Home
Search
Collections
Journals
About
Contact us
My IOPscience
AlN hollow-nanofilaments by electrospinning
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 085603 (http://iopscience.iop.org/0957-4484/26/8/085603) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 132.239.1.231 This content was downloaded on 15/06/2017 at 16:42 Please note that terms and conditions apply.
You may also be interested in: Fabrication of carbon nanofiber-reinforced aluminum matrix composites assisted by aluminum coating formed on nanofiber surface by in situ chemical vapor deposition Fumio Ogawa and Chitoshi Masuda Synthesis of continuous boron nitride nanofibers by solution coating electrospun templatefibers Yejun Qiu, Jie Yu, Jing Yin et al. Electrospinning of superconducting YBCO nanowires Edgar A Duarte, Nicholas G Rudawski, Pedro A Quintero et al. Carbon-coated hexagonal magnetite nanoflakes production by spray CVD of alcohols in mixture with water Marisol Reyes-Reyes, Daniel Hernández-Arriaga and Román López-Sandoval Formation mechanism and photoluminescence of AlN nanowhiskers H T Chen, X L Wu, X Xiong et al. Low-temperature collapsing boron nitride nanospheres into nanoflakes and their photoluminescence properties\ Jie Li, Han Luo, Jing Lin et al. Synthesis and characterization of electrospun superconducting (La,Sr)CuO4 nanowires and nanoribbons X L Zeng, M R Koblischka and U Hartmann 352 nm ultraviolet emission from high-quality crystalline AlN whiskers Baodan Liu, Yoshio Bando, Aimin Wu et al.
Nanotechnology Nanotechnology 26 (2015) 085603 (7pp)
doi:10.1088/0957-4484/26/8/085603
AlN hollow-nanofilaments by electrospinning Tony Gerges1, Vincent Salles1, Samuel Bernard2, Catherine Journet1, Xavier Jaurand3, Rodica Chiriac1, Gabriel Ferro1 and Arnaud Brioude1 1
Laboratoire des Multimatériaux et Interfaces (UMR 5615 Université Lyon 1-CNRS), 43 Bd du 11 novembre 1918, F-69622, Villeurbanne Cedex, France 2 IEM (Institut Europeen des Membranes), UMR 5635 (CNRS-ENSCM-UM2), Université Montpellier 2, Place E. Bataillon, F-34095, Montpellier, France 3 Centre Technologique des Microstructures, Université Claude Bernard Lyon1, 5 rue Raphael DuboisBâtiment Darwin B, F-69622, Villeurbanne Cedex, France E-mail: [email protected] Received 31 October 2014, revised 21 December 2014 Accepted for publication 12 January 2015 Published 4 February 2015 Abstract
We present for the first time an original method to elaborate AlN nanofilaments (NFs) by using a preceramic-based electrospinning process. Initially, an Al-containing precursor (poly (ethylimino)alane) is mixed with an organic spinnable polymer to be electrospun and generate polymeric filaments with a homogeneous diameter. A ceramization step at 1000 °C under ammonia and a crystallization step at 1400 °C under nitrogen are performed to get the final product made of AlN NFs with a diameter ranging from 150 to 200 nm. Studies carried out by high resolution electron microscopy and 3D tomography show their regular morphology, with high chemical purity and polycrystalline nature. S Online supplementary data available from stacks.iop.org/NANO/26/085603/mmedia Keywords: aluminum nitride, nanofilaments, polymer-derived ceramics, electrospinning (Some figures may appear in colour only in the online journal) 1. Introduction
By reducing the size of AlN material to a submicronscale as a one-dimensional (1D) nanostructure, the contact surface (i.e. specific surface area) is considerably increased and the expected properties of a composite (or a device) containing those nanoscaled objects should be enhanced. Moreover, the aspect ratio of the nanofilaments (NFs) should help to adjust the above-mentioned properties in specific directions. As already demonstrated with another kind of inorganic filler [8], very thin functional materials or devices can be prepared. Surprisingly, AlN nanomaterials have been poorly studied up to nowadays. In the literature, the very few related materials can be presented through their respective elaboration processes as following: chemical vapor deposition (CVD) [9–17], direct nitridation [1, 9, 15], direct sublimation [13], carbothermal reduction and nitridation (CRN) [10, 18, 19], rapid nitridation [11], combustion synthesis [20], and plasma-enhanced atomic layer deposition [12]. Those
Aluminum nitride (AlN) is a very important compound of the group III nitrides, characterized by the widest band gap, 6.2 eV (hexagonal phase) [1]. Moreover, AlN possesses high thermal conductivity (320 W m−1 K−1), high electrical resistivity (1013 Ω cm), small dielectric constant (8.8 at 1 MHz) [2] and particular piezoelectric behavior. As a consequence, it also plays an important role in the fields of electronics and optoelectronics [3], as sensors and actuators, thanks to its piezoelectric properties [4, 5], to fabricate surface acoustic wave detectors for instance [6]. AlN fillers have interested more researchers in the last few years to prepare thermal conductive and electric insulating composites for electronic applications by using a polymer matrix. A recent paper shows very interesting results with polyimide (PI)-modified AlN composites which can be used extensively for electronic packaging applications [7]. 0957-4484/15/085603+07$33.00
1
© 2015 IOP Publishing Ltd Printed in the UK
T Gerges et al
Nanotechnology 26 (2015) 085603
methods are used to obtain nanofibers (or NFs) [1, 11–13], nanotubes [14, 18], nanotips [10, 17], nanobelts [15], nanoneedles [16], nanotowers [9] and nanowhiskers [20]. Electrospinning is a suitable shaping process to fabricate very long filaments with adjustable diameters from tens of nm to several μm which cannot be easily obtained with other methods. Self-supported materials as well as short filaments (or wires or tubes) can be prepared depending on the final targeted application [21, 22]. Compared to other fabricating methods, the electrospinning process presents also the possibility to obtain a higher production rate of filaments, with high aspect ratio, and without using any catalyst as in CVD method. The high purity of the material has a positive influence on its final properties. This is true for all scales from bulk to 1D AlN materials. To the best of our knowledge, only two papers describe AlN NFs fabrication by electrospinning. Firstly, Al-containing tubes were previously prepared by depositing trimethylaluminum on electrospun nylon filaments and then by treating under air the assembly in order to remove the polymer part [12]. However, the resulting material is composed of about 40 at% of oxygen. Another paper based on electrospinning showed the ability to produce AlN tubes with a higher purity by introducing aluminum nitrate into the spinnable aqueous solution. The resulting as-spun material was then converted into AlN after thermal treatment at 1600 °C with non-negligible amount of remaining carbon under N2 using a CRN process. The authors mentioned that nitridation occurs above 1000 °C. The obtained tubes were polycrystalline with a diameter of 500 nm [18]. We present in this paper, an original and versatile method based on electrospinning process to get highly pure AlN NFs with a tunable nanometric diameter. This experimental strategy is based on the polymer-derived ceramics (PDCs) route which is known to produce non-oxide ceramics, as previously described for boron nitride NFs for instance [23]. This combined method allows to produce pure and stable AlN NFs starting from a poly(N-ethylimino)alane (PEIA), and to tune their structural properties from the amorphous state to a high crystallization degree.
electrospinning in order to use them as template. All chemicals were used without further purification. 2.2. From PAN template to AlN NFs
PAN was first spun before being covered by the aluminum precursor. The electrospinning experiment was performed by loading with the PAN solution a syringe equipped with a 21 Gauge stainless-steel needle. The electric field was applied between the needle, connected to a positive high voltage power supply with an alligator clip, and an aluminum foil serving as a collector, connected to the ground. The applied voltage and the working distance between the needle tip and the collector were 12 kV and 15 cm respectively, with a feeding rate of 1 mL h−1. The electrospun PAN filaments were air-dried at RT for 12 h and then stabilized in air at 280 °C for 2 h, with a heating rate of 2 °C min−1. The 2 cm2 surface PAN filaments template, stabilized at 280 °C, was consecutively inserted into the glove box and then immersed in 2 ml of a solution containing 2 wt% of AlN precursor. The impregnation step was performed at ambient temperature for 2 min. After PEIA deposition, the sample was rinsed into 3 ml of chloroform and then dried for several minutes on an absorbent paper. The coated filaments were firstly heated under ammonia flow at 1000 °C for ceramization with a heating rate of 1 °C min−1 and subsequently heated to 1400 °C for 5 h under nitrogen flow for crystallization with a heating rate of 3 °C min−1. It has to be noticed that a dwell at 50 °C for 1 h before the heat treatment at high temperatures is necessary for the precursor to allow time to be decomposed without changing the filaments morphology. The final samples were stored in a glove box under argon. 2.3. Characterization techniques
Thermogravimetric analyses were performed on a TGA/ SDTA 851e Mettler Toledo apparatus. A few mg of sample were introduced in an alumina crucible with a pierced lid and the temperature was increased from 30 to 1000 °C at 5 °C min−1 under air (50 mL min−1). In order to check the morphology of the samples, the first characterization is conducted by scanning electron microscopy (SEM) (FEI-Quanta 250). The samples were fixed on an aluminum holders covered by a carbon tape before platinum deposition (5 nm) by sputtering. Then, their nanostructure is investigated by transmission electron microscopy (TEM) (Jeol-2100F). In order to prevent the oxidation of the AlN filaments, a specific TEM PicoIndenter is used. It is put into the glove box and charged with the sample by keeping it away from air until its introduction inside the TEM chamber. The samples composition and phases were determined by x-ray diffraction (XRD) patterns (Philipps PW 3040/60 PANalytical X’Pert PRO (CuKα radiation; λ = 1.5406 Å at 40 kV and 30 mA). A μ-Raman spectrometer (LabRAM ARAMIS HORIDA) equipped with a 20 mW HeNe laser beam (λ = 633 nm) with a spot of few μm2 in a co-focal configuration for polytypes identification. Each NFs mean diameter was determined from measures performed on 100 filaments, using the software ImageJ.
2. 2. Materials and experimental procedure 2.1. Materials
The AlN filaments were prepared using a raw precursor poly (N-ethylimino)alane ((HAlNEt)n, PEIA) which synthesis was previously described [24]. This precursor is formed after reaction between lithium tetrahydro aluminate (LiAlH4) and ammonium chloride (NH4Cl) at −78 °C in methylene oxide. The as-prepared precursor is a white and moisture sensitive solid, thus stored and handled in an argon-filled glove box. Its solubility into chloroform (CHCl3) allows preparing a transparent solution with 2 wt% of Al precursor. A second solution containing a spinnable polymer, polyacrylonitrile (PAN) (Mw = 150 000 g mol−1, Sigma Aldrich) in N,N-dimethylformamide (Sigma Aldrich), was used to fabricate PAN NFs by 2
T Gerges et al
Nanotechnology 26 (2015) 085603
Figure 1. SEM image of PAN fibers after impregnation for 2 min in Al precursor solution (a) low magnification, (b) high magnification.
3. Results and discussion 3.1. Before annealing process
The morphology of as-spun PAN filaments stabilized at 280 °C under air has been investigated by SEM (SI1). Their mean diameter has been evaluated at 400 nm and the surface roughness is quite low and uniform. After 2 min of the mat impregnation with the PEIA solution, the filaments are covered by the latter (figure 1). It has to be noticed that, compared to as-spun filaments, their diameter is increased of about 40 nm, to obtain a final diameter around 440 nm. Their surface is decorated with small nanoparticles regularly deposited showing clearly the impregnation reaction between PEIA and the PAN. Figure 2. SEM image of the filaments after the ceramization step in a
3.2. Ceramization step
furnace at 1000 °C under ammonia.
The ceramization step, performed in a furnace at 1000 °C under ammonia, has mainly two purposes: (1) to remove the carbon coming from both the PAN and PEIA; (2) to enrich PEIA with nitrogen and convert the latter into AlN. As a consequence of this treatment, the PAN core is then totally decomposed. A 3D tomography study has been specially conducted to observe the morphology of the filaments after heat treatment at 1000 °C (SI2). The video in attachment shows clearly the central cavity inside an isolated filament, confirming the decomposition of the PAN part of the initial filament, and leading to the formation of tubes as expected before. A typical SEM observation of the sample is also shown in figure 2. It evidences that the hollow filaments are uniform and without aggregates on several tens of micrometers. The diameter of those new nanostructures ranges from 110 to 200 nm and the mean value is centered at 170 nm. The reduction of diameter from the original 400 nm (as-spun PAN filaments) is explained by the loss of carbon during the heat treatment and the reticulation of the precursor with the polymer. The obtained filaments are constituted of small aggregated
crystallites (mean diameter of 10 nm), which suggests a polycrystalline structure. In order to form highly crystallized AlN hollow filaments, the previous sample was heated further for 5 h at 1400 °C under nitrogen gas. The figure 3 shows SEM images of the filaments obtained in this way, with a mean diameter estimated close to 150 nm. Their macroscopic morphology is kept constant while a closer look shows that the tubes are formed of larger nanoparticles aggregates with a greater porosity. Indeed, as a consequence of the temperature induced crystallization effect, the particles forming the filaments grow and increase the porosity of the sample. In order to check the purity of the sample, a quantitative analysis by energy-dispersive spectroscopy (EDS) has been made. In supporting information (SI3), the EDS spectrum shows that the sample components are only Al and N with an atomic ratio close to 1. Moreover, this synthesis was performed in an isolated enclosure without oxygen; therefore, no quantifiable amount 3
T Gerges et al
Nanotechnology 26 (2015) 085603
Figure 3. (a) SEM image of the hollow filaments after the crystallization step in a furnace at 1400 °C under nitrogen (b) SEM image shows a magnification of the central region of image (a), with a magnified part of the picture (inset).
Figure 4. (a) TEM image of the hollow filaments after the ceramization step at 1000 °C under ammonia. Inset: diffraction pattern of the
corresponding area. (b) HRTEM image of a filament.
reported with enough accuracy. The HRTEM mode was used to more precisely focus on ultrafine structure and on very small nanocrystallites with sizes inferior to 10 nm embedded in an amorphous matrix (figure 4(b)). Thus, crystallographic planes related to the (100) interplanar distance have been observed, confirming the initial stage of AlN formation (figure 4(b)) at only 1000 °C. We have performed the same study on the corresponding sample annealed at 1400 °C under nitrogen. On figure 5, the tubular structure is more clearly exhibited by the contrast image. Larger particles can be identified and the high rate of crystallization is sufficient to distinguish the atomic columns of the AlN structure. The diffraction pattern associated to large area (inset of figure 5(a)) shows the main interplanar distances of AlN. Contrary to the previous figure, very thin rings can be distinguished and attributed to
of oxygen was detected. The sample remained stable even after four weeks with environmental air exposure as shown by the FTIR analysis (SI4), that shows the Al–N band between 625 and 670 cm−1 [25, 26] without any impurities, even hydroxyl group (−OH)). It is known that the presence of oxygen or carbon in the AlN structures can have a deleterious effect on their thermal, electrical and optical properties [27, 28]. In the present study, the purity of the AlN NFs obtained indicates their high quality. The structural properties of the samples have been investigated by TEM in conventional and high resolution mode. SAED patterns recorded on a large zone of the samples (inset of figure 4(a)) show that filaments annealed at only 1000 °C are mainly amorphous. The rings present in the diffraction pattern are quite large and only the one corresponding to the (100) direction of the AlN bulk material (JCPDS NO.25-1133) has been reasonably 4
T Gerges et al
Nanotechnology 26 (2015) 085603
Figure 5. (a) TEM image of the hollow filaments after the crystallization step at 1400 °C under nitrogen. Inset: diffraction pattern of the
corresponding area. (b) HRTEM image of filament nanoparticles.
Figure 6. X-ray diffraction patterns for AlN nanofilaments treated at 1000 °C (a) and 1400 °C (b).
Figure 7. Raman spectrum of AlN nanofilaments treated at 1400 °C under nitrogen.
the hexagonal phase of AlN. The AlN filaments are composed of many interconnected particles randomly oriented. The mechanical strength of the entire filament is insured by this particular polycrystalline structure where all the grains (mean size around 20 nm) are linked together with strong grain boundaries (SI6). The Moiré fringes, which correspond to the superposition of two AlN nanoparticles, are also a proof of the high level of crystallinity of these particles. XRD (figure 6) and Raman spectroscopy measurements (figure 7) have also been done to further studying the structural properties of the samples. Figure 6(a) shows the XRD pattern of AlN hollow NFs after heat-treatment at 1000 °C under ammonia. The peaks confirmed that the obtained product can be identified as
hexagonal AlN (h-AlN), in accordance with the JCPDS card (NO.25-1133). By applying the Scherrer equation, a particle mean diameter of 11.6 nm can be calculated. This value is consistent with the result previously obtained by TEM. After heat-treatment at 1400 °C under nitrogen, the samples present XRD peaks much narrower (figure 6(b)). This indicates a significant improvement of the crystallinity of AlN filaments. The calculated mean particle size is thus increasing to 22.5 nm as expected from TEM observations. All of the diffraction peaks in the pattern can be assigned to the hexagonal würtzite AlN phase, with the following lattice parameters: a = 3.109 Å and c = 4.979 Å. No other peaks due to impurities are detected. 5
T Gerges et al
Nanotechnology 26 (2015) 085603
For AlN, six Raman-active modes may be present [29, 30], and they are all present for the present material (figure 7): the modes E2(low) at 248 cm−1, A1(TO) at 611 cm−1, E2(high) at 655 cm−1, E1(TO) at 667 cm−1, A1(LO) at 893 cm−1 and E1(LO) at 906 cm−1 which exactly correspond to the würtzite-AlN (W-AlN) are clearly identified [24, 29]. It is well known that the structure of AlN can be detected by the FWHM of the curve of E2(high) mode. For high quality AlN bulk crystals, a value of 3.2 cm−1 was measured [31]. In our case, the value of 12.3 cm−1 found, could be an index of the polycristallinity obtained [32]. The caxis orientation of the NFs can be deduced from the ratio between the area of the E2(high) mode and that of the A1(TO) mode [33]. The proportion of the two areas obtained on our spectrum denotes a crystal orientation majority along to the caxis with a small inhomogeneous orientation probably in the polycrystalline NFs. In addition, a c-axis orientation is an important parameter indicating the good piezoelectricity response of the AlN NFs [34]. Finally, Hsu et al [35] do not detect any changes of the peak position as well as of the FWHM of the E2 (high) mode, whatever the NFs diameter. One of the advantages of the entire elaboration process is that the PEIA is synthesized prior the electrospinning experiments. This last point is of main importance in order to adjust the composition and the diameter of the final product. Two strategies can take place to modify the tubes diameter. The first one concerns the initial diameter of the PAN NFs used as template before impregnation. This allows modifying the inner and consequently the outer diameters of the final AlN tubes (SI7). The second one corresponds to the concentration of the PEIA inside the solution used for NFs impregnation. In this latter case we have verified that the obtained filaments can be glued to each other if a higher amount of 2 wt% of precursor is used into the solution. The SEM and TEM images of the two types of filaments show that the morphology (hollow structure) and the crystallization state with the presence of small well-crystallized particles are preserved for all diameters obtained (100–500 nm) (SI7). In terms of implementation, the filaments can be used at room temperature without requiring any special inert environment. This last point is of main importance with regard to future applications. An ageing study was made to follow the effect of the temperature in an oxidative environment on the surface state of the filaments. Figure 8 shows the TGA under air of AlN NFs with a mass gain starting at 550 °C to reach a value of 11.85% at 1000 °C implying an oxidation of 50% of the sample. The oxidation was slightly detected by XRD spectra (SI5) indicating a global presence of AlN with two small additional peaks attributed to γ-alumina. Compared to the literature, these results confirm a high stability of the AlN NFs against oxidation up to around 550 °C. These AlN NFs seem to be, under oxidative atmosphere, at least as good as AlN materials previously investigated in the literature with other dimensions (micropowders [36], films [37], bulk [38]) . This result is very important taking into account the maximum working temperature of about 300–400 °C, before implying the polymer degradation (most commonly polyimide or epoxy resins).
Figure 8. TGA under air for the nanofilaments after annealing at 1400 °C showing the mass gain from 550 °C to 1000 °C.
4. Conclusion We present for the first time an original and versatile method based on electrospinning process coupled with the PDCs route to get pure hollow AlN NFs with a tunable nanometric diameter. Starting from a PEIA and a well-adapted annealing treatment under ammonia and then nitrogen up to 1400 °C, AlN NFs with a tailored diameter in the range 100–500 nm, and a controlled crystallization state from amorphous to polycrystalline are generated. This method brings an opportunity to fabricate AlN NFs which seem to be suitable and efficient for future use as thermal conductive fillers in polymer matrices for electronic applications. Their high stability under air up to 550 °C indicates that these AlN nanostructures can be handled under air without losing their physical properties.
References [1] Zhang P G, Wang K Y and Guo S M 2010 Ceram. Int. 36 2209 [2] Kuang J C, Zhang C R, Zhou X G, Liu Q C and Ye C 2005 Mater. Lett. 59 2006–10 [3] Kandaswamy P K, Machhadani H, Bellet-Amalric E, Nevou L, Tchernycheva M, Lahourcade L, Julien F H and Monroy E 2009 Microelectron. J. 40 336–8 [4] Mortet V, Soltani A, Talbi A, Pobedinskas P, Haenen K, De Jaeger J-C, Pernod P and Wagner P 2009 Procedia Chem. 1 40–3 [5] Giordano C, Ingrosso I, Todaro M T, Maruccio G, De Guido S, Cingolani R, Passaseo A and De Vittorio M 2009 Microelectron. Eng. 86 1204–7 [6] Lloreta F, Araújoa D, Villara M P, Rodríguez-Madridb J G, Iriarteb G F, Williamsc O A and Calle F 2013 Microelectron. Eng. 112 193–7 [7] Zhou Y, Yao Y, Chen C Y, Moon K, Wang H and Wong C 2014 Sci. Rep. 4 4779 [8] Fiorido T et al 2014 Sensors Actuators A 211 105–14 [9] Zhang P G, Wang K Y, Liang J and Guo S M 2011 Physica E 43 934–7 [10] Jung W-S 2009 Bull. Korean Chem. Soc. 30 1563 6
T Gerges et al
Nanotechnology 26 (2015) 085603
[25] Dimitrova V, Manova D, Paskova T, Uzunov T, Ivanov N and Dechev D 1998 Vacuum 51 161–4 [26] Antsiferov V N, Gilyov V G and Karmanov V I 2002 Vib. Spectrosc. 30 169–73 [27] Tang Y, Cong H, Li F and Cheng H-M 2007 Diam. Relat. Mater. 16 537–41 [28] Schowalter L J, Schujman S B, Liu W, Goorsky M and Shahedipour-Sandvik F 2006 Phys. Status Solidi (A) 203 1667–71 [29] Cao Y G, Chen X L, Lan Y C, Li J Y, Xu Y, Xu T, Liu Q and Liang J K 2000 J. Cryst. Growth 213 198–202 [30] Tian Y, Jia Y, Bao Y and Chen Y 2007 Diam. Relat. Mater. 16 302–5 [31] Kuball M, Hayes J M, Prins A D, van Uden N W A, Dunstan D J, Shi Y and Edgar J H 2001 Appl. Phys. Lett. 78 724 [32] Lughi V and Clarke D R 2006 Appl. Phys. Lett. 89 241911 [33] Chen D, Xu D, Wang J, Zhao B and Zhang Y 2008 Thin Solid Films 517 986–9 [34] Sanz-Hervás A, Clement M, Iborra E, Vergara L, Olivares J and Sangrador J 2006 Appl. Phys. Lett. 88 161915 [35] Hsu H-C, Hsu G-M, Lai Y-S, Feng Z C, Tseng S-Y, Lundskog A, Forsberg U, Janzén E, Chen K-H and Chen L-C 2012 Appl. Phys. Lett. 101 121902 [36] Gu Z, Edgar J H, Wang C and Coffey D W 2006 J. Am. Ceram. Soc. 89 2167–71 [37] Kang H C, Seo S H, Kim J W and Noh D Y 2002 Appl. Phys. Lett. 80 1364 [38] EDGAR J H, Gu Z and Taggart K 2006 Mater. Res. Soc. Symp. Proc. 892 FF21–02
[11] Lee T H, Nersisyan H H, Jeong H G, Lee K H, Noh J S and Lee J H 2011 Chem. Eng. J. 174 461–6 [12] Ozgit-Akgun C, Kayaci F, Donmez I, Uyar T and Biyikl N 2013 J. Am. Ceram. Soc. 96 916–22 [13] Lei M, Song B, Guo X, Guo Y F, Li P G and Tang W H 2009 J. Eur. Ceram. Soc. 29 195–200 [14] Stan G, Ciobanu C V, Thayer T 0 P, Wang G T, Creighton J R, Purushotham K P, Bendersky L A and Cook F 2009 Nanotechnology 20 035706 [15] Tang Y, Cong H, Li F and Cheng H-M 2007 Diam. Relat. Mater. 16 537 [16] Shah S, Nabi G, Khan W, Majid A, Ali S, Hussain M, Nabi A and Butt F K 2013 Mater. Lett. 107 255–8 [17] Shi S C, Chattopadhyay S, Chena C F, Chen K H and Chen L C 2006 Chem. Phys. Lett. 418 152–7 [18] Sun Y, Li J Y, Tan Y and Zhang L 2009 J. Alloys Compd. 471 400–3 [19] Suehiro T, Tatami J, Meguro T and Komeya K 2002 J. Am. Ceram. Soc. 85 715–7 [20] Shi Z, Radwan M, Kirihara S, Miyamoto Y and Jin Z 2009 J. Alloys Compd. 476 360–5 [21] Greiner A and Wendorff J H 2007 Angew. Chem., Int. Ed. Engl. 46 5670–703 [22] Julijana P, Biljana K, Baumgartner J and Kocbek P 2013 Int. J. Pharmaceutics 456 125–34 [23] Salles V, Bernard S, Brioude A, Cornu D and Miele P 2010 Nanoscale 2 215–7 [24] Termoss H, Bechelany M, Toury B, Brioude A, Bernard S l, Cornu D and Miele P 2009 J. Eur. Ceram. Soc. 29 857–61
7
Search
Collections
Journals
About
Contact us
My IOPscience
AlN hollow-nanofilaments by electrospinning
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 085603 (http://iopscience.iop.org/0957-4484/26/8/085603) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 132.239.1.231 This content was downloaded on 15/06/2017 at 16:42 Please note that terms and conditions apply.
You may also be interested in: Fabrication of carbon nanofiber-reinforced aluminum matrix composites assisted by aluminum coating formed on nanofiber surface by in situ chemical vapor deposition Fumio Ogawa and Chitoshi Masuda Synthesis of continuous boron nitride nanofibers by solution coating electrospun templatefibers Yejun Qiu, Jie Yu, Jing Yin et al. Electrospinning of superconducting YBCO nanowires Edgar A Duarte, Nicholas G Rudawski, Pedro A Quintero et al. Carbon-coated hexagonal magnetite nanoflakes production by spray CVD of alcohols in mixture with water Marisol Reyes-Reyes, Daniel Hernández-Arriaga and Román López-Sandoval Formation mechanism and photoluminescence of AlN nanowhiskers H T Chen, X L Wu, X Xiong et al. Low-temperature collapsing boron nitride nanospheres into nanoflakes and their photoluminescence properties\ Jie Li, Han Luo, Jing Lin et al. Synthesis and characterization of electrospun superconducting (La,Sr)CuO4 nanowires and nanoribbons X L Zeng, M R Koblischka and U Hartmann 352 nm ultraviolet emission from high-quality crystalline AlN whiskers Baodan Liu, Yoshio Bando, Aimin Wu et al.
Nanotechnology Nanotechnology 26 (2015) 085603 (7pp)
doi:10.1088/0957-4484/26/8/085603
AlN hollow-nanofilaments by electrospinning Tony Gerges1, Vincent Salles1, Samuel Bernard2, Catherine Journet1, Xavier Jaurand3, Rodica Chiriac1, Gabriel Ferro1 and Arnaud Brioude1 1
Laboratoire des Multimatériaux et Interfaces (UMR 5615 Université Lyon 1-CNRS), 43 Bd du 11 novembre 1918, F-69622, Villeurbanne Cedex, France 2 IEM (Institut Europeen des Membranes), UMR 5635 (CNRS-ENSCM-UM2), Université Montpellier 2, Place E. Bataillon, F-34095, Montpellier, France 3 Centre Technologique des Microstructures, Université Claude Bernard Lyon1, 5 rue Raphael DuboisBâtiment Darwin B, F-69622, Villeurbanne Cedex, France E-mail: [email protected] Received 31 October 2014, revised 21 December 2014 Accepted for publication 12 January 2015 Published 4 February 2015 Abstract
We present for the first time an original method to elaborate AlN nanofilaments (NFs) by using a preceramic-based electrospinning process. Initially, an Al-containing precursor (poly (ethylimino)alane) is mixed with an organic spinnable polymer to be electrospun and generate polymeric filaments with a homogeneous diameter. A ceramization step at 1000 °C under ammonia and a crystallization step at 1400 °C under nitrogen are performed to get the final product made of AlN NFs with a diameter ranging from 150 to 200 nm. Studies carried out by high resolution electron microscopy and 3D tomography show their regular morphology, with high chemical purity and polycrystalline nature. S Online supplementary data available from stacks.iop.org/NANO/26/085603/mmedia Keywords: aluminum nitride, nanofilaments, polymer-derived ceramics, electrospinning (Some figures may appear in colour only in the online journal) 1. Introduction
By reducing the size of AlN material to a submicronscale as a one-dimensional (1D) nanostructure, the contact surface (i.e. specific surface area) is considerably increased and the expected properties of a composite (or a device) containing those nanoscaled objects should be enhanced. Moreover, the aspect ratio of the nanofilaments (NFs) should help to adjust the above-mentioned properties in specific directions. As already demonstrated with another kind of inorganic filler [8], very thin functional materials or devices can be prepared. Surprisingly, AlN nanomaterials have been poorly studied up to nowadays. In the literature, the very few related materials can be presented through their respective elaboration processes as following: chemical vapor deposition (CVD) [9–17], direct nitridation [1, 9, 15], direct sublimation [13], carbothermal reduction and nitridation (CRN) [10, 18, 19], rapid nitridation [11], combustion synthesis [20], and plasma-enhanced atomic layer deposition [12]. Those
Aluminum nitride (AlN) is a very important compound of the group III nitrides, characterized by the widest band gap, 6.2 eV (hexagonal phase) [1]. Moreover, AlN possesses high thermal conductivity (320 W m−1 K−1), high electrical resistivity (1013 Ω cm), small dielectric constant (8.8 at 1 MHz) [2] and particular piezoelectric behavior. As a consequence, it also plays an important role in the fields of electronics and optoelectronics [3], as sensors and actuators, thanks to its piezoelectric properties [4, 5], to fabricate surface acoustic wave detectors for instance [6]. AlN fillers have interested more researchers in the last few years to prepare thermal conductive and electric insulating composites for electronic applications by using a polymer matrix. A recent paper shows very interesting results with polyimide (PI)-modified AlN composites which can be used extensively for electronic packaging applications [7]. 0957-4484/15/085603+07$33.00
1
© 2015 IOP Publishing Ltd Printed in the UK
T Gerges et al
Nanotechnology 26 (2015) 085603
methods are used to obtain nanofibers (or NFs) [1, 11–13], nanotubes [14, 18], nanotips [10, 17], nanobelts [15], nanoneedles [16], nanotowers [9] and nanowhiskers [20]. Electrospinning is a suitable shaping process to fabricate very long filaments with adjustable diameters from tens of nm to several μm which cannot be easily obtained with other methods. Self-supported materials as well as short filaments (or wires or tubes) can be prepared depending on the final targeted application [21, 22]. Compared to other fabricating methods, the electrospinning process presents also the possibility to obtain a higher production rate of filaments, with high aspect ratio, and without using any catalyst as in CVD method. The high purity of the material has a positive influence on its final properties. This is true for all scales from bulk to 1D AlN materials. To the best of our knowledge, only two papers describe AlN NFs fabrication by electrospinning. Firstly, Al-containing tubes were previously prepared by depositing trimethylaluminum on electrospun nylon filaments and then by treating under air the assembly in order to remove the polymer part [12]. However, the resulting material is composed of about 40 at% of oxygen. Another paper based on electrospinning showed the ability to produce AlN tubes with a higher purity by introducing aluminum nitrate into the spinnable aqueous solution. The resulting as-spun material was then converted into AlN after thermal treatment at 1600 °C with non-negligible amount of remaining carbon under N2 using a CRN process. The authors mentioned that nitridation occurs above 1000 °C. The obtained tubes were polycrystalline with a diameter of 500 nm [18]. We present in this paper, an original and versatile method based on electrospinning process to get highly pure AlN NFs with a tunable nanometric diameter. This experimental strategy is based on the polymer-derived ceramics (PDCs) route which is known to produce non-oxide ceramics, as previously described for boron nitride NFs for instance [23]. This combined method allows to produce pure and stable AlN NFs starting from a poly(N-ethylimino)alane (PEIA), and to tune their structural properties from the amorphous state to a high crystallization degree.
electrospinning in order to use them as template. All chemicals were used without further purification. 2.2. From PAN template to AlN NFs
PAN was first spun before being covered by the aluminum precursor. The electrospinning experiment was performed by loading with the PAN solution a syringe equipped with a 21 Gauge stainless-steel needle. The electric field was applied between the needle, connected to a positive high voltage power supply with an alligator clip, and an aluminum foil serving as a collector, connected to the ground. The applied voltage and the working distance between the needle tip and the collector were 12 kV and 15 cm respectively, with a feeding rate of 1 mL h−1. The electrospun PAN filaments were air-dried at RT for 12 h and then stabilized in air at 280 °C for 2 h, with a heating rate of 2 °C min−1. The 2 cm2 surface PAN filaments template, stabilized at 280 °C, was consecutively inserted into the glove box and then immersed in 2 ml of a solution containing 2 wt% of AlN precursor. The impregnation step was performed at ambient temperature for 2 min. After PEIA deposition, the sample was rinsed into 3 ml of chloroform and then dried for several minutes on an absorbent paper. The coated filaments were firstly heated under ammonia flow at 1000 °C for ceramization with a heating rate of 1 °C min−1 and subsequently heated to 1400 °C for 5 h under nitrogen flow for crystallization with a heating rate of 3 °C min−1. It has to be noticed that a dwell at 50 °C for 1 h before the heat treatment at high temperatures is necessary for the precursor to allow time to be decomposed without changing the filaments morphology. The final samples were stored in a glove box under argon. 2.3. Characterization techniques
Thermogravimetric analyses were performed on a TGA/ SDTA 851e Mettler Toledo apparatus. A few mg of sample were introduced in an alumina crucible with a pierced lid and the temperature was increased from 30 to 1000 °C at 5 °C min−1 under air (50 mL min−1). In order to check the morphology of the samples, the first characterization is conducted by scanning electron microscopy (SEM) (FEI-Quanta 250). The samples were fixed on an aluminum holders covered by a carbon tape before platinum deposition (5 nm) by sputtering. Then, their nanostructure is investigated by transmission electron microscopy (TEM) (Jeol-2100F). In order to prevent the oxidation of the AlN filaments, a specific TEM PicoIndenter is used. It is put into the glove box and charged with the sample by keeping it away from air until its introduction inside the TEM chamber. The samples composition and phases were determined by x-ray diffraction (XRD) patterns (Philipps PW 3040/60 PANalytical X’Pert PRO (CuKα radiation; λ = 1.5406 Å at 40 kV and 30 mA). A μ-Raman spectrometer (LabRAM ARAMIS HORIDA) equipped with a 20 mW HeNe laser beam (λ = 633 nm) with a spot of few μm2 in a co-focal configuration for polytypes identification. Each NFs mean diameter was determined from measures performed on 100 filaments, using the software ImageJ.
2. 2. Materials and experimental procedure 2.1. Materials
The AlN filaments were prepared using a raw precursor poly (N-ethylimino)alane ((HAlNEt)n, PEIA) which synthesis was previously described [24]. This precursor is formed after reaction between lithium tetrahydro aluminate (LiAlH4) and ammonium chloride (NH4Cl) at −78 °C in methylene oxide. The as-prepared precursor is a white and moisture sensitive solid, thus stored and handled in an argon-filled glove box. Its solubility into chloroform (CHCl3) allows preparing a transparent solution with 2 wt% of Al precursor. A second solution containing a spinnable polymer, polyacrylonitrile (PAN) (Mw = 150 000 g mol−1, Sigma Aldrich) in N,N-dimethylformamide (Sigma Aldrich), was used to fabricate PAN NFs by 2
T Gerges et al
Nanotechnology 26 (2015) 085603
Figure 1. SEM image of PAN fibers after impregnation for 2 min in Al precursor solution (a) low magnification, (b) high magnification.
3. Results and discussion 3.1. Before annealing process
The morphology of as-spun PAN filaments stabilized at 280 °C under air has been investigated by SEM (SI1). Their mean diameter has been evaluated at 400 nm and the surface roughness is quite low and uniform. After 2 min of the mat impregnation with the PEIA solution, the filaments are covered by the latter (figure 1). It has to be noticed that, compared to as-spun filaments, their diameter is increased of about 40 nm, to obtain a final diameter around 440 nm. Their surface is decorated with small nanoparticles regularly deposited showing clearly the impregnation reaction between PEIA and the PAN. Figure 2. SEM image of the filaments after the ceramization step in a
3.2. Ceramization step
furnace at 1000 °C under ammonia.
The ceramization step, performed in a furnace at 1000 °C under ammonia, has mainly two purposes: (1) to remove the carbon coming from both the PAN and PEIA; (2) to enrich PEIA with nitrogen and convert the latter into AlN. As a consequence of this treatment, the PAN core is then totally decomposed. A 3D tomography study has been specially conducted to observe the morphology of the filaments after heat treatment at 1000 °C (SI2). The video in attachment shows clearly the central cavity inside an isolated filament, confirming the decomposition of the PAN part of the initial filament, and leading to the formation of tubes as expected before. A typical SEM observation of the sample is also shown in figure 2. It evidences that the hollow filaments are uniform and without aggregates on several tens of micrometers. The diameter of those new nanostructures ranges from 110 to 200 nm and the mean value is centered at 170 nm. The reduction of diameter from the original 400 nm (as-spun PAN filaments) is explained by the loss of carbon during the heat treatment and the reticulation of the precursor with the polymer. The obtained filaments are constituted of small aggregated
crystallites (mean diameter of 10 nm), which suggests a polycrystalline structure. In order to form highly crystallized AlN hollow filaments, the previous sample was heated further for 5 h at 1400 °C under nitrogen gas. The figure 3 shows SEM images of the filaments obtained in this way, with a mean diameter estimated close to 150 nm. Their macroscopic morphology is kept constant while a closer look shows that the tubes are formed of larger nanoparticles aggregates with a greater porosity. Indeed, as a consequence of the temperature induced crystallization effect, the particles forming the filaments grow and increase the porosity of the sample. In order to check the purity of the sample, a quantitative analysis by energy-dispersive spectroscopy (EDS) has been made. In supporting information (SI3), the EDS spectrum shows that the sample components are only Al and N with an atomic ratio close to 1. Moreover, this synthesis was performed in an isolated enclosure without oxygen; therefore, no quantifiable amount 3
T Gerges et al
Nanotechnology 26 (2015) 085603
Figure 3. (a) SEM image of the hollow filaments after the crystallization step in a furnace at 1400 °C under nitrogen (b) SEM image shows a magnification of the central region of image (a), with a magnified part of the picture (inset).
Figure 4. (a) TEM image of the hollow filaments after the ceramization step at 1000 °C under ammonia. Inset: diffraction pattern of the
corresponding area. (b) HRTEM image of a filament.
reported with enough accuracy. The HRTEM mode was used to more precisely focus on ultrafine structure and on very small nanocrystallites with sizes inferior to 10 nm embedded in an amorphous matrix (figure 4(b)). Thus, crystallographic planes related to the (100) interplanar distance have been observed, confirming the initial stage of AlN formation (figure 4(b)) at only 1000 °C. We have performed the same study on the corresponding sample annealed at 1400 °C under nitrogen. On figure 5, the tubular structure is more clearly exhibited by the contrast image. Larger particles can be identified and the high rate of crystallization is sufficient to distinguish the atomic columns of the AlN structure. The diffraction pattern associated to large area (inset of figure 5(a)) shows the main interplanar distances of AlN. Contrary to the previous figure, very thin rings can be distinguished and attributed to
of oxygen was detected. The sample remained stable even after four weeks with environmental air exposure as shown by the FTIR analysis (SI4), that shows the Al–N band between 625 and 670 cm−1 [25, 26] without any impurities, even hydroxyl group (−OH)). It is known that the presence of oxygen or carbon in the AlN structures can have a deleterious effect on their thermal, electrical and optical properties [27, 28]. In the present study, the purity of the AlN NFs obtained indicates their high quality. The structural properties of the samples have been investigated by TEM in conventional and high resolution mode. SAED patterns recorded on a large zone of the samples (inset of figure 4(a)) show that filaments annealed at only 1000 °C are mainly amorphous. The rings present in the diffraction pattern are quite large and only the one corresponding to the (100) direction of the AlN bulk material (JCPDS NO.25-1133) has been reasonably 4
T Gerges et al
Nanotechnology 26 (2015) 085603
Figure 5. (a) TEM image of the hollow filaments after the crystallization step at 1400 °C under nitrogen. Inset: diffraction pattern of the
corresponding area. (b) HRTEM image of filament nanoparticles.
Figure 6. X-ray diffraction patterns for AlN nanofilaments treated at 1000 °C (a) and 1400 °C (b).
Figure 7. Raman spectrum of AlN nanofilaments treated at 1400 °C under nitrogen.
the hexagonal phase of AlN. The AlN filaments are composed of many interconnected particles randomly oriented. The mechanical strength of the entire filament is insured by this particular polycrystalline structure where all the grains (mean size around 20 nm) are linked together with strong grain boundaries (SI6). The Moiré fringes, which correspond to the superposition of two AlN nanoparticles, are also a proof of the high level of crystallinity of these particles. XRD (figure 6) and Raman spectroscopy measurements (figure 7) have also been done to further studying the structural properties of the samples. Figure 6(a) shows the XRD pattern of AlN hollow NFs after heat-treatment at 1000 °C under ammonia. The peaks confirmed that the obtained product can be identified as
hexagonal AlN (h-AlN), in accordance with the JCPDS card (NO.25-1133). By applying the Scherrer equation, a particle mean diameter of 11.6 nm can be calculated. This value is consistent with the result previously obtained by TEM. After heat-treatment at 1400 °C under nitrogen, the samples present XRD peaks much narrower (figure 6(b)). This indicates a significant improvement of the crystallinity of AlN filaments. The calculated mean particle size is thus increasing to 22.5 nm as expected from TEM observations. All of the diffraction peaks in the pattern can be assigned to the hexagonal würtzite AlN phase, with the following lattice parameters: a = 3.109 Å and c = 4.979 Å. No other peaks due to impurities are detected. 5
T Gerges et al
Nanotechnology 26 (2015) 085603
For AlN, six Raman-active modes may be present [29, 30], and they are all present for the present material (figure 7): the modes E2(low) at 248 cm−1, A1(TO) at 611 cm−1, E2(high) at 655 cm−1, E1(TO) at 667 cm−1, A1(LO) at 893 cm−1 and E1(LO) at 906 cm−1 which exactly correspond to the würtzite-AlN (W-AlN) are clearly identified [24, 29]. It is well known that the structure of AlN can be detected by the FWHM of the curve of E2(high) mode. For high quality AlN bulk crystals, a value of 3.2 cm−1 was measured [31]. In our case, the value of 12.3 cm−1 found, could be an index of the polycristallinity obtained [32]. The caxis orientation of the NFs can be deduced from the ratio between the area of the E2(high) mode and that of the A1(TO) mode [33]. The proportion of the two areas obtained on our spectrum denotes a crystal orientation majority along to the caxis with a small inhomogeneous orientation probably in the polycrystalline NFs. In addition, a c-axis orientation is an important parameter indicating the good piezoelectricity response of the AlN NFs [34]. Finally, Hsu et al [35] do not detect any changes of the peak position as well as of the FWHM of the E2 (high) mode, whatever the NFs diameter. One of the advantages of the entire elaboration process is that the PEIA is synthesized prior the electrospinning experiments. This last point is of main importance in order to adjust the composition and the diameter of the final product. Two strategies can take place to modify the tubes diameter. The first one concerns the initial diameter of the PAN NFs used as template before impregnation. This allows modifying the inner and consequently the outer diameters of the final AlN tubes (SI7). The second one corresponds to the concentration of the PEIA inside the solution used for NFs impregnation. In this latter case we have verified that the obtained filaments can be glued to each other if a higher amount of 2 wt% of precursor is used into the solution. The SEM and TEM images of the two types of filaments show that the morphology (hollow structure) and the crystallization state with the presence of small well-crystallized particles are preserved for all diameters obtained (100–500 nm) (SI7). In terms of implementation, the filaments can be used at room temperature without requiring any special inert environment. This last point is of main importance with regard to future applications. An ageing study was made to follow the effect of the temperature in an oxidative environment on the surface state of the filaments. Figure 8 shows the TGA under air of AlN NFs with a mass gain starting at 550 °C to reach a value of 11.85% at 1000 °C implying an oxidation of 50% of the sample. The oxidation was slightly detected by XRD spectra (SI5) indicating a global presence of AlN with two small additional peaks attributed to γ-alumina. Compared to the literature, these results confirm a high stability of the AlN NFs against oxidation up to around 550 °C. These AlN NFs seem to be, under oxidative atmosphere, at least as good as AlN materials previously investigated in the literature with other dimensions (micropowders [36], films [37], bulk [38]) . This result is very important taking into account the maximum working temperature of about 300–400 °C, before implying the polymer degradation (most commonly polyimide or epoxy resins).
Figure 8. TGA under air for the nanofilaments after annealing at 1400 °C showing the mass gain from 550 °C to 1000 °C.
4. Conclusion We present for the first time an original and versatile method based on electrospinning process coupled with the PDCs route to get pure hollow AlN NFs with a tunable nanometric diameter. Starting from a PEIA and a well-adapted annealing treatment under ammonia and then nitrogen up to 1400 °C, AlN NFs with a tailored diameter in the range 100–500 nm, and a controlled crystallization state from amorphous to polycrystalline are generated. This method brings an opportunity to fabricate AlN NFs which seem to be suitable and efficient for future use as thermal conductive fillers in polymer matrices for electronic applications. Their high stability under air up to 550 °C indicates that these AlN nanostructures can be handled under air without losing their physical properties.
References [1] Zhang P G, Wang K Y and Guo S M 2010 Ceram. Int. 36 2209 [2] Kuang J C, Zhang C R, Zhou X G, Liu Q C and Ye C 2005 Mater. Lett. 59 2006–10 [3] Kandaswamy P K, Machhadani H, Bellet-Amalric E, Nevou L, Tchernycheva M, Lahourcade L, Julien F H and Monroy E 2009 Microelectron. J. 40 336–8 [4] Mortet V, Soltani A, Talbi A, Pobedinskas P, Haenen K, De Jaeger J-C, Pernod P and Wagner P 2009 Procedia Chem. 1 40–3 [5] Giordano C, Ingrosso I, Todaro M T, Maruccio G, De Guido S, Cingolani R, Passaseo A and De Vittorio M 2009 Microelectron. Eng. 86 1204–7 [6] Lloreta F, Araújoa D, Villara M P, Rodríguez-Madridb J G, Iriarteb G F, Williamsc O A and Calle F 2013 Microelectron. Eng. 112 193–7 [7] Zhou Y, Yao Y, Chen C Y, Moon K, Wang H and Wong C 2014 Sci. Rep. 4 4779 [8] Fiorido T et al 2014 Sensors Actuators A 211 105–14 [9] Zhang P G, Wang K Y, Liang J and Guo S M 2011 Physica E 43 934–7 [10] Jung W-S 2009 Bull. Korean Chem. Soc. 30 1563 6
T Gerges et al
Nanotechnology 26 (2015) 085603
[25] Dimitrova V, Manova D, Paskova T, Uzunov T, Ivanov N and Dechev D 1998 Vacuum 51 161–4 [26] Antsiferov V N, Gilyov V G and Karmanov V I 2002 Vib. Spectrosc. 30 169–73 [27] Tang Y, Cong H, Li F and Cheng H-M 2007 Diam. Relat. Mater. 16 537–41 [28] Schowalter L J, Schujman S B, Liu W, Goorsky M and Shahedipour-Sandvik F 2006 Phys. Status Solidi (A) 203 1667–71 [29] Cao Y G, Chen X L, Lan Y C, Li J Y, Xu Y, Xu T, Liu Q and Liang J K 2000 J. Cryst. Growth 213 198–202 [30] Tian Y, Jia Y, Bao Y and Chen Y 2007 Diam. Relat. Mater. 16 302–5 [31] Kuball M, Hayes J M, Prins A D, van Uden N W A, Dunstan D J, Shi Y and Edgar J H 2001 Appl. Phys. Lett. 78 724 [32] Lughi V and Clarke D R 2006 Appl. Phys. Lett. 89 241911 [33] Chen D, Xu D, Wang J, Zhao B and Zhang Y 2008 Thin Solid Films 517 986–9 [34] Sanz-Hervás A, Clement M, Iborra E, Vergara L, Olivares J and Sangrador J 2006 Appl. Phys. Lett. 88 161915 [35] Hsu H-C, Hsu G-M, Lai Y-S, Feng Z C, Tseng S-Y, Lundskog A, Forsberg U, Janzén E, Chen K-H and Chen L-C 2012 Appl. Phys. Lett. 101 121902 [36] Gu Z, Edgar J H, Wang C and Coffey D W 2006 J. Am. Ceram. Soc. 89 2167–71 [37] Kang H C, Seo S H, Kim J W and Noh D Y 2002 Appl. Phys. Lett. 80 1364 [38] EDGAR J H, Gu Z and Taggart K 2006 Mater. Res. Soc. Symp. Proc. 892 FF21–02
[11] Lee T H, Nersisyan H H, Jeong H G, Lee K H, Noh J S and Lee J H 2011 Chem. Eng. J. 174 461–6 [12] Ozgit-Akgun C, Kayaci F, Donmez I, Uyar T and Biyikl N 2013 J. Am. Ceram. Soc. 96 916–22 [13] Lei M, Song B, Guo X, Guo Y F, Li P G and Tang W H 2009 J. Eur. Ceram. Soc. 29 195–200 [14] Stan G, Ciobanu C V, Thayer T 0 P, Wang G T, Creighton J R, Purushotham K P, Bendersky L A and Cook F 2009 Nanotechnology 20 035706 [15] Tang Y, Cong H, Li F and Cheng H-M 2007 Diam. Relat. Mater. 16 537 [16] Shah S, Nabi G, Khan W, Majid A, Ali S, Hussain M, Nabi A and Butt F K 2013 Mater. Lett. 107 255–8 [17] Shi S C, Chattopadhyay S, Chena C F, Chen K H and Chen L C 2006 Chem. Phys. Lett. 418 152–7 [18] Sun Y, Li J Y, Tan Y and Zhang L 2009 J. Alloys Compd. 471 400–3 [19] Suehiro T, Tatami J, Meguro T and Komeya K 2002 J. Am. Ceram. Soc. 85 715–7 [20] Shi Z, Radwan M, Kirihara S, Miyamoto Y and Jin Z 2009 J. Alloys Compd. 476 360–5 [21] Greiner A and Wendorff J H 2007 Angew. Chem., Int. Ed. Engl. 46 5670–703 [22] Julijana P, Biljana K, Baumgartner J and Kocbek P 2013 Int. J. Pharmaceutics 456 125–34 [23] Salles V, Bernard S, Brioude A, Cornu D and Miele P 2010 Nanoscale 2 215–7 [24] Termoss H, Bechelany M, Toury B, Brioude A, Bernard S l, Cornu D and Miele P 2009 J. Eur. Ceram. Soc. 29 857–61
7
