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DOI 10.1007/s11182-015-0579-5 Russian Physics Journal, Vol. 58, No. 6, October, 2015 (Russian Original No. 6, June, 2015)
ELASTIC PROPERTIES OF OXIDE NANOWHISKERS PREPARED FROM ELECTROLYTICALLY DEPOSITED COPPER A. N. Priezzheva,1 M. V. Dorogov,1 S. Vlassov,1 I. Kink,2 A. A. Vikarchuk,1 L. M. Dorogin,1 R. Lõkmus,2 and A. E. Romanov3,4
CuO nanowhiskers prepared by copper electrodeposition and subsequent annealing in air are experimentally investigated. A method of manufacturing nanowhisker structures is described. The bending strength of CuO nanowhiskers is estimated, the results of fatigue testing of nanowhiskers are presented, and the strengths of CuO and ZnO nanowhiskers are compared. Keywords: nanowhiskers, electrodeposition, heat treatment, annealing, differential scanning calorimetry, strength, resistivity. INTRODUCTION Needle-like crystals (“moustaches” or whiskers) have actively been investigated since the 1940s of the last century. For a long time the formation of whiskers was considered to be an adverse phenomenon that led to failure of the device. Investigations of whiskers demonstrated that they frequently possess unique properties and their strength characteristics are close to the theoretical limit. Many methods of synthesis of metal and oxide whisker structures have been developed by the present time. Among them, great attention is focused on CuO nanowhiskers that have a number of advantages: copper is a rather inexpensive raw material; it has wide practical application and can be easily processed to prepare oxide structures. Metal and oxide whisker structures are widely used as nano- and microwires, contacts, probes, and catalysts. A review of oxide whiskers was presented in , where various methods of synthesis, possible mechanisms of growth, fields of nanowhisker application, and different nanowhisker properties (electric, optical, and magnetic) were described. However, the literature data on the mechanical properties of nanowhiskers are still few in number and badly systematized, which motivated the present study. EXPERIMENTAL PROCEDURE Copper oxide (CuO) nanowhisker structures were grown on the surface of copper particles manufactured by the electrodeposition method on the metal grid carrier made from stainless steel or nickel alloy with cell size of 30 m and wire diameter of 40 m and treated thermally in an oxygen-containing atmosphere. Copper was electrodeposited in the potentiostatic regime at cathode overvoltage of 160 mV during 15 min. The electrolyte composition and the electrodeposition procedure were described in more detail in [2, 3]. The annealing temperature was chosen based on the
Togliatti State University, Tolyatti, Russia, e-mail: [email protected]; [email protected]; [email protected]; [email protected]; 2Institute of Physics, University of Tartu, Tartu, Estonia, e-mail: [email protected]; 3A. F. Ioffe PhysicalTechnical Institute of the Russian Academy of Sciences, Saint Petersburg, Russia, e-mail: [email protected]; 4Saint Petersburg National Research University of Information Technologies, Mechanics and Optics, Saint Petersburg, Russia, e-mail: [email protected] Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 95– 99, June, 2015. Original article submitted March 6, 2015. 1064-8887/15/5806-0843 2015 Springer Science+Business Media New York
Figg. 1. Copper paarticles on a metal grid.
DSC). In DSC experiments, sspecimens werre heated in ann oxygen atmossphere data of differential scanning calorimetry (D iin ceramic crucibles with a raate of 10°C/miin at temperatuures ranging froom 25 to 700°С С. The specimens were isothermallyy annealed at a temperature of 400°С choosen using the DSC in the oxygen o atmosphere byy the methodd analogous tto that describbed in . T The morpholoogy of the paarticle surfacee after electrodepositiion and annealling was investtigated by the method of scaanning electronn microscopy ((SEM). Bendin ng and fatigue tests off whiskers werre performed uusing a FEI Hellios Nanolab 6600 high-resoluution scanning electron micro oscope 4 and a Kleindiek manipulatoor arranged in its working cchamber. Indivvidual whiskerrs were subjected to about 1.210 1 lloading cycless. Distributionss of mechanical bending strresses were callculated by the finite elemennt method usinng the software packaage COMSOL Multiphysics.
R RESULTS AN ND DISCUSSION Figuree 1 shows a micrograph m off copper partiicles formed oon the metal ggrid surface aas a result of metal electrodepositiion. As is welll known, the D DSC method alllows the strucctural and phasse transformatiions in the exaamined objects to be ddetermined. Ann analysis of thhe specimens obtained allow wed us to detecct two characteeristic regions in the D DSC curve (Fig. 2): the firstt region with tthe minimum bbeginning at 4400°С and term minating at 4077°С (with a minimal m ttemperature off 401.7°С) andd the second reegion with thee peak beginniing at 445.6°С С and terminatiing at 529.8°С С (with a maximal tem mperature of 476.6°С). 4 Exacctly at these ttemperatures w we observed nnanowhisker grrowth and nannopore formation. Bassed on these reesults, an optim mal specimen annnealing tempeerature of 4000°С was establlished. SEM studies demonnstrated that aftter annealing inn air of electroodeposited coppper at 400°С fo for 4 h, the surfface of tthe grid was covered by dense d nanowhisker “forest” (with density of the order of 1010–1011 cm–2) (Fig. 3)) with characteristic nnanowhisker diameter d ranginng from 30 to 1100 nm and lenngth up to 10 m. As indicatted above, at present p tthere are few ddata on the meechanical propperties of oxidee nanowhiskerss in the literatuure. Meanwhille, the appearaance of forest whiskers causes intereest to the probleems of their strrength and fatigue resistance.. Fatiguue tests of the CuO nanowhhiskers involveed manipulationns with whiskkers in the SEM M working ch hamber uusing the Kleinndiek manipullator. Figure 4 shows an indiividual whiskeer before and dduring tests. Affter repeated looading iin the course of the experim ment (1.2104 ccycles) whiskeers did not chaange initial shaapes and sizess. In addition, in the w whisker bendinng zone (indiccated by the arrrow) no defectts, for examplee, cracks or rupptures characteeristic for thesse tests w were revealed.. From the viewpointt of revealing tthe strength naanowhisker chharacteristics, oof interest is too analyze the elastic bbehavior of thhe whisker rigiidly fixed from m one end. In this case, a ccomplex of appproaches to thhe problem bassed on ttheory of elasttic beams  or direct num merical calculattions within thhe limits of thee elasticity theeory can be ussed. In pparticular, we studied the bennding strength of the whiskerr fixed from onne end under thhe action of thee manipulator probe. The whiskers experienced a maximal strress in the reegion of the ffixed basis because of the employed bouundary
F Fig. 2. DSC annealing a curvve for copper particles recoorded with an X-DSC7000 Exstar ddifferential scaanning calorim meter.
F Fig. 3. CuO nanowhisker structures obbtained on thhe grid carrieer by annealiing of eelectrodepositeed copper.
conditions. Thherefore, the liimited bendingg strength of whiskers was determined exxactly in this region (in thee basis zzone). An exam mple of calculaations perform med is given bellow for the whhisker shown inn Fig. 5. To callculate the elasstic stress distrribution, a num mber of preset and measured parameters, suuch as elastic moduli m (the Young m modulus E and the Poisson ccoefficient ν) aand whisker geeometry and ddisplacement δ (Fig. 5) havee to be ttaken into accoount. Moreoveer, it is sufficiennt to consider the length L off the whisker ffragment from its basis to thee point of contact witth the nanomaanipulator proobe, because eexactly this whhisker fragmeent was subjeccted to deform mation. K Knowing the Y Young moduluus from the refe ference data forr massive CuO O specimens (E E = 81.6 GPa ) and choosiing the ttypical value oof the Poisson coefficient c ν = 0.3, the distribbution of elastiic stresses in thhe whisker cann be calculated by the m method of finite elements. Siince the shape of the cross seection of the exxamined whiskker could not bee precisely idenntified iin our experim ment, we took into i account thhe statistics of observed CuO O nanowhiskerss, according too which these objects o hhad typically rrectangular (cloose to squared)) or circular crooss sections.
F Fig. 4. SEM image of a w whisker before the fatigue tests(a), duringg test (b), andd after rremoval of loaad (c).
F Fig. 5. SEM im mage of whiskkers with the ssuperimposed scheme of measuring of the strain pparameters. Here H L denotees the length of the whiskker fragment and δ denotees the ddisplacement.
SEM image reccorded at the last moment bbefore the whiisker fracture w was used to calculate the whisker w The S llength L and thhe displacemennt δ. These datta were then ussed to model thhe stressed statte by the methood of finite elem ments. The diameter oof the chosen whisker w was m measured beforre the bending tests. The bendding strength σmax of the exaamined w whisker was ddetermined as the t maximal teension stress inn the region of the subsequennt fracture. As a result, the sttrength calculated for the examined nanowhisker n aand averaged over two chosenn (cylindrical aand rectangularr) cross sectionns was σmax ≈ (6.6 ± 00.2) GPa. In  the data on tthe strengths oof ZnO nanowiires ranging froom 1.0 to 9.0 G GPa were pressented. F From compariison of the callculated strenggth of the CuO O nanowires annd the known strength of thhe ZnO nanow wires it follows that thheir mechanicaal properties (inn particular, thhe calculated beending strengthh) lie in the saame ranges. It should s bbe noted that the elastic prooperties of nannodimensional whiskers diffeered from thosse of the bulk CuO because of the anisotropy andd the size effecct that can leadd to somewhatt greater values of strength thhan those pressented above . On tthe example oof the whiskerr analyzed herre, the examinned CuO nanooobjects demonnstrated sufficciently high beending strength. The ffatigue tests deemonstrated thaat the CuO nannowhiskers witthstand 1.2104 loading cycles.
CONCLUSIONS In this work, the procedure of manufacturing of nanowhisker CuO structures by the method of metal electrodeposition and subsequent annealing has been described. The optimal technological regimes of manufacturing of the whisker structure were determined by the method of differential scanning spectroscopy. It was established that the maximal whisker density was formed at the annealing temperature around 400°С. The fatigue tests of the CuO nanowhiskers demonstrated that they withstanded 1.2104 loading cycles. Calculations of the bending strength of the CuO nanowhiskers yielded high values comparable to that of the analogous ZnO nanoobjects. This work was supported in part by the Ministry of Education and Science of the Russian Federation (Decree No. 220), Federal State Budget Educational Institution of Higher Professional Education “Togliatti State University” (Agreement No. 14.V25.31.0011), and by the Project “Structure Sensitive Interaction Mechanisms in Functional Materials at Nanoscale (IUT2-25).”
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