatment and 71.8% CaC2 yield in the case of microwave heat treatment. The reaction enhancement and high CaC2 yield in the case of microwave heat treatment is correlated to the volumetric heating nature of microwaves. During microwave heating, the selective absorption of electromagnetic energy raises the temperature of graphite particles, which not only enhances the reaction between C and CaO to form CaC2 but also helps to form the eutectic mixture of CaO and CaC2. The microwave absorption in liquid is significantly higher than that in the solid, which in turn promotes the diffusion of CaO to form more CaC2. One can also argue that the observed high CaC2 yield in the case of microwave heat treatment is simply a thermal−kinetic effect resulting from high reaction temperature that can be rapidly achieved by microwave heating. Practical difficulties in measuring the actual reaction temperature often misleads the interpretation of the experimental results. Reliable and accurate measurement of temperature in a microwave environment is very challenging.34 Both direct and indirect methods are used to measure temperature during microwave processing. Examples for direct methods are thermocouple and optical radiation pyrometer, and indirect methods include thermally activated chemical paint sensors, impurity diffusion, oxidation, and decomposition of materials.35 Although thermocouples have been successfully used to measure temperature in conventional furnaces, its response time is too slow (100 °C/sec) of microwave furnaces.35 Arcing at the tip of the thermocouple is another problem. Even though one can suppress the arcing by using an electrically grounded sheath, it still reduces the accuracy of the thermocouple. Optical radiation pyrometer is suggested as an alternative to thermocouples due to its fast response time and immunity to electromagnetic interference.35 However, one requires the knowledge of the emissivity of the material at the operating wavelength for accurate results. Moreover, the pyrometer provides information about the surface temperature which is lower than the actual temperature at the center of the sample because of reverse thermal gradients exhibited by microwave heating. Thus, Zhu and Chen10 emphasized that comparison of microwave and

charge density on the atom due to a variation in effective coordination number of the atom.33 The other reasons could be changes in oxide structures and interface regions. Panels a and b of Figure 10 show the O 1s spectrum of the microwave and conventionally heat-treated samples, respectively. The peaks at 530.46 and 530.15 eV are attributed to calcium oxide (CaO), which are in agreement with values reported by Andersson et al.30 The low intensity of the CaO peak in the case of microwave-synthesized sample indicates that more calcium oxide is converted into calcium carbide compared to the conventionally synthesized sample. The peak at 531.63 eV in the conventional and 531.29 eV in the microwave-synthesized samples designate calcium carbonate (CaCO3), which are comparable with the 531.4 eV value reported by Blanchard et al.32 The preliminary XRD and XPS quantitative analysis results clearly indicate that microwave heat treatment enhances the reaction rate of CaO and C to form CaC2 compared to the conventional method. 3.4. Microwave Reaction Mechanism. Li et al.3 reported the reaction mechanisms of CaC2 formation from CaO and coke by an autothermal process. At the initial stage of the reaction, a solid layer of CaC2 starts to form at the interface of C and CaO above 1460 °C. The as-formed CaC2 layer prevents the diffusion of CaO into C. However, above 1660 °C, CaC2 and CaO forms a eutectic mixture which further promotes the diffusion of CaO and formation of CaC2. Figure 11 presents a schematic diagram of heating of C and CaO samples by conventional and microwave techniques. Fine graphite and CaO powder compacts were heat treated at 1700 °C for 30 min. In the case of conventional heating, heat transfer takes place from the surface to the interior of the sample through a

Figure 11. Schematic diagram showing conventional and microwave heating mechanisms of C−CaO samples. H


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temperature than the experimentally measured temperature. The XRD quantitative analysis revealed 14.1% CaC2, 55.4% unreacted graphite, and 30.5% unreacted CaO in the case of conventional heat-treated samples at 1700 °C for 30 min and 62.8% CaC2, 32.3% unreacted graphite, and 4.9% unreacted CaO in the case of conventional heat-treated samples at 1750 °C for 60 min. The microwave heat-treated samples at 1700 °C for 30 min showed 71.8% CaC2, 24.5 % unreacted graphite, and 3.7% unreacted CaO, suggesting that microwave heat treatment is more efficient than conventional heat treatment. The XPS quantitate analysis results revealed 30.75% CaC2, 51.65% graphite, 7.42% CaO, and 10.18% CaCO3 in the case of conventional heat-treated samples and 54.02% CaC2, 13.78% graphite, 3.66% CaO, and 28.54% CaCO3 in the case of microwave-heated samples. The experimental results of the present study show that microwave heating is a promising alternative route for the synthesis of CaC2 from CaO and graphite powders.

conventional heating should be performed under exactly similar conditions to avoid misinterpretation of results, which is practically very difficult. 3.5. Scale-up of Microwave Process to Synthesize CaC2. Clark36 presented an excellent overview of some recent successful scale-up and commercialization of microwave processing technologies other than food processing which includes rubber vulcanization, curing of structural wood products, and drying and sintering of ceramic white wares. The rubber vulcanization process uses a combination of microwave and hot air heating. The microwave is applied to heat the rubber rapidly to its curing temperature, and the hot air is used to keep the temperature constant throughout the curing stage. The Parallam process produces wood products with superior quality which otherwise could not be possible using conventional heating methods. The poor thermal conductivity of wood and resin restricted the thickness of conventionally produced billets to 2 in., whereas microwave produced 20 in. thick billets due to its deep penetrating capability.36 EA Technologies, U.K.,37 demonstrated an economic and commercially viable rapid process which combines microwave heating with an existing continuous process to fire large ceramic bodies with uniform microstructure and low thermal stress. Evans and Hamlyn38 reported that electrical-to-microwave power conversion efficiency of magnetrons operating at 2.45 GHz is 50% while the that of magnetrons operating at 915 MHz is 90%. Thus, one can increase the overall efficiency of microwave processing by selecting the operating system at 915 MHz instead of 2.45 GHz. Another advantage of using 915 MHz is that the penetration depth can be increased 2-fold, which in turn improves the uniformity of the fired products. The above cited examples clearly point to the fact that it is more beneficial to combine the 915 MHz based microwave heating systems with conventional gas firing or electrical resistance heating to achieve rapid uniform volumetric heating with minimum heat loss from the surface of the samples. The major benefits are significant reduction in processing time, energy, and cost. Moreover, hightemperature microwave transparent insulation is too costly and a secondary heating system will be more economical and synergistic at commercial scale. The cost of industrial microwave heating system is estimated to be US$ 4000−7000 per kW, and using standalone microwave systems for material processing is not economical.39 In the near future, a detailed reaction kinetic study will be performed to further explore the possibility of scaling up microwave technology to synthesize CaC2.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 304-293-3272. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This technical effort was performed in support of the US−China Clean Energy Research Center−Advanced Coal Technology Consortium's (CERC-ACTC) ongoing research in clean coal conversion processes under the DOE Contract DEPI0000017. The authors would like to thank Dr. Wei Ding and Dr. Marcela Redigolo for their assistance in the WVU Shared Research Facilities. Mr. Patrick Iyere and Mr. Aneeruddha Bulbule are also thankfully acknowledged for their contributions to the original work.

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REFERENCES

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4. CONCLUSIONS Calcium carbide was prepared from CaO and graphite powders in a microwave reactor in less than 180 min compared to 1200 min in a conventional tube furnace. Microwaves very efficiently coupled with the graphite powder through the activation of the weak interlayer bonds, and the resonance condition for microwave absorption is met through the excitation of stateto-state rotational modes. The fast heating rate in the microwave method was favorable in producing powders with small particle sizes compared to those produced by conventional methods. The CaC2 powder prepared through the microwave route showed both monoclinic and tetragonal crystal structure, whereas conventional heat-treated samples exhibited monoclinic crystal structure, indicating that the actual reaction occurred in the microwave furnace at a higher I


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Solid-State Synthesis of Calcium Carbide by Using 2.45 GHz

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