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Synthesis and characterization of nanoparticles of wurtzite aluminum nitride from various nut shells
Journal of Alloys and Compounds 708 (2017) 67e72
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Synthesis and characterization of nanoparticles of wurtzite aluminum nitride from various nut shells S.B. Qadri*, E.P. Gorzkowski, B.B. Rath, C.R. Feng, R. Amarasinghe Materials Science and Component Technology Directorate, Naval Research Laboratory, Washington D.C., USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 October 2016 Received in revised form 8 February 2017 Accepted 1 March 2017 Available online 2 March 2017
Nanoparticles of Aluminum Nitride (AlN) were synthesized from a thermal treatment of mixtures of aluminum oxide (Al2O3) and either shells of almond, cashew, coconuts, pistachio, or walnuts in a nitrogen atmosphere at temperatures in excess of 1450 C. By selecting the appropriate ratios of each nutshell powder to Al2O3, it is demonstrated that stoichiometric aluminum nitride is produced via carbothermal reduction in nitrogen atmosphere. In addition, results show the formation of Al from Al2O3 before transformation to AlN. On the other hand, when Al was mixed with nutshell powder first, mixed phases of AlN and Al4C3 were formed before complete transformation to AlN. X-ray diffraction analysis, Raman scattering and Fourier Transform Infrared spectroscopy confirmed the wurtzite phase of aluminum nitride. Transmission electron microscopy indicated the formation of AlN nanoparticles. The formation of AlN from nutshells offers a simple route and avoids multiple-step processes involving carbon rich agents at elevated temperatures. © 2017 Published by Elsevier B.V.
Keywords: Aluminum nitride X-ray diffraction Agriculture waste Raman spectroscopy Wide band gap semiconductors
1. Introduction Aluminum nitride belongs to a class of III-N materials and is a wide gap semiconductor with band gap between 6.01 and 6.05 eV at room temperature [1]. It exhibits both wurtzite and zincblende phases as do other III-N compounds and has many potential applications in microelectronics due to its relatively high thermal conductivity (70e210 W m1 K1 to 285 W m1 K1) [2]. Additionally, it has unique physical properties including high electrical resistivity, low thermal expansion, resistance to erosion and corrosion, excellent thermal shock resistance and chemical stability in air up to 1380 C with surface oxidation occurring at 780 C. Moreover, epitaxially grown thin films of crystalline aluminum nitride are used for surface acoustic wave sensors (SAWs) deposited on silicon wafers because of AlN's piezoelectric properties [3]. Another important application for AlN is as an RF filter, also called a thin film bulk acoustic resonator (FBAR), which is widely used in mobile phones [4]. AlN in bulk form is synthesized by the carbothermal reduction of aluminum oxide in the presence of gaseous nitrogen or ammonia or by direct nitridation of aluminum. In order to get fully dense form, Y2O3 or CaO are required as additives during the hot pressing. * Corresponding author. E-mail address: [email protected] (S.B. Qadri). http://dx.doi.org/10.1016/j.jallcom.2017.03.003 0925-8388/© 2017 Published by Elsevier B.V.
There are two types of agriculture waste, one containing silica and carbonaceous matter and the other containing mostly carbonaceous matter and no silica. Among the first type, the prominent ones are rice husk, wheat husk, corn husk, sorghum leaves and peanut shells. We have demonstrated in our previous work that these are useful in the synthesis of industrially important materials such as SiO2, SiC, Si3N4, and zinc silicate by pyrolizing them in air, argon or in nitrogen atmospheres [5e11]. The second type of agriculture waste which contain only carbon matter and no silica are nutshells such as almond, walnuts, pistachio, coconuts, macadamia, and cashew. Billions of pounds of nutshells, which are currently discarded as waste products, are available if they can be harnessed in the synthesis of industrially important materials. Various tree nut shells are frequently used for organic composts, animal feedstock, or discarded as trash [13]. The present study demonstrates a more effective and practical way of utilizing tree nut shells by utilizing them as a carbon source in synthesizing AlN, that will have widespread industrial implications. Recently, it was reported that mixed phases of SiC and Si3N4 can be produced by carbothermal reduction and nitridation of a mixture of silica and macadamia powder [12]. We demonstrated in our previous work that by adding ZnO to powder of wheat or rice husk, pure zinc silicate is produced with photo-luminescent properties [11]. In the present paper, we report on the formation of AlN by adding nanocrystalline powders of Al2O3 and Al to the
1000
2
6
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2
4
Energy (keV)
MnKα
FeKβ NiΚα
FeΚα
400
CuKα
ZnKα
600
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CuKα
FeKβ NiΚα
ZnKα
FeΚα
4
CrKα MnKα
200
CaKβ
400
CaKα, Κβ
KKα
600
800
ArKα KKα CaKα, Κβ CaKβ TiKα
Intensity (arbitrary units)
800
0
Coconut
Almond
ArKα
Intensity (arbitrary units)
1000
6 Energy (keV)
8
1000
1000
Walnut
2
4
Energy (keV)
0
8
10
2
4
ZnKα CuKα
FeKβ
6
NiΚα
CrKα MnKα
FeΚα
CaKα, ΚΚβ
400
CaKβ
ZnKα
6
600
200
CuKα
FeΚβ NiKα
CrKα MnKα
250
FeKα
500
800
ArKα KKα
Intensity (arbitrary units)
750
ArKα KKα CaKα, Κβ CaKβ
Intensity (arbitrary units)
Pistachio
0
10
8
10
Energy (keV)
Fig. 1. Energy dispersive x-ray fluorescence of almond, coconut, macadamia, and walnut showing the presence of trace amounts of K, Ca, Cr, Mn, Fe, Ni, Cu, and Zn.
Fig. 2. X-ray diffraction scans of Al2O3 mixed with selected nut shell powders along with Rietveld whole profiles analysis of the diffraction patterns for the AlN sample derived from nut shells powder after pyrolising in a nitrogen atmosphere followed by treatment in air at 800 C.
S.B. Qadri et al. / Journal of Alloys and Compounds 708 (2017) 67e72
69
Table 1 Lattice parameters and Crystallite sizes for AlN derived from various nuts. Agricultural source
Lattice parameters (Å)
Coconut Pistachio Almond Walnut Macadamia
a a a a a
¼ ¼ ¼ ¼ ¼
3.112(1); 3.111(1); 3.112(1); 3.112(1); 3.113(1);
b b b b b
¼ ¼ ¼ ¼ ¼
Phase
3.112(1); 3.111(1); 3.112(1); 3.112(1); 3.113(1);
c c c c c
¼ ¼ ¼ ¼ ¼
4.981(1); 4.980(1); 4.982(1); 4.982(1); 4.982(1);
a ¼ 90; b ¼ 90; g ¼ 120 a ¼ 90; b ¼ 90; g ¼ 120 a ¼ 90; b ¼ 90; g ¼ 120 a ¼ 90; b ¼ 90; g ¼ 120 a ¼ 90; b ¼ 90; g ¼ 120
powdered nut shells of almond, walnut, coconut, macadamia, pistachio and cashew and pyrolising them in nitrogen atmosphere at 1400 to 1500 C. Energy dispersive X-ray spectroscopy showed pure AlN is formed. The x-ray diffraction and Rietveld analysis and Raman spectroscopy analysis of the pyrolised samples showed formation of pure wurtzite phase of AlN. In addition, transmission electron microscopy confirmed the nanocrystallinity of AlN.
1.1. Experimental details Samples were prepared from powders of raw nuts of almonds, coconuts, macadamia, pistachios, and walnuts after mixing them with nanocrystalline Al2O3 powder using ball milling with a SPEX 8000 M including stainless steel milling media. The Al2O3 sample along with the specific nut shell was combined and milled to obtain a uniform powder. A hydraulic press was used to pressurize the homogenous powder into 1 cm diameter disks with a 2.5e3 mm depth. The pellets were heat-treated (pyrolised) in a convention furnace at temperatures exceeding 1400 C for an interval of 5e6 h in a nitrogen atmosphere. In order to eliminate the residual carbon, the pellets were then placed in air at 670 C. In addition, the samples were given HF treatment and H2O2 rinsing to remove trace amounts of metal impurities that were present in the starting material. XRD scans were made using a Rigaku 18 kW rotating anode generator and a high resolution powder diffractometer. The diffraction scans were collected using monochromatic CuKa radiation. Energy dispersive x-ray fluorescence measurements were performed using a Thermo Electron Corporation system using white radiation from Rh target and a Pd filter of appropriate thickness. Raman spectra were collected on an inVia Raman Microscope (Renishaw) using a 514 nm laser line. Scans were obtained at ca. 15 mW laser power at the sample and an integration period of
E2 (high)
Intensity (arbitrary units)
656 cm-1
E1 (TO) 669 cm-1 A1 (TO) -1
896 cm 400
500
600
700
800
mc mc mc mc mc
Crystallite sizes (nm) (186) (186) (186) (186) (186)
31.3 (2.1) 33.9 (1.9) 30.1(1.5) 32.6(1.7) 25.2 (1.5)
30 s. Fourier Transform Infrared (FTIR) spectra were collected using Thermo Scientific Nicolet FT-IR spectrometer with Diffuse Reflectance Infrared Transform Spectroscopy (DRIFTS) accessory. In order to conduct the TEM analysis, ethyl alcohol was mixed with the pyrolyzed sample; the mixture was then set in an ultrasonic cleaner. A carbon covered 200 mesh copper grid was submerged into the mixture to collect AlN particles. A FEI Tecnai G2 TEM was utilized to examine the sample at 300 kV. Energy dispersive x-ray spectroscopy (EDS) was used as an analytic technique to determine elemental composition on JEOL scanning electron microscope (SEM). 2. Results and discussion The samples were prepared in the shape of circular pellets from the milled nuts by applying uniaxial pressures in excess of 2.0 GPa in a hydraulic press using a 1 cm diameter die. The elemental compositions were analyzed using energy dispersive x-ray fluorescence spectra and are presented in Fig. 1. All the nut shells showed trace amounts of K, Ca, Cr, Mn, Fe, Ni, Cu, and Zn with a slight variation between them. The Ar peak is present due the measurement performed in the air environment. The nut powders, each weighing 1 g, were mixed with 0.1 g of 50 nm nanocrystallites of Al2O3 and then pyrolised in nitrogen atmosphere in excess of 1450 C. Following the heat treatment in nitrogen atmosphere, the samples were treated in air at 800 C to remove any excess unreacted carbon. X-ray diffraction scans were taken of the asprepared pellet samples of each mixed nut and alumina powder prior to any heat treatment and showed peaks from the corundum phase of Al2O3 as given in PDF#04-002-8135. The crystallite sizes for Al2O3 as determined from Rietveld analysis of the diffraction pattern were of the order of 47.3(1.3) nm. The as-prepared samples also showed the presence of amorphous peak due to carbonaceous carbon present in the shell. The x-ray diffraction analysis of the heat processed samples showed peaks corresponding to only wurtzite phase of AlN with a space group of P63mc (186) [14]. A Rietveld analysis of the diffraction scans of the processed samples gave Rfactors in the range 4e6% and are shown in Fig. 2 for selected nut shell samples. The observed and calculated intensity profiles are displaced with respect to each other to illustrate the goodness of the fit and the difference between the observed and calculated profiles are given at the lower portions of the plots. The lattice parameters, crystallite size and the structural parameters of the AlN obtained for various nut shells are given in Table 1. The crystallite sizes varied in the range between 25 and 34 nm for all AlN samples
E1 (LO)
A1 (LO)
616 cm-1
P63 P63 P63 P63 P63
905 cm-1
900
1000
-1
Wavenumber (cm ) Fig. 3. Raman spectra of AlN derived from pistachio showing the different Raman active modes.
Table 2 Raman frequencies observed for AlN in comparison to literature values. Reference
A1(TO)
A1(LO)
E1(TO)
E1(LO)
E22
Pistachio (This Study) Almond (This Study) Ref. [15] Ref. [16] Ref. [19]
616 616 612, 608 614 610
896 896 898, 890 893 890
669 669 665, 660 673 669
908 905 910, 902 916 911
656 656 654, 655 660 656
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1.0
Pistachio 0.8
Absorbence (a. u.)
0.6
0.4
0.2
0.0 400
600
800
1000
1200
1400
1600
1800
2000
-1
Wavenumber (cm ) Fig. 4. FTIR spectra of AlN derived from pistachio showing a broad band of 699 cm1.
derived from various nut shells. The formation of the wurtzite phase of AlN was confirmed using Raman spectroscopy. Fig. 3 illustrates the Raman spectra for AlN derived from pistachio powder. All the Raman modes are labelled with their corresponding values. The observed active Raman modes for AlN derived from pistachio and almond shells are listed in Table 2 along with the literature values. These values for Raman modes are in agreement with the wurtzite phase values reported in the literature [15,16]. The Raman scattering in solids is attributed to the inelastic scattering of photons by the crystal lattice vibrations (optical phonons). The Raman frequency can slightly differ from its ideal value due to factors such as stress, interfaces, structural defects, impurities, and the crystallite size. It is specially manifested in oriented films grown on different substrates such as Si and silica [15]. We find slight difference in E1 mode for pistachio and almond derived AlN that is attributed to small difference in the crystallite sizes. The IR spectrum of the AlN sample from pistachio nut sample is shown in Fig. 4. The data was obtained after normalization. The most prominent infrared active phonon band was observed at 699 cm1 for the pistachio sample and 714 cm1 for the almond sample. These bands are in contrast to what was observed at 698 cm1 for bulk AlN powder. The slight shift in the case of almond could be attributed to the crystallite size of 30 nm compared to
34 nm for pistachio. This shift in peak position is consistent with the shifts reported by Balasubramanian et al. for nanoparticles and nanotubes of AlN [17,18]. The transmission electron micrographs for almond and walnut shells are presented in Fig. 5 showing the nanodimensionality of the AlN crystallites. The average crystallite size is of the order of 30 nm in reasonable agreement with the x-ray measurements. The energy dispersive x-ray spectra of the post processed samples are given in Fig. 6 along with that of pure commercial AlN obtained from commercially from Aldrich chemical company. The spectra show peaks corresponding to only Al and N and absence of any of the trace elements which were present in the as-prepared samples. The atomic % content of Al and N were calculated from the peak intensities in comparison with pure AlN sample. In the case of pure AlN from the Aldrich Chemical Company source the ratio of Al:N was 70:30 in comparison to 72:28 and 69:31 for AlN samples from almond and walnut source, respectively. The results indicates that the ratio of Al:N in our samples is nearly 1:1 ratio. Moreover, we found that the processed samples did not contain any carbon or oxygen peaks or any trace amounts of K,Ca, Zn, Cr, Mn, Fe, Ni, and Cu which were present in the starting samples. The full EDS spectrum of AlN is shown in Fig. 7. This indicates that our processing results in the formation of pure AlN without any impurities and is as good as the commercially available AlN. In order to determine the pathways to the final product of AlN, we performed experiments with Al2O3 and Al powders, each mixed with pistachio powder. Both these samples were pyrolised in N2 atmosphere for 1 h. Fig. 8 shows an overlay of the diffraction scans of Al2O3 sample in which we observed the formation of mostly Al and a small quantity of AlN indicating that Al melt forms first which then reacts with free nitrogen to form AlN. A full 6 h period of heating in nitrogen resulted in AlN wurtzite phase. This indicates that the sequence of transformation to AlN in the case of Al2O3 goes as Al2O3 / Al / AlN. Fig. 9 shows an overlay of diffraction scans for Al-pistachio mixed sample where we found that in the first stage Al4C3, Al2O3 and AlN phases are formed and the 6 h heating in N2 atmosphere resulted only in the wurtzite AlN phase. The presence of crystalline corundum phase of alumina is due to a thin oxide layer that will be present for Al powder and is usually in the amorphous state and is not manifested in the diffraction scan. However, it becomes crystalline as we heat treat the Al powder. The Al2O3 eventually converts to AlN by carbo-thermal reduction and nitridation process. On the other hand, AlN has lower total energy compared to Al4C3 and therefore as an over time it also converts to AlN. Thus for Al mixed pistachio powder sample the sequence of transformation appears to follow Al/(Al4C3þAlN þ Al2O3) / AlN.
Fig. 5. TEM micrographs of AlN from almond and walnut samples showing nanocrystallites of the order of 25e50 nm.
S.B. Qadri et al. / Journal of Alloys and Compounds 708 (2017) 67e72
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Fig. 6. Selected EDX spectrum of pure AlN (Aldrich Company), AlN from walnut, almond and pistachio nut shells showing the peaks from Al and N.
The mechanism of AlN formation through carbothermal reduction of Al2O3 in N2 atmosphere has been discussed by various authors [20e22] and takes place at much elevated temperatures of 1700e200 C based on the following equation:
Al2O3 þ 3C þ N2 / 2AlN þ 3CO AlN has two polymorphs wurtzite and zinc-blende cubic similar to other III-N nitrides such as GaN and InN. The wurtzite is the more
Intensity (arbitrary units)
8000
6000
4000
2000
0
2
4
6
8
Energy (keV) Fig. 7. Full EDX spectrum of post processed AlN sample from almond showing absence of trace elements found in Fig. 1 of as prepared sample. Only Al and N peaks are observed.
Fig. 8. An overlay of x-ray diffractions of Al2O3-pistachio mixed sample. (a) diffraction scan of as-prepared samples showing the corundum phase (b) diffraction scan after heating in nitrogen atmosphere for 1 h showing peaks from AlN wurtzite phase and Al (c) complete transformation to AlN wurtzite phase after 6 h.
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spectroscopy and FTIR studies confirmed the wurtzite phase of AlN whereas the TEM results indicated that the crystallite sizes are of the order of 30 nm. In the case in the case of Al2O3 the sequence of transformation proceeds as Al2O3/Al/AlN. Similarly, for Al mixed pistachio powder sample the sequence of transformation follows Al/(Al4C3þAlN þ Al2O3)/AlN. This nanostructure of wurtzite AlN will be very useful in a variety of engineering applications and in opto-electronics as heat sink/electrically insulating material.
Acknowledgments We like to thank to K.P. Fears of NRL, Chemistry Division, for helping us get the Raman data.
References
Fig. 9. An overlay of x-ray diffractions of Al-pistachio mixed sample. (a) diffraction scan of as-prepared samples showing Al peak (b) diffraction scan after heating in nitrogen atmosphere for 1 h showing peaks from Al4C3 (PDF#01-071-3787) marked with an asterisk(*), wurtzite AlN phase and Al2O3 (corundum phase). (c) Complete transformation to AlN wurtzite phase after 6 h.
stable of the two configurations. Developments in film growth technology have led to advanced devices consisting of AlN and GaN such as blue-green lasers, light emitting diodes (LEDs), ultraviolet photodetectors, field effect transistors (FET), and acoustic wave devices. In addition, AlN has large electromechanical coupling coefficient and exhibit piezoelectric properties in conjunction with GaN which are uniquely suitable for high temperature piezoelectric and acoustic devices. AlN although can be synthesized by various routes. The most widely used methods for AlN production are direct nitridation and carbothermal reduction and followed by nitridation. Each of these methods involves several steps including the ignition of Al powder and several milling steps. In this paper we have established that AlN can easily be produced by using a natural source of carbon from nut shells and mixing them with alumina. In this process, both direct nitridation and carbothermal reduction followed by nitridation are built in. The simplicity of this technique results in less energy consumption and is less labor intensive. Thus, we have demonstrated that large quantities of pure wurtzite phase of nanocrystalline AlN can be obtained from the nut shells of almond, coconut, pistachio, macadamia, walnut and cashew. 3. Conclusions Nanoparticles of wurtzite AlN are obtained from the mixture of Al2O3 or Al powders with powdered shells of almond, coconut, pistachio, macadamia, walnut and cashew by pyrolysis in nitrogen atmosphere at 1400 C. The formation of pure AlN from nutshells offers a simple route involving carbon rich agents at elevated temperatures of 1700e2000 C. X-ray diffraction, Raman
[1] H. Yamashita, K. Fukui, S. Misawa, S. Yoshida, J. Appl. Phys. 50 (2) (1979) 896e898. [2] T.B. Jackson, A.V. Virkar, K.L. More, R.B. Dinwiddie, R.A. Cutler, J. Am. Ceram. Soc. 80 (6) (1997) 1421e1435. [3] R. Aigner, S. Marksteiner, L. Elbrecht, W. Nessler, TRANSDUCERS, solid-state sensors, actuators and microsystems, in: 12th International Conference on, vol. 1, 2003, pp. 891e894. ve, [4] T. Aubert, O. Elmazria, B. Assouar, E. Blampain, A. Hamdan, D. Gene S. Weber, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 59 (5) (2012) 999e1005. [5] S. B. Qadri, A. Fliflet, M. A. Imam, B.B Rath, and E.P. Gorzkowski, Silicon Carbide Synthesis from Agricultural Waste. USPatent (2013) 20130272947 A1. [6] S.B. Qadri, A. Fliflet, M.A. Imam, B.B. Rath, E.P. Gorzkowski, Silicon Carbide Synthesis, 2013. US20140287907 A1. [7] S.B. Qadri, E.P. Gorzkowski, M.A. Imam, A. Fliflet, R. Goswami, H. Kim, B.B. Rath, Industrial Crops Prod. 51 (2013) 158e162. [8] S.B. Qadri, E.P. Gorzkowski, B.B. Rath, J. Feng, S.N. Qadri, H. Kim, M.A. Imam, J. Appl. Phys. 117 (2015) (2015), 044306-044306. [9] S.B. Qadri, B.B. Rath, E.P. Gorzkowski, J. Wollmershauser, C. Feng, J. Appl. Phys. 118 (2015) 104904. [10] S.B. Qadri, M. Imam, A. Fliflet, B.B. Rath, R. Goswami, J. Caldwell, J. Appl. Phys. 111 (2012) 073523. [11] S.B. Qadri, E.P. Gorzkowski, B.B. Rath, C.R. Feng, R. Amarasinghe, J.A. Freitas Jr., J.C. Culbertson, J.A. Wollmershauser, Submitted to Industrial Crops & Products, 2016. [12] R. Rajarao, V. Sahajwalla, J. Clean. Prod. 133 (2016) 1277e1282. [13] R.L. Smith Jr., R.M. Malaluan, W.B. Setianto, H. Inomata, K. Arai, Bioresour. Technol. 88 (1) (2003) 1e7. [14] M. Von Stackelberg, K.F. Spiess, Phys. Chem. Abt. A Chem. Therm. Kin. Elek., Eig. v175 (1936) 140. [15] M.S. Liu, K.W. Nugent, S. PraweR, L.A. Bursill, J.L. Peng, Y.Z. Tong, P. Jewsbury, Int. J. Mod. Phys. B 12 (19) (1998) 1963e1974. [16] L.E. McNeil, M. Grimsditch, R.H. French, J. Am. Ceram. Soc. 76 (5) (1993) 1132e1136. [17] C. Balasubramanian, S. Bellucci, G. Cinque, A. Marcelli, M.C. Guidi, M. Piccinini, A. Popov, A. Soldatov, P. Onorato, J. Phys. Condens. Matter 18 (33) (2006) S2095. [18] K. Jagannadham, A.K. Sharma, Q. Wei, R. Kalyanraman, J. Narayan, J. Vac. Sci. Technol. A 16 (5) (1998) 2804e2815. [19] M. Kuball, J.M. Hayes, A.D. Prins, N.W.A. Van Uden, D.J. Dunstan, Y. Shi, J.H. Edgar, Appl. Phys. Lett. 78 (6) (2001) 724e726. [20] P. Lefort, M. Billy, J. Am. Ceram. Soc. 76 (1993) 2295e2299. [21] Y. Baik, K. Shanker, J.R. McDermid, R.A.L. Drew, J. Am. Ceram. Soc. 77 (1994) 2165e2172. [22] M.A. Rhamdhani, M.A. Dewan, G.A. Brooks, B.J. Monaghan, L. Prentice, Mineral processing and extractive metallurgy, Trans. Inst. Min. Metall. C 122 (2) (2013) 87e104.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Synthesis and characterization of nanoparticles of wurtzite aluminum nitride from various nut shells S.B. Qadri*, E.P. Gorzkowski, B.B. Rath, C.R. Feng, R. Amarasinghe Materials Science and Component Technology Directorate, Naval Research Laboratory, Washington D.C., USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 October 2016 Received in revised form 8 February 2017 Accepted 1 March 2017 Available online 2 March 2017
Nanoparticles of Aluminum Nitride (AlN) were synthesized from a thermal treatment of mixtures of aluminum oxide (Al2O3) and either shells of almond, cashew, coconuts, pistachio, or walnuts in a nitrogen atmosphere at temperatures in excess of 1450 C. By selecting the appropriate ratios of each nutshell powder to Al2O3, it is demonstrated that stoichiometric aluminum nitride is produced via carbothermal reduction in nitrogen atmosphere. In addition, results show the formation of Al from Al2O3 before transformation to AlN. On the other hand, when Al was mixed with nutshell powder first, mixed phases of AlN and Al4C3 were formed before complete transformation to AlN. X-ray diffraction analysis, Raman scattering and Fourier Transform Infrared spectroscopy confirmed the wurtzite phase of aluminum nitride. Transmission electron microscopy indicated the formation of AlN nanoparticles. The formation of AlN from nutshells offers a simple route and avoids multiple-step processes involving carbon rich agents at elevated temperatures. © 2017 Published by Elsevier B.V.
Keywords: Aluminum nitride X-ray diffraction Agriculture waste Raman spectroscopy Wide band gap semiconductors
1. Introduction Aluminum nitride belongs to a class of III-N materials and is a wide gap semiconductor with band gap between 6.01 and 6.05 eV at room temperature [1]. It exhibits both wurtzite and zincblende phases as do other III-N compounds and has many potential applications in microelectronics due to its relatively high thermal conductivity (70e210 W m1 K1 to 285 W m1 K1) [2]. Additionally, it has unique physical properties including high electrical resistivity, low thermal expansion, resistance to erosion and corrosion, excellent thermal shock resistance and chemical stability in air up to 1380 C with surface oxidation occurring at 780 C. Moreover, epitaxially grown thin films of crystalline aluminum nitride are used for surface acoustic wave sensors (SAWs) deposited on silicon wafers because of AlN's piezoelectric properties [3]. Another important application for AlN is as an RF filter, also called a thin film bulk acoustic resonator (FBAR), which is widely used in mobile phones [4]. AlN in bulk form is synthesized by the carbothermal reduction of aluminum oxide in the presence of gaseous nitrogen or ammonia or by direct nitridation of aluminum. In order to get fully dense form, Y2O3 or CaO are required as additives during the hot pressing. * Corresponding author. E-mail address: [email protected] (S.B. Qadri). http://dx.doi.org/10.1016/j.jallcom.2017.03.003 0925-8388/© 2017 Published by Elsevier B.V.
There are two types of agriculture waste, one containing silica and carbonaceous matter and the other containing mostly carbonaceous matter and no silica. Among the first type, the prominent ones are rice husk, wheat husk, corn husk, sorghum leaves and peanut shells. We have demonstrated in our previous work that these are useful in the synthesis of industrially important materials such as SiO2, SiC, Si3N4, and zinc silicate by pyrolizing them in air, argon or in nitrogen atmospheres [5e11]. The second type of agriculture waste which contain only carbon matter and no silica are nutshells such as almond, walnuts, pistachio, coconuts, macadamia, and cashew. Billions of pounds of nutshells, which are currently discarded as waste products, are available if they can be harnessed in the synthesis of industrially important materials. Various tree nut shells are frequently used for organic composts, animal feedstock, or discarded as trash [13]. The present study demonstrates a more effective and practical way of utilizing tree nut shells by utilizing them as a carbon source in synthesizing AlN, that will have widespread industrial implications. Recently, it was reported that mixed phases of SiC and Si3N4 can be produced by carbothermal reduction and nitridation of a mixture of silica and macadamia powder [12]. We demonstrated in our previous work that by adding ZnO to powder of wheat or rice husk, pure zinc silicate is produced with photo-luminescent properties [11]. In the present paper, we report on the formation of AlN by adding nanocrystalline powders of Al2O3 and Al to the
1000
2
6
8
0
10
2
4
Energy (keV)
MnKα
FeKβ NiΚα
FeΚα
400
CuKα
ZnKα
600
200
CuKα
FeKβ NiΚα
ZnKα
FeΚα
4
CrKα MnKα
200
CaKβ
400
CaKα, Κβ
KKα
600
800
ArKα KKα CaKα, Κβ CaKβ TiKα
Intensity (arbitrary units)
800
0
Coconut
Almond
ArKα
Intensity (arbitrary units)
1000
6 Energy (keV)
8
1000
1000
Walnut
2
4
Energy (keV)
0
8
10
2
4
ZnKα CuKα
FeKβ
6
NiΚα
CrKα MnKα
FeΚα
CaKα, ΚΚβ
400
CaKβ
ZnKα
6
600
200
CuKα
FeΚβ NiKα
CrKα MnKα
250
FeKα
500
800
ArKα KKα
Intensity (arbitrary units)
750
ArKα KKα CaKα, Κβ CaKβ
Intensity (arbitrary units)
Pistachio
0
10
8
10
Energy (keV)
Fig. 1. Energy dispersive x-ray fluorescence of almond, coconut, macadamia, and walnut showing the presence of trace amounts of K, Ca, Cr, Mn, Fe, Ni, Cu, and Zn.
Fig. 2. X-ray diffraction scans of Al2O3 mixed with selected nut shell powders along with Rietveld whole profiles analysis of the diffraction patterns for the AlN sample derived from nut shells powder after pyrolising in a nitrogen atmosphere followed by treatment in air at 800 C.
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69
Table 1 Lattice parameters and Crystallite sizes for AlN derived from various nuts. Agricultural source
Lattice parameters (Å)
Coconut Pistachio Almond Walnut Macadamia
a a a a a
¼ ¼ ¼ ¼ ¼
3.112(1); 3.111(1); 3.112(1); 3.112(1); 3.113(1);
b b b b b
¼ ¼ ¼ ¼ ¼
Phase
3.112(1); 3.111(1); 3.112(1); 3.112(1); 3.113(1);
c c c c c
¼ ¼ ¼ ¼ ¼
4.981(1); 4.980(1); 4.982(1); 4.982(1); 4.982(1);
a ¼ 90; b ¼ 90; g ¼ 120 a ¼ 90; b ¼ 90; g ¼ 120 a ¼ 90; b ¼ 90; g ¼ 120 a ¼ 90; b ¼ 90; g ¼ 120 a ¼ 90; b ¼ 90; g ¼ 120
powdered nut shells of almond, walnut, coconut, macadamia, pistachio and cashew and pyrolising them in nitrogen atmosphere at 1400 to 1500 C. Energy dispersive X-ray spectroscopy showed pure AlN is formed. The x-ray diffraction and Rietveld analysis and Raman spectroscopy analysis of the pyrolised samples showed formation of pure wurtzite phase of AlN. In addition, transmission electron microscopy confirmed the nanocrystallinity of AlN.
1.1. Experimental details Samples were prepared from powders of raw nuts of almonds, coconuts, macadamia, pistachios, and walnuts after mixing them with nanocrystalline Al2O3 powder using ball milling with a SPEX 8000 M including stainless steel milling media. The Al2O3 sample along with the specific nut shell was combined and milled to obtain a uniform powder. A hydraulic press was used to pressurize the homogenous powder into 1 cm diameter disks with a 2.5e3 mm depth. The pellets were heat-treated (pyrolised) in a convention furnace at temperatures exceeding 1400 C for an interval of 5e6 h in a nitrogen atmosphere. In order to eliminate the residual carbon, the pellets were then placed in air at 670 C. In addition, the samples were given HF treatment and H2O2 rinsing to remove trace amounts of metal impurities that were present in the starting material. XRD scans were made using a Rigaku 18 kW rotating anode generator and a high resolution powder diffractometer. The diffraction scans were collected using monochromatic CuKa radiation. Energy dispersive x-ray fluorescence measurements were performed using a Thermo Electron Corporation system using white radiation from Rh target and a Pd filter of appropriate thickness. Raman spectra were collected on an inVia Raman Microscope (Renishaw) using a 514 nm laser line. Scans were obtained at ca. 15 mW laser power at the sample and an integration period of
E2 (high)
Intensity (arbitrary units)
656 cm-1
E1 (TO) 669 cm-1 A1 (TO) -1
896 cm 400
500
600
700
800
mc mc mc mc mc
Crystallite sizes (nm) (186) (186) (186) (186) (186)
31.3 (2.1) 33.9 (1.9) 30.1(1.5) 32.6(1.7) 25.2 (1.5)
30 s. Fourier Transform Infrared (FTIR) spectra were collected using Thermo Scientific Nicolet FT-IR spectrometer with Diffuse Reflectance Infrared Transform Spectroscopy (DRIFTS) accessory. In order to conduct the TEM analysis, ethyl alcohol was mixed with the pyrolyzed sample; the mixture was then set in an ultrasonic cleaner. A carbon covered 200 mesh copper grid was submerged into the mixture to collect AlN particles. A FEI Tecnai G2 TEM was utilized to examine the sample at 300 kV. Energy dispersive x-ray spectroscopy (EDS) was used as an analytic technique to determine elemental composition on JEOL scanning electron microscope (SEM). 2. Results and discussion The samples were prepared in the shape of circular pellets from the milled nuts by applying uniaxial pressures in excess of 2.0 GPa in a hydraulic press using a 1 cm diameter die. The elemental compositions were analyzed using energy dispersive x-ray fluorescence spectra and are presented in Fig. 1. All the nut shells showed trace amounts of K, Ca, Cr, Mn, Fe, Ni, Cu, and Zn with a slight variation between them. The Ar peak is present due the measurement performed in the air environment. The nut powders, each weighing 1 g, were mixed with 0.1 g of 50 nm nanocrystallites of Al2O3 and then pyrolised in nitrogen atmosphere in excess of 1450 C. Following the heat treatment in nitrogen atmosphere, the samples were treated in air at 800 C to remove any excess unreacted carbon. X-ray diffraction scans were taken of the asprepared pellet samples of each mixed nut and alumina powder prior to any heat treatment and showed peaks from the corundum phase of Al2O3 as given in PDF#04-002-8135. The crystallite sizes for Al2O3 as determined from Rietveld analysis of the diffraction pattern were of the order of 47.3(1.3) nm. The as-prepared samples also showed the presence of amorphous peak due to carbonaceous carbon present in the shell. The x-ray diffraction analysis of the heat processed samples showed peaks corresponding to only wurtzite phase of AlN with a space group of P63mc (186) [14]. A Rietveld analysis of the diffraction scans of the processed samples gave Rfactors in the range 4e6% and are shown in Fig. 2 for selected nut shell samples. The observed and calculated intensity profiles are displaced with respect to each other to illustrate the goodness of the fit and the difference between the observed and calculated profiles are given at the lower portions of the plots. The lattice parameters, crystallite size and the structural parameters of the AlN obtained for various nut shells are given in Table 1. The crystallite sizes varied in the range between 25 and 34 nm for all AlN samples
E1 (LO)
A1 (LO)
616 cm-1
P63 P63 P63 P63 P63
905 cm-1
900
1000
-1
Wavenumber (cm ) Fig. 3. Raman spectra of AlN derived from pistachio showing the different Raman active modes.
Table 2 Raman frequencies observed for AlN in comparison to literature values. Reference
A1(TO)
A1(LO)
E1(TO)
E1(LO)
E22
Pistachio (This Study) Almond (This Study) Ref. [15] Ref. [16] Ref. [19]
616 616 612, 608 614 610
896 896 898, 890 893 890
669 669 665, 660 673 669
908 905 910, 902 916 911
656 656 654, 655 660 656
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1.0
Pistachio 0.8
Absorbence (a. u.)
0.6
0.4
0.2
0.0 400
600
800
1000
1200
1400
1600
1800
2000
-1
Wavenumber (cm ) Fig. 4. FTIR spectra of AlN derived from pistachio showing a broad band of 699 cm1.
derived from various nut shells. The formation of the wurtzite phase of AlN was confirmed using Raman spectroscopy. Fig. 3 illustrates the Raman spectra for AlN derived from pistachio powder. All the Raman modes are labelled with their corresponding values. The observed active Raman modes for AlN derived from pistachio and almond shells are listed in Table 2 along with the literature values. These values for Raman modes are in agreement with the wurtzite phase values reported in the literature [15,16]. The Raman scattering in solids is attributed to the inelastic scattering of photons by the crystal lattice vibrations (optical phonons). The Raman frequency can slightly differ from its ideal value due to factors such as stress, interfaces, structural defects, impurities, and the crystallite size. It is specially manifested in oriented films grown on different substrates such as Si and silica [15]. We find slight difference in E1 mode for pistachio and almond derived AlN that is attributed to small difference in the crystallite sizes. The IR spectrum of the AlN sample from pistachio nut sample is shown in Fig. 4. The data was obtained after normalization. The most prominent infrared active phonon band was observed at 699 cm1 for the pistachio sample and 714 cm1 for the almond sample. These bands are in contrast to what was observed at 698 cm1 for bulk AlN powder. The slight shift in the case of almond could be attributed to the crystallite size of 30 nm compared to
34 nm for pistachio. This shift in peak position is consistent with the shifts reported by Balasubramanian et al. for nanoparticles and nanotubes of AlN [17,18]. The transmission electron micrographs for almond and walnut shells are presented in Fig. 5 showing the nanodimensionality of the AlN crystallites. The average crystallite size is of the order of 30 nm in reasonable agreement with the x-ray measurements. The energy dispersive x-ray spectra of the post processed samples are given in Fig. 6 along with that of pure commercial AlN obtained from commercially from Aldrich chemical company. The spectra show peaks corresponding to only Al and N and absence of any of the trace elements which were present in the as-prepared samples. The atomic % content of Al and N were calculated from the peak intensities in comparison with pure AlN sample. In the case of pure AlN from the Aldrich Chemical Company source the ratio of Al:N was 70:30 in comparison to 72:28 and 69:31 for AlN samples from almond and walnut source, respectively. The results indicates that the ratio of Al:N in our samples is nearly 1:1 ratio. Moreover, we found that the processed samples did not contain any carbon or oxygen peaks or any trace amounts of K,Ca, Zn, Cr, Mn, Fe, Ni, and Cu which were present in the starting samples. The full EDS spectrum of AlN is shown in Fig. 7. This indicates that our processing results in the formation of pure AlN without any impurities and is as good as the commercially available AlN. In order to determine the pathways to the final product of AlN, we performed experiments with Al2O3 and Al powders, each mixed with pistachio powder. Both these samples were pyrolised in N2 atmosphere for 1 h. Fig. 8 shows an overlay of the diffraction scans of Al2O3 sample in which we observed the formation of mostly Al and a small quantity of AlN indicating that Al melt forms first which then reacts with free nitrogen to form AlN. A full 6 h period of heating in nitrogen resulted in AlN wurtzite phase. This indicates that the sequence of transformation to AlN in the case of Al2O3 goes as Al2O3 / Al / AlN. Fig. 9 shows an overlay of diffraction scans for Al-pistachio mixed sample where we found that in the first stage Al4C3, Al2O3 and AlN phases are formed and the 6 h heating in N2 atmosphere resulted only in the wurtzite AlN phase. The presence of crystalline corundum phase of alumina is due to a thin oxide layer that will be present for Al powder and is usually in the amorphous state and is not manifested in the diffraction scan. However, it becomes crystalline as we heat treat the Al powder. The Al2O3 eventually converts to AlN by carbo-thermal reduction and nitridation process. On the other hand, AlN has lower total energy compared to Al4C3 and therefore as an over time it also converts to AlN. Thus for Al mixed pistachio powder sample the sequence of transformation appears to follow Al/(Al4C3þAlN þ Al2O3) / AlN.
Fig. 5. TEM micrographs of AlN from almond and walnut samples showing nanocrystallites of the order of 25e50 nm.
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Fig. 6. Selected EDX spectrum of pure AlN (Aldrich Company), AlN from walnut, almond and pistachio nut shells showing the peaks from Al and N.
The mechanism of AlN formation through carbothermal reduction of Al2O3 in N2 atmosphere has been discussed by various authors [20e22] and takes place at much elevated temperatures of 1700e200 C based on the following equation:
Al2O3 þ 3C þ N2 / 2AlN þ 3CO AlN has two polymorphs wurtzite and zinc-blende cubic similar to other III-N nitrides such as GaN and InN. The wurtzite is the more
Intensity (arbitrary units)
8000
6000
4000
2000
0
2
4
6
8
Energy (keV) Fig. 7. Full EDX spectrum of post processed AlN sample from almond showing absence of trace elements found in Fig. 1 of as prepared sample. Only Al and N peaks are observed.
Fig. 8. An overlay of x-ray diffractions of Al2O3-pistachio mixed sample. (a) diffraction scan of as-prepared samples showing the corundum phase (b) diffraction scan after heating in nitrogen atmosphere for 1 h showing peaks from AlN wurtzite phase and Al (c) complete transformation to AlN wurtzite phase after 6 h.
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spectroscopy and FTIR studies confirmed the wurtzite phase of AlN whereas the TEM results indicated that the crystallite sizes are of the order of 30 nm. In the case in the case of Al2O3 the sequence of transformation proceeds as Al2O3/Al/AlN. Similarly, for Al mixed pistachio powder sample the sequence of transformation follows Al/(Al4C3þAlN þ Al2O3)/AlN. This nanostructure of wurtzite AlN will be very useful in a variety of engineering applications and in opto-electronics as heat sink/electrically insulating material.
Acknowledgments We like to thank to K.P. Fears of NRL, Chemistry Division, for helping us get the Raman data.
References
Fig. 9. An overlay of x-ray diffractions of Al-pistachio mixed sample. (a) diffraction scan of as-prepared samples showing Al peak (b) diffraction scan after heating in nitrogen atmosphere for 1 h showing peaks from Al4C3 (PDF#01-071-3787) marked with an asterisk(*), wurtzite AlN phase and Al2O3 (corundum phase). (c) Complete transformation to AlN wurtzite phase after 6 h.
stable of the two configurations. Developments in film growth technology have led to advanced devices consisting of AlN and GaN such as blue-green lasers, light emitting diodes (LEDs), ultraviolet photodetectors, field effect transistors (FET), and acoustic wave devices. In addition, AlN has large electromechanical coupling coefficient and exhibit piezoelectric properties in conjunction with GaN which are uniquely suitable for high temperature piezoelectric and acoustic devices. AlN although can be synthesized by various routes. The most widely used methods for AlN production are direct nitridation and carbothermal reduction and followed by nitridation. Each of these methods involves several steps including the ignition of Al powder and several milling steps. In this paper we have established that AlN can easily be produced by using a natural source of carbon from nut shells and mixing them with alumina. In this process, both direct nitridation and carbothermal reduction followed by nitridation are built in. The simplicity of this technique results in less energy consumption and is less labor intensive. Thus, we have demonstrated that large quantities of pure wurtzite phase of nanocrystalline AlN can be obtained from the nut shells of almond, coconut, pistachio, macadamia, walnut and cashew. 3. Conclusions Nanoparticles of wurtzite AlN are obtained from the mixture of Al2O3 or Al powders with powdered shells of almond, coconut, pistachio, macadamia, walnut and cashew by pyrolysis in nitrogen atmosphere at 1400 C. The formation of pure AlN from nutshells offers a simple route involving carbon rich agents at elevated temperatures of 1700e2000 C. X-ray diffraction, Raman
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