Highly aligned carbon nanotube forests coated by superconducting NbC

Transcription

1 Received 24 Nov 21 Accepted 14 Jul 211 Pulished 16 Aug 211 DOI: 1.138/ncomms1438 Highly ligned cron nnotue forests coted y superconducting NC G.F. Zou 1,2, H.M. Luo 3, S. Bily 4, Y.Y. Zhng 2, N.F. Herkorn 4, J. Xiong 2, E. Buer 5, T.M. McCleskey 5, A.K. Burrell 5, L. Civle 4, Y.T. Zhu 6, J.L. McMnus-Driscoll 7 & Q.X. Ji 2 The formtion of cron nnotue nd superconductor composites mkes it possile to produce new nd/or improved functionlities tht the individul mteril does not possess. Here we show tht coting cron nnotue forests with superconducting nioium cride (NC) does not destroy the microstructure of the nnotues. NC lso shows much improved superconducting properties such s higher irreversiility nd upper criticl field. An upper criticl field vlue of ~5 T t 4.2 K is much greter thn the 1.7 T reported in the literture for pure ulk NC. Furthermore, the ligned cron nnotues induce nisotropy in the upper criticl field, with higher upper criticl field occurring when the mgnetic field is prllel to the cron nnotue growth direction. These results suggest tht highly oriented cron nnotues emedded in superconducting NC mtrix cn function s defects nd effectively enhnce the superconducting properties of the NC. 1 School of Physics Science nd Technology, Soochow University, Suzhou 215, Chin. 2 Center for Integrted Nnotechnologies, Los Almos Ntionl Lortory, Los Almos, New Mexico 87545, USA. 3 Deprtment of Chemicl Engineering, New Mexico Stte University, Ls Cruces, New Mexico 883, USA. 4 Superconductivity Technology Center, Los Almos Ntionl Lortory, Los Almos, New Mexico 87545, USA. 5 Mterils Chemistry, Los Almos Ntionl Lortory, Los Almos, New Mexico 87545, USA. 6 Deprtment of Mterils Science & Engineering, North Crolin Stte University, Rleigh, North Crolin 27695, USA. 7 Deprtment of Mterils Science nd Metllurgy, University of Cmridge, Cmridge CB2 3QZ, UK. Correspondence nd requests for mterils should e ddressed to G.F.Z. (emil: zouguifu@sud.edu.cn) or to Q.X.J. (emil: qxji@lnl.gov).

2 nture communictions DOI: 1.138/ncomms1438 In contrst to the power ppliction required of high-temperture superconducting mterils 1, the mterils under considertion for hot-electron olometric superconducting mixers 2 nd superconductive rdio-frequency cvities 3 require higher superconducting trnsition temperture nd high conductivity (or low resistivity) in the norml stte t low temperture. On the sis of this criterion, nioium cride (NC), which exhiits high melting point (3,61 C), good chemicl stility, excellent hrdness, nd superior wer resistnce, hs een considered s one of the potentil mterils. Unfortuntely, the growth of cermic-like (or ulk) NC with desired functionlities hs een ig chllenge ecuse rection temperture ove 1, C is generlly required. Such high processing temperture leves no plyground to mnipulte the microstructure nd/or defect lndscpe in NC. In other words, innovtive pproches re needed to synthesize NC with controlled microstructures/defects so tht enhnced superconducting properties (such s the irreversiility field, H irr, the highest useful field ove which there is no flux pinning nd where criticl current density is zero), reduced electricl resistivities in the norml stte t low tempertures, nd improved mechnicl strengths cn e ccomplished for the pplictions of olometric mixers nd/or coted cvities. Cron nnotues (CNTs) re known to e the strongest nd stiffest mterils in terms of tensile strength nd elstic modulus, respectively 4. Furthermore, the metllic tues re highly conductive. A successful comintion of NC nd CNTs cn therefore potentilly led to much improved functionlities nd/or crete emergent ehviours through coupled competing order prmeters. The chllenge is, however, to mintin the integrity of individul components fter the post processes, ecuse superconducting NC mterils re typiclly grown t high temperture nd CNTs re synthesized in the reduced environment. Here for the first time in the field, we report the integrtion of superconducting NC with the highly ligned CNT forests using chemicl solution pproch 5. The formtion of superconducting NC coting on the CNT forests does not destroy the microstructure of CNTs. Importntly, NC shows much improved superconducting properties such s higher irreversiility field (H irr ) nd upper criticl field (H c2 ). A vlue of H c2 round 5 T t 4.2 K is much greter thn 1.7 T for the pure ulk NC reported in the literture 6. Moreover, the ligned CNTs induce nisotropy in H c2, with higher H c2, when the mgnetic field is prllel to the CNT growth direction. These results suggest tht highly oriented CNTs emedded in superconducting NC cn function s defects nd effectively enhnce the superconducting properties of the NC. It should e noted tht introduction of rtificil defects into high-temperture superconducting YB 2 Cu 3 O 7 x (YBCO) hs een rodly investigted to modify the superconducting properties. For exmple, much improved superconducting properties hve een ccomplished through introducing columnr defects y high-energy irrdition 7,8 nd volume defects y inclusion of nnoprticles or of self-ssemled, verticlly ligned moo rrys 9,1. Results Structurl properties of CNT NC composites. Figure 1 shows the schemtic drwing of the processing steps to form the CNT NC composites. The highly ligned CNT forests were grown on the silicon sustrte y chemicl vpour deposition 11. The CNT forests were then soked y homogeneous N precursor solution. After detching the CNT forests from the silicon sustrte (simply y peeling off the CNT forest from the Si sustrte using tweezer), we converted the N precursor to NC y nneling the smple under n ethylene environment. Ethylene is lso used s the cron source for the growth of CNT forests 11. The integrity of oth CNT forests nd NC is mintined fter the formtion of CNT NC composite. Figure 2 shows the SiO 2 /Si Fe film CNT NC composite CVD growth Anneling CNT forest Solution permetion CNT coted with N precursor solution Figure 1 The schemtic processing steps for the synthesis of CNT NC composites. The NC is the mtrix nd CNT forest is the second phse mteril distriuted in the mtrix. CVD, chemicl vpour deposition. c 1 µm 1 µm 1 µm 1 µm morphologies nd the microstructures of the pure CNT nd the NC-coted CNT forests. As cn e seen from the scnning electron microscopy (SEM) imge shown in Figure 2, the CNTs prepred y chemicl vpour deposition re highly ligned long the growth direction. The enlrged view (inset in Fig. 2) clerly shows tht the CNTs in the forest re closely pcked nd homogeneously distriuted. The high-resolution trnsmission electron microscope (HRTEM) imge shown in Figure 2 illustrtes tht the CNTs hve multiple wlls nd n inner dimeter of ~. Detiled HRTEM nlysis revels tht the CNT wll thickness is round 1.8 nm (see inset in Fig. 2). As compred with the s-grown CNT forest, the whole ssemly of NC-coted CNT forest is even denser s reveled y the SEM imge shown in Figure 2c. Furthermore, the cross-section of the NC-coted CNT forest is quite uniform. 2 nm 2 nm 18 nm Figure 2 Morphology nd microstructure of the prepred CNT forest nd CNT NC composite. () SEM imge of highly ligned CNT forest, () HRTEM imge of CNTs, (c) SEM imge of s-synthesized CNT NC composite, nd (d) HRTEM imge of NC-coted CNT. Insets show the mgnified SEM imge of CNT forest (), the high-resolution electron microscopy imge of the s-grown multi-wll CNTs (), nd the mgnified SEM imge CNT NC composite (c), respectively. d

3 nture communictions DOI: 1.138/ncomms1438 ARTICLE X-ry detector Intensity (.u.) CNT (2) θ (degree) Intensity (.u.) X-ry detector NC (111) NC (2) NC (111) NC (2) CNT (1) Resistivity (µω cm) H (T) H // CNTs 5 T 4 T 3 T 2 T 1 T.5 T T H // CNTs H c2 H irr θ (degree) Figure 3 X-ry diffrction ptterns of s-synthesized CNT NC composites. () X-ry ws irrdited long the side of the smple, nd () X-ry ws irrdited from the top of the smple Figure 4 Superconducting properties of CNT NC composite. () Resistivity versus temperture chrcteristics t different pplied mgnetic fields, where the mgnetic field is prllel to the CNT growth direction, () irreversiility field nd upper criticl field s function of temperture otined from those curves, where the irreversiility field H irr is defined s the field t which the ρ/ρ N = 3% nd the upper criticl field s the field t which the ρ/ρ N = 8%. The mgnified imge (inset in Fig. 2c) clerly shows tht wellligned CNTs re still mintined in the CNT NC composite. Figure 2d shows HRTEM imge of NC-coted CNT. The lttice pttern on the wll mtches well with tht of the NC. In ddition, tuulr shdow is clerly seen from this HRTEM imge. The inner dimeter of the tue shdow is round tht is the sme s the dimeter of the s-synthesized CNTs shown in Figure 2. The full retention of the highly ligned nture of CNT forests in the CNT NC composite is confirmed y the X-ry diffrction mesurement. To illustrte this, we nneled the N precursorcoted CNT forest in pure Ar (in the sence of ethylene) under the sme temperture used for the synthesis of CNT NC composites. X-ry diffrction θ 2θ scns (not shown here) reveled (11) N nd (1) CNT peks only. The ppernce of N nd CNT peks only implies tht the integrity of the cron nnotue is retined. In other words, the chemicl rection etween N nd CNT should, if there is ny t ll, e negligile under such processing conditions. Figure 3 shows the typicl diffrction pttern of the CNT NC composite when the incident X-ry ws irrdited long the side of the smple (see the schemtic drwing in Fig. 3). In ddition to diffrction peks of (111) nd (2) from NC, the (2) pek from the CNTs is visile. On the other hnd, when the incident X-ry ws irrdited from the top of the smple (see the schemtic drwing in the Fig. 3), the (1) pek t 42.4 from the CNTs, nd the sme diffrction peks from the NC, re oserved s shown in Figure 3. These results clerly illustrte tht the highly ligned CNT forest retins its verticl lignment fter the formtion of CNT NC composite. It should e noted tht the interplnr spcing clculted sed on the diffrction peks is =.4469 nm for NC nd.34 nm for (2) CNT, mtching well with the ulk vlues of NC nd CNT, respectively. Superconducting properties of CNT NC composites. The formtion of NC coting on the CNT forests not only mintins the microstructure of CNTs s discussed ove, ut more importntly, the CNT NC composite shows much improved superconducting properties compred with the pure ulk NC. First, the CNT NC composite shows enhnced H irr nd H c2. Figure 4 shows the resistivity ρ(t) versus temperture chrcteristics s function of mgnetic field (H), where the H is pplied prllel to the CNT growth direction. Figure 4 shows H irr nd H c2 s function of temperture otined from those curves. We hve used the field t ρ/ρ N = 3% to define the H irr, nd elow H irr, finite criticl current flows, resulting from flux pinning 12. We hve used the ρ/ρ N = 8% criterion to define H c2 to void the influence of the long til extending to higher tempertures oserved for higher ρ/ρ N frctions, where ρ N is the norml stte resistivity ner the trnsition. The H irr vlue for our CNT NC composite reches s high s 5 T t 4.2 K. The H c2 is even higher. On the sis of our knowledge, these re the highest reported H irr nd H c2 for the ulk NC. Hence, the inclusion of highly ligned CNTs in the NC significntly improves the performnce of the NC under

4 nture communictions DOI: 1.138/ncomms1438 µ H (T) T c2 [K] H c2 // CNTs ( ) H c2 CNTs (9 ) 45 9 Angle ( ) Figure 5 Anisotropic properties of the upper criticl mgnetic field tested for nother CNT NC composite. The inset shows the ngulr dependence of the T c2 (the temperture t which ρ/ρ N = 8%) t n pplied field of 3 T, where nd 9 correspond to the field prllel nd norml to the CNT growth direction, respectively. mgnetic field. It should e noted tht thin film NC cn show very different superconducting properties, in comprison with its counterprt of ulk NC. For exmple, our very thin epitxil NC films, grown y polymer-ssisted deposition (not shown here), hve similr H irr vlue s the ulk CNT NC composites. Similr to the high-temperture superconductor, YBCO, where the film shows much improved superconducting properties 13, the enhnced superconducting properties of our epitxil NC films cn come from the lttice strin, the inclusion of second phses, nd/or the relted defects during the growth. A second importnt effect is tht the superconducting properties of the CNT NC composite ecome nisotropic compred with pure NC. Figure 5 shows the H c2 (T) for nother CNT NC smple, where the H c2 (T) is otined from resistivity versus temperture curves t different mgnetic fields. It is pprent tht H c2 (T) is higher when H is prllel to the ligned CNT forests thn when it is perpendiculr to them. The inset shows the ngulr dependence of the trnsition, T c2 (Θ), t H = 3 T. We cn estimte the nisotropy s γ ~H c2 (//CNT)/H c2 ( CNT)~1.5. Discussion The enhnced H c2 implies shorter superconducting coherence length (ξ). This is consistent with reduction in the electronic men free pth (l) due to the incresed disorder, which turns the mteril towrds the superconducting dirty limit nd reduces the effective ξ (ref. 14). As the properties should e isotropic within the plne perpendiculr to the CNT, for H//CNT, we cn use ξ = (Φ / 2πH c2// ) 1/2 ; so for T~5 K, we otin ξ ~7 nm (ref. 12), where Φ is the mgnetic flux quntum. For H CNT, we hve H c2 = Φ /2πξ ξ // ; thus, t T~5 K, we estimte ξ = γ ξ ~1 nm. The fct tht ξ > ξ my e consequence of l in this direction not eing constrined y the electron scttering with the CNT. Similrly to the strong correlted pinning in high-temperture superconducting YBCO crystls produced y ligned columnr defects creted through high-energy hevy ion irrdition 7, or in YBCO films y self-ssemled second phse BZrO 3 (refs 9,1), it is resonle to speculte tht the ligned CNT forests emedded in the superconducting NC cn function s effective correlted pinning centres for the vortices in the NC. Moreover, in the CNT NC composite, we cn not only mintin well-ligned CNTs, ut lso well-controlled dimeter of the CNTs round 12 ± 2 nm s shown in the Figure 2. This dimeter is well suited to pin vortices in NC, s it is similr to the dimeter of the vortex core, ~2ξ (ref. 15). In high-temperture superconductors such s YBCO, the irreversiility field H irr is ssocited with thermodynmic solid liquid phse trnsition in the vortex lttice. In the prticulr cse of ligned columnr defects, Bose glss trnsition with mximum in H irr for H defects is oserved 15. In contrst, the lower T c nd lrger ξ in the CNT NC composite result in Ginzurg numer Gi~1 7, severl orders of mgnitude smller thn in YBCO; thus, n extended vortex liquid phse is not expected 12, nd H irr just reflects pinning effects. This is consistent with the sence of rodening in the resistivity versus temperture chrcteristics with incresing H, s seen in Figure 4. In contrst to the cse of columnr defects introduced in nisotropic high-temperture superconductors, in the CNT NC composite oth the nisotropy in H c2 nd the vortex pinning re induced y the CNT; thus, detiled studies of the ngulr dependence of the criticl current density elow the H irr (T, Θ) line will e necessry to elucidte the pinning mechnisms. It should e noted tht the re density of columnr defects is nother importnt prmeter one hs to optimize to effectively pin the flux lines 7. As the re density of the CNT forest is round cm 2, corresponding to reltively low mtching fields (B Φ, the field t which the density of vortices nd defects re the sme) of.2 2 T, we elieve tht we still hve room to optimize the CNT volume density in the CNT NC composite. For exmple, we cn mnipulte the dimeter of the CNTs s well s the re density of the CNT in the forest y controlling the lyer thickness of the ctlyst nd the growth temperture of the CNT forests. We would like to point out tht the results demonstrted here my not e directly pplied to mixers nd cvities. For exmple, n enhncement of conductivity through percoltion norml to the CNT growth direction would not e expected to e chieved. However, n enhnced conductivity would e expected long the CNT growth direction ecuse the CNTs re continuous from the top to the ottom of the forest. As comprison, resistivity of 7 µω cm ws reported for NC single crystls 16. Our epitxil NC film hs resistivity of 6 µω.cm t 2 K (ref. 17). The resistivities for the CNT NC composite in this work re ~145 nd ~11 µω cm norml nd prllel to the CNT growth direction, respectively. More efforts re needed to pply CNT NC composites developed here for hotelectron olometric superconducting mixers 3 nd superconductive rdio-frequency cvities 4. However, the process developed here is dptle for the synthesis of other formt of CNT NC composites. We hve recently demonstrted tht polymer-ssisted deposition cn e used to grow high-performnce superconducting NC films 17. It is possile tht one cn use the process developed here to cot NC on CNT rions, if one cn successfully pull CNT rions nd ttch them on sustrte. Therefore, the much improved superconducting properties chieved y forming CNT NC composites could e the first step towrds potentil new pplictions where mechnicl strengths nd high performnce superconducting properties re required. In summry, we hve demonstrted tht well-ligned CNT forest cn e emedded into superconducting NC mtrix, where CNTs cn function s the effective vortex pinning centres. The CNT NC composite shows much enhnced nd nisotropic H c2 nd irreversiility field (5 T t 4.2 K). These vlues re fr higher thn those reported for the ulk NC nd other NC comintions. Methods Preprtion of precursor N solution. The precursor used for the deposition of NC ws n queous solution of N ion ound to polyethyleneimine. To prepre such homogeneous solution, high purity ( > 99%) NCl 5, NH 4 OH, nd 2% HF were dissolved in wter, where the wter ws purified using the Milli-Q wter tretment system. Ultrfiltrtion ws crried out under 6 psi nitrogen pressure using Amicon-stirred cells tht hve 3, moleculr weight cutoff. In detils, 2 g of NCl 5 were converted to N(OH) 5 y dding mmonium hydroxide into the solution. The N(OH) 5 ws then dissolved in 3 ml of deionized wter nd 7.5 ml of

5 nture communictions DOI: 1.138/ncomms1438 ARTICLE 2% HF. Polyethyleneimine ws then dded in 3 g liquots (totl 3. g) nd mixed fter ech ddition. After stirring, the solution ws plced in n Amicon filtrtion unit designed to pss mterils with moleculr weight < 3, g mol 1. The solution ws diluted 3 times to 2 ml nd then concentrted to 35 ml in volume. Inductively coupled plsm-tomic emission spectroscopy showed tht the finl solution ws 4 mm N. Synthesis of ligned cron nnotue forests. The CNT forests were synthesized y chemicl vpour deposition in qurtz tue furnce. The ctlyst used for the growth of CNTs ws thin lyer of Fe (~1. nm) deposited on Al 2 O 3 (~1 nm)/sio 2 (~1 µm)/si wfers. Forming gs (Ar + 6% H 2 ) ws used s crrier gs, nd ethylene served s cron source. The rection ws crried out t 75 C for 12 min with 14 sccm forming gs nd 3 sccm ethylene. The length of synthesized CNTs is round.6 mm. Coting NC on highly ligned cron nnotue forests. The CNT NC composites were formed through therml rection under controlled environment. As-synthesized CNT forests were soked y the N precursor solution. After the solution ws permeted into CNT forests, the CNT forest ws detched from the Si wfer simply y peeling off the CNT forest from the Si sustrte using tweezer. The CNT forests contining N precursor solution were heted to 65 C t rte of 1 C min 1 in the mixture gses of ethylene (1 sccm) nd forming gs (1 sccm). The smple ws nneled t 65 C under such n environment for 1 h. The mixture gses were switched to Ar (1 sccm) while the furnce ws rmped to 1, C in 1 h. The smple ws nneled t this temperture in Ar for 3 h. Finlly, the smple ws nturlly cooling to room temperture y turning off the power supply to the furnce. The thickness of synthesized CNT NC composite is close to.6 mm, which is similr to the length of CNT forest. Chrcteriztion. The N concentrtion in the precursor solution ws mesured using Hori Join Yvon Ultim II inductively coupled plsm-tomic emission spectrometer. X-ry diffrction ws used to chrcterize the crystllogrphic structure of the smples. The morphologies nd microstructures of the smples were nlysed y SEM nd HRTEM. The resistivity versus temperture chrcteristics were mesured using stndrd four-proe technique y Quntum Design Physicl Properties Mesurement System. References 1. Lrlestier, D., Gurevich, A., Mtthew Feldmnn, D. & Polynskii, A. High-T c superconducting mterils for electric power pplictions. Nture 414, (21). 2. Gol tsmn, G. N. & Gershenzon, E. M. Phono-cooled hot-electron olometric mixer: overview of recent results. Appl. Superconduct. 6, (1999). 3. Plmieri, V. New Mterils for Superconducting Rdiofrequency Cvities The 1th Workshop on RF Superconductivity, Tsuku, Jpn (21). 4. Zhng, X. F. Ultrstrong, stiff, nd lightweight cron-nnotue fiers. Adv. Mter. 19, (27). 5. Ji, Q. X. et l. Polymer-ssisted deposition of metl-oxide films. Nt. Mter. 3, (24). 6. Fink, H. J., Thorsen, A. C., Prker, E., Zcky, V. F. & Toth, L. High-field superconductivity of crides. Phys. Rev. 138, A117 A1173 (1965). 7. Civle, L. et l. Vortex confinement y columnr defects in YB 2 Cu 3 O 7 crystls: enhnced pinning t high fields nd tempertures. Phys. Rev. Lett. 67, (1991). 8. Bugoslvsky, Y. et l. Enhncement of the high-mgnetic-field criticl current density of superconducting MgB 2 y proton irrdition. Nture 411, (21). 9. Mcmnus-Driscoll, J. L. et l. Strongly enhnced current densities in superconducting coted conductors of YB 2 Cu 3 O 7 x + BZrO 3. Nt. Mter. 3, (24). 1. Goyl, A. et l. Irrdition-free, columnr defects comprised of self-ssemled nnodots nd nnorods resulting in strongly enhnced flux-pinning in YB 2 Cu 3 O 7 δ films. Supercond. Sci. Technol. 18, (25). 11. Zhng, Y. Y. et l. Tiloring the morphology of cron nnotue rrys: from spinnle forests to undulting foms. ACS Nno 3, (29). 12. Bltter, G., Feigelmn, M. V., Geshkenein, V. B., Lrkin, A. I. & Vinokur, V. M. Vortices in high-temperture superconductors. Rev. Mod. Phys. 66, (1994). 13. Foltyn, S. R. et l. Mterils science chllenges for high-temperture superconducting wire. Nt. Mter. 6, (27). 14. Tinkhm, M. Introduction to Superconductivity (McGrw Hill, 1975). 15. Nelson, D. R. & Vinokur, V. M. Boson loction nd pinning y correlted disorder in high-temperture superconductors. Phys. Rev. Lett. 68, (1992). 16. Allison, C. Y., Finch, C. B., Foegelle, M. D. & Modine, F. A. Low-temperture electricl resistivity of trnsition-metl crides. Solid Stte Commun. 68, (1988). 17. Zou, G. F. et l. A chemicl solution pproch for superconducting nd hrd epitxil NC film. Chem. Commun. 46, (21). Acknowledgements This work ws performed, in prt, t the Center for Integrted Nnotechnologies, U.S. Deprtment of Energy, Office of Bsic Energy Sciences user fcility. Los Almos Ntionl Lortory, n ffirmtive ction equl opportunity employer, is operted y Los Almos Ntionl Security, LLC, for the Ntionl Nucler Security Administrtion of the U.S. Deprtment of Energy under contrct DE-AC52-6NA The work of L.C. nd N.H. ws supported y the US Deprtment of Energy, Office of Bsic Energy Sciences, Division of Mterils Sciences nd Engineering. Author contriutions G.F.Z., H.M.L., S.B., J.X. nd N.F.H. chrcterized the trnsport properties of the mterils nd discussed the dt. Y.Y.Z., Y.T.Z. nd J.L.M. operted nd discussed some of experiments. E.B., T.M.M., nd A.K.B. prepred the precursor solution. G.F.Z., L.C., nd Q.X.J. co-wrote this mnuscript. The work ws directed y G.F.Z. nd Q.X.J. Additionl informtion Competing finncil interests: The uthors declre no competing finncil interests. Reprints nd permission informtion is ville online t reprintsndpermissions/ How to cite this rticle: Zou, G.F. et l. Highly ligned cron nnotue forests coted y superconducting NC. Nt. Commun. 2:428 doi: 1.138/ncomms1438 (211).