one-step synthesis of a 12cao7al2o3 electride via the spark

8/10/2019 One-Step Synthesis of a 12CaO7Al2O3 Electride via the Spark

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Electrochemical and Solid-State Letters,14(12) E41-E43 (2011) E411099-0062/2011/14(12)/E41/3/$28.00The Electrochemical Society

One-Step Synthesis of a 12CaO 7Al2O3Electride via the SparkPlasma Sintering (SPS) Method

Jun Ho Chung,a Jeong Ho Ryu,b, Jong Won Eun,a Bong Geun Choi,a and Kwang Bo Shima, z

aDivision of Materials Science and Engineering, Hanyang University, Seongdong-Gu, Seoul 133-791, South KoreabDepartment of Materials Science and Engineering, Chungju National University, Chungji-si, Chungbuk 380-702,South Korea

Polycrystalline C12A7 electrides were successfully synthesized via spark plasma sintering (SPS) process. C22 ions, generated fromgraphite by the spark plasma, presumably serve as a template for the formation of the C12A7 phase during the re-crystallizationprocess and are spontaneously released from the cage of C12A7 during the SPS process, remaining mobile electrons in the cage.The polycrystalline C12A7 electride processed at 1100C exhibits an electrical resistivity of 1.7 101 cm1, an electronconcentration of 5.3 1019/cm3 and optical absorptions of 0.4 eV and 2.7 eV at room temperature. 2011 The Electrochemical Society. [DOI: 10.1149/2.021112esl] All rights reserved.

Manuscript submitted July 29, 2011; revised manuscript received September 8, 2011. Published October 25, 2011.

Electrides are ionic materials in which an electron occupies acrystallographic site, acting as an anion, the structural features of

which allow them to serve as a promising material for various appli-cations, such as reducing agents, cold-cathode electron field emittersand refrigeration devices.14 However, most electrides are composedof organic materials and are unstable at room temperature. These in-stabilities have restricted the practical applications of such material.5

In order to overcomethese instabilities, inorganic electrides withgoodstability at room temperature have been studied. Hosonoet al. reportedthe fabrication of a 12CaO 7Al2O3(C12A7) inorganic electride by re-placing the extra oxygen ions in the framework with electrons.6

C12A7 is a constituent of alumina cement that has an uniquecrystal structure, composed of 12 sub-nanometer-sized cages with aninner free space diameter of 0.4 nm connected to a neighboring cagethrough shared six-atom rings consisting of Ca-O-Al-O-Al-O.6 Theunit cell, called the framework, is composed of positively-charged[Ca24Al28O64]4+. To compensate for the positive charge of the frame-

work, two O2 ions, free oxygen ions, are loosely bound in the cages(C12A7:O2). These free oxygen ions can be substituted with mono-valent ions, such as halogen ions (F, Cl),7,8 hydroxide ions (OH)9

or super radical hydrogen ions (H),10 and converted to electride byreplacing free oxygen ions with electrons.6

The C12A7 electride has been fabricated via metal vaportreatment6,11 or a melt-solidification process.12,13 However, the metalvapor treatment requires a single crystal C12A7:O2 and long an-nealing time, and the melt-solidification process employs a complexprocedure for the decomposition of C12A7:O2 to C3A+CA andrecrystallization to C12A7 electride. In order to overcome these diffi-culties, theSPS process is proposed fora simpler andfaster fabricationprocess of the C12A7 electride.

TheSPS process is a fast sinteringtechnique where thepulsed elec-

tric currents induce joule heating and spark plasma to pass through thesurfaces of the loaded powders, which rapidly increases the heatingrate and drastically reduces the reaction time.14,15 During the SPS pro-cess, a carbon-rich atmosphere can be made by graphite die, the sparkplasma can activate the carbon ions, and the electric field can promotediffusion of the activated carbon ions into the samples. In addition,rapid joule heating can maintain a homogeneous temperature.1618

In this work, we firstly demonstrate the SPS techniques as a sim-ple and fast synthetic route for fabrication of C12A7 electrides. Theelectrical and optical properties of the SPS processed C12A7 electridewas evaluated, and the electron replacement mechanism of the SPSprocess was presented in detail.

Electrochemical Society Active Member.z E-mail: [email protected]

Experimental

The starting C12A7 powder was prepared using a conventionalcitrate-gel method. Synthesized C12A7 powders (1 g) were loadedinto a cylindrical graphite die with a diameter of 15 mm and heatedusing the SPS (Dr. Sinter model R, SPS-2080, Japan) process within atemperature range of 900 to 1100C, with a heating rate of 100C/minand an uni-axial pressure of 40 MPa in vacuum (106 Torr). Thesamples were held for 10 min at each temperature. The direct currentwas 1500 A with a pulsed duration of 12 ms and a pulse interval of2 ms. After SPS process, the samples are grinded and polished forremoved carbon films on the surface using SiC abrasive paper. Thethickness samples were about 2 mm.

Phase analysis was performed using an X-ray diffractometer(Rigaku D/MAX2C, Japan, Cu- (=1.5046 )). Optical absorptionspectra were measured by UV-Vis spectroscopy (UV 3600, Shimadzu,Japan) in a range of 0.4 - 4.0 eV, and Raman spectra were measured

(NRS-3000, laser ( = 532nm)) in a range of 4002000 cm1

atroomtemperature. The electron concentration and electrical resistivityweremeasured usingfour-probe typeHallmeasurement (RESISTEST8300, DongYang Tech, Japan).

Results and Discussion

Photographs of the C12A7 samples at different processing temper-atures are presented in Figure 1. The colors of the samples changeddepending on the temperature; white at 900C, gray at 1000C anddark green color at 1100C. This change was suggested to be inducedby the optical absorption level (2.7 eV) generated by the electrons thatreplaced the free oxygen ions.20

Figure 2 shows optical absorption results of the C12A7 samples

depending on the processing temperature. The sample processed at900C showed no specific absorption peak; however, the samplesprocessed at 1000C and 1100C showed absorption peaks near the0.4 and 2.7 eV regions, indicating electron trapping in the C12A7

Figure 1. Photograph of C12A7 specimens via SPS at different processingtemperatures.

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E42 Electrochemical and Solid-State Letters,14(12) E41-E43 (2011)

4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0

0

1

2

3

4

5

Absorption(Kubelak

a-Munk)

Phothon Energy(eV)

2.7 eV 0.4 eV

900oC

1000oC

1100oC

Figure 2. UV-Vis spectra of the C12A7 specimens processed via SPS at

different processing temperatures.

cage structure.20 The energy level of the 2.7 eV peak reflects the colorchange from white to dark green, as shown in Figure 1. The opticalabsorption levels of 0.4 eV and 2.7 eV can be caused by an inter-cagetransition (0.4 eV) and intra-cage transition (2.7 eV), respectively.2022

Figure 3 shows the electric resistivities and electron concentra-tions of the C12A7 samples at different processing temperatures.The electric resistivity of the C12A7 sample processed at 900C was3.2 1011 cm, indicating a perfect insulator. The electric resis-tivities of the samples processed at 1000C and 1100C dramaticallydecreased to 2.6 102 and 1.7 101 cm1, respectively. Whilethe electron concentrations of the samples processed at 900C were6.1 108/cm3, those of the C12A7 samples processed at 1000C and

1100

Cwere3.2 1017

and 5.3 1019

/cm3

, respectively. The resultsof the sample processed at 1100C were comparable with previousresults19 and confirmed that the SPS process can successfully convertC12A7 to C12A7 electrides.

Figure 4a represents the XRD results of the C12A7 samples de-pending on the processing temperature. Compared with the powdersample, the SPS processed sample at 900C showed mixed phasescomposed of 3CaO Al2O3(C3A) and CaO Al2O3(CA) with theC12A7 phase. Increasingprocessing temperature to 1000Cdecreasedthe peak intensities of the C3A+CA phases, which are completely dis-appeared at 1100C, indicating that the C12A7 phase decomposed to

900 1000 110010

-2

101

104

107

1010

1013

Sintering Temp(oC)

ElectricResistivity(c

m)

105

109

1013

1017

1021

Electronconcentration(cm

-3)

3.2 x 1011cm

2.6 x 102cm

1.7 x 10-1cm

3.2 x 1017/cm3

5.3 x 1019/cm3

6.1 x 108/cm3

Figure 3. Electric resistivity and electron concentration of the C12A7 speci-mens at room temperature (300 K).

Figure 4. (a) X-ray diffraction patterns and (b) raman spectroscopy of theC12A7 specimens produced via SPS at different processing temperatures.

C3A+CA phases at 900C and re-crystallized at 1100C during theSPS process. Raman spectra of the SPS processed samples are shownin Figure 4b. At 900C, the CaC2 band at 1870 cm1, which is at-tributed to C3A+CA phases, was formed through the decompositionof the C12A7 phase. The CaC2 band was still present at 1000C;however, its intensity decreased compared to that at 900C due to adecrease in the amount of C22 ions and the gradual re-crystallizationfrom C3A+CA phases to the C12A7 phase. This CaC2 band disap-peared at 1100C because the decomposed C3A+CA phases werecompletely re-crystallized to C12A7 phase. These results confirmedthat the SPS processed specimen at 1100C was successfully con-verted to the C12A7 electride.

Figures 5a5d schematically show the fabrication procedures ofthe C12A7 electride during the SPS process. In the SPS system, thegraphite die filled with C12A7 powders was placed between the lowerand upper electrodes, as shown in Figure 5a. An external DC pulseddischarge power source provided the electric current which generatedthe electric field.1618 Through charging and discharging processes bythe electric current, a high temperature spark plasma was momentar-ily generated between the surfaces of the C12A7 particles23,24 and

then activated carbon ions like C22

from thegraphite. Therefore,C12A7 particles have locally high temperatureregions about severalthousandC, which can be decomposed from C12A7 to C3A+CA, as

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Electrochemical and Solid-State Letters,14(12) E41-E43 (2011) E43

Figure 5. Schematics of a one-step fabrication route for C12A7 electride using the PECS process. (a) The C12A7:O2 powders were placed between two graphiterams in a cylindrical graphite die, (b) decomposition from C12A7 phase to C3A+CA phases due to spark plasma-induced electric current, (c) diffusion of carbonions (C22) into decomposed C3A+CA phases and re-crystallization from C3A+CA to C12A7:C22 by C22 ions and (d) transition from C12A7:C22 toC12A7:e and emission of carbon ions as 2C or 2CO into the atmosphere.

shown in Figure 5b. The decomposed C3A+CA phases were com-bined with C22 ions diffused into samples by the electric field andre-crystallized to the C12A7 phase by C22 ions, as shown in Figures5c5d according to the following reaction,

C3A+CA+C22 C12A7 : C2

2 [1]

As shown in Figures 4a, processed below 1000C, the decom-posed C3A+CA phases were not perfectly re-crystallized to C12A7because of insufficient temperatures for re-crystallization. However,above 1100C, only a single C12A7 phase existed, indicating that thedecomposed C3A+CA phases were perfectly re-crystallized to theC12A7 phase, as shown Figure 5d. During the SPS process, C22 ionsare diffused into the C3A+CA phases by the electric field, and thenthe decomposed C3A+CA phases can be re-crystallized to the C12A7

phasebyC22 ions. Therefore, the C22 ions instead of O2 ions serveas a template for compensating positive charges of [Ca24Al28O64]4+

because the ionic radius of the C22 ion (1.2 ) is similar to thatof O2 (1.4 ).19 The C22 ions trapped in the cages of the C12A7were only stable in the initial stage of re-crystallization and were thenreleased from the cages during the re-crystallization procedure. In thisprocess, the electrons were generated in cages of the C12A7 via thefollowing reactions,

C22 (graphite) C2

2 (cage) 2C (solid) + 2e (cage) [2]

and/or

C22 (cage)+ 2O2

2 (cage) 2CO(atmosphere)+ 6e (cage)[3]

Through above process, C22 ions generate electrons to the cages

and release 2C or 2CO into the atmosphere.19

Conclusions

Polycrystalline C12A7 electride was successfully fabricated viatheSPSprocess.Theopticalabsorptionbandsat0.4eVand2.7eVandelectric properties such as electric resistivity and electron concentra-tion confirmed that C12A7:O2 was successfully converted to C12A7electride. The C12A7 electride synthesized via the one-step SPS pro-cess is attributable to the carbon-related anion, C22, generated byspark plasma induced by the charging-discharging process. The C22

ion serves as the template anion, instead of the O2 anions, to stabi-lize the C12A7 phase and then is removed from the cage, remainingelectrons in the cages of C12A7 during the re-crystallization process.

The resultant polycrystalline C12A7 electride showed an electricalresistivity and electron concentration of 1.7 101 cm1 and5.3 1019 /cm3, respectively. Therefore, we concluded that the SPSprocess is an effective and simple technique for the fabrication of aC12A7 electride.

References

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one-step synthesis of a 12cao7al2o3 electride via the spark

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