三自由度Delta并聯(lián)機器人的設計與仿真(含CADCREO三維圖)

畢 業(yè) 設 計 論 文 外 文 參 考 資 料 及 譯 文 譯文題目 由表面應力引起的納米多孔金 懸臂梁的宏觀彎曲 學生姓名 專 業(yè) 所在學院 指導教師 職 稱 Surface Stress Induce d Macros copic Bendi ng of Nano porou s Gold Cantilevers Dominik Kramer Raghavan Nadar Viswanath and Jo1rg Weissmu1ller Institut fu r Nanotechnologie Forschungszentrum Karlsruhe GmbH 76021 Karlsruhe Germany and Fachrichtung Technische Physik UniVersita t des Saarlandes 66041 Saarbru cken Germany Received January 13 2004 ABSTRACT NANO LETTERS 2004 Vol 4 No 5 793 796 We report the preparation of composite foils consisting of two layers one solid gold and one nanoporous gold Tip displacements of several millimeters are observed when the foils are immersed in aqueous electrolytes and the electrochemical potential varied This suggests that nanoporous metals could be used as the active component in actors and it demonstrates for the first time that changes in the surface stress f of the metal electrolyte interface can induce a macroscopic strain orders of magnitude larger than the amplitudes which are reached in conventional cantilever bending experiments used to measure f Changes of the shape of liquid mercury electrodes in response to changes of the electrical potential have been observed as early as the 19th century In 1872 Gabriel Lippmann invented his capillary electrometer in which small voltage differences can be measured by observation of the displacement of a mercury meniscus The Lippmann equation relates the surface tension of a liquid electrode to the electrode potential and it is also a good approximation for solids 1 However the surface stress f of a solid is not even approximately equal to its surface tension and it exhibits a different generally stronger 2 dependence on the potential 1 Furthermore due to the stiffness of solids potential dependent changes in the position or shape of solid surfaces are much smaller than those of liquid electrodes Highly sensitive extensometers3 were used to monitor the strain and in the past decade surface stress changes have been measured using atomic force microscope type techniques thin metal films on the cantilevers are used as electrodes and techniques as for instance laser beam deflection allow the tip displacement in the lower nanometer range e g in ref 4 induced by changes in the surface stress to be measured 5 10 Because the surface stress in solids could hitherto only be detected in a laboratory environment using sophisticated equipment it might be considered as an exotic phenomenon of little practical relevance Even in thin film growth where the Corresponding author E mail Dominik Kramer int fzk de Ad dress Dr Dominik Kramer Forschungszentrum Karlsruhe GmbH Institut fu r Nanotechnologie PO Box 3640 D 76021 Karlsruhe Germany Tel 49 0 7247 82 6379 Fax 49 0 7247 82 6369 Forschungszentrum Karlsruhe Universita t des Saarlandes 10 1021 nl049927d CCC 27 50 2004 American Chemical Society interface induced stress may be large its importance remains the subject to current research 11 More recently surface stress induced length changes of 1 5 m have been observed in nanoporous mm sized platinum cubes an indication that the capillary effects can be enhanced by increasing the surface to volume ratio 12 which takes on exceptionally large values in porous nano structures Since the pressure in the bulk required to balance the surface stress scales with independent of the geometry of the microstructure 13 large volume changes and a considerable mechanical work density result from changes in the surface stress of the nanoporous metal 12 Therefore it has been suggested that such materials may be attractive for use as actuators 14 However integration of the porous metal into a device requires that it can be precisely and reproducibly shaped and that it can be bonded to the parts of the device that transmit displacement and load It has not been demonstrated so far how this can be achieved using nano powder compacts furthermore while powder compacts support a considerable hydrostatic pressure their resistance to shear stress may be poor Here we show that nanoporous metals prepared by dealloying a bulk solid solution exhibit similarly large strain amplitudes as nanopowder compacts and that the porous material can be joined to solid metal foils to form a composite cantilever beam actuator The charge induced expansion or contraction of the porous metal gives rise to a biaxial stress component that results in a large bending of the foil In this way the effect of the interface induced stress is amplified so that the deflection becomes visible to the naked eye the tip moves by 3 mm an increase Published on Web 03 31 2004 Figure 1 Scanning electron micrograph of the nanoporous gold structure obtained by etching silver gold alloy in perchloric acid by the factor 106 compared to previous cantilever bending experiments using a planar surface This demonstrates that changes in the surface stress of nanoporous metals can be exploited to do work in cantilever bending analogously to what was recently reported for carbon nanotubes 15 vanadium oxide nanofibers 16 and conducting polymers 17 Dealloying the selective dissolution of the less noble component from a solid solution is well known to result in nanoporous structures 18 Dealloying is attractive as a tech nique for preparing nanoporous solids since it can be applied irrespective of the shape of the active part of a device including conceivably lithographically shaped miniaturized components Our samples were obtained by the dealloying of Ag75Au25 master alloy sheets see Methods Figure 1 shows a scanning electron microscopy image of the nano porous gold microstructure The ligament size is ca 20 nm Cuboids of porous gold of dimension 1 2 1 2 1 mm3 were investigated in a commercial dilatometer equipped with an in situ electrochemical cell Figure 2A is the cyclic voltammogram current vs potential curve of a nanoporous gold sample immersed in 50 mM sulfuric acid recorded in situ in the dilatometer cell The potential limits are given by the onset of hydrogen evolution ca 0 25 V and gold oxidation above 1 V The voltammogram in Figure 2A is typical of a polycrystalline gold surface The current is almost constant over the entire potential range indicating a continuous capacitive double layer charging and discharging in agreement with the known tendency of SO4 anions to interact only weakly with Au 19 Figure 2B shows the change L in sample length versus the time as the potential is cycled between 0 26 and 1 05 V in 50 mM H2SO4 The length changes periodically and reversibly with the potential with a small irreversible shrinking superimposed to that When the reversible part of L is plotted versus the potential Figure 2C it is apparent that the length of the sample can be changed reproducibly by controlling the potential with a small hysteresis of 0 1 V or 0 02 m The charge was obtained by integration of the current of Figure 2A and by setting the potential of zero charge pzc to 0 25 V compare ref 20 The graph of strain versus charge Figure 2E is highly reversible and linear both Figure 2 In situ dilatometry using 15 succesive cycles of the potential of a cuboid nanoporous gold sample in 50 mM sulfuric acid A Cyclic voltammogram current I versus the electrochemical potential E B Length change L versus time t during the 15 cycles of A C Reversible part of L versus E obtained by subtraction of an constant arbitrary value for each cycle D Total charge Q versus E E L L0 versus Q A and C E display results of all 15 cycles superimposed 794 Nano Lett Vol 4 No 5 2004 in the negatively and positively charged regimes it exhibits a change in slope near the pzc A similar linear correlation has been observed for a Au 111 surface by STM 21 22 but the break near the pzc of Figure 2E was not resolved there It is a matter of debate in how far the potential dependence of the surface stress reflects the details of the bonding of adsorbates to the surface see ref 10 and references therein We have carried out experiments using perchloric acid as the electrolyte and found the results to be in qualitative agreement with Figure 2 see Supporting Information Since the ClO4 ion adsorbs even more weakly than SO4 this finding is compatible with the notion that the potential induced strain does not intrinsically require the formation of the chemical bonds involved in specific adsorption this would imply that the change in surface stress reflects the modified bonding in the space charge layer within the metal surface 2 12 Two further observations in support of this notion are i whereas we find Au to contract at negative potential carbon nanotubes show the opposite effect expansion upon negative charging 15 which indicates that the change in surface stress is strongly related to the nature of the bonding in the solid and ii in situ X ray adsorption near edge spectroscopy XANES data show a significant change in d band occupancy in Pt nanoparticles as the Pt electrolyte interface is charged confirming that the superficial electronic structure of the solid can be changed 23 If the change in surface stress and the surface induced strain in our samples are indeed a consequence of the modified bonding in the metal then the results provide support for a more general concept 24 by controlling the net charge in space charge layers at metal surfaces one can modify the electronic density of states and thereby the local properties of the matter at the surface In nanomaterials which have a large surface to volume ratio this will result in changes of the overall properties opening a way for tuning all those materials properties that depend on the density of states The action of the surface stress can be amplified by use of bilayer foils Each of the foils consists of a layer of porous Au bonded to a layer of solid Au see the cross sections in Figure 3A When the foil is immersed in an electrolyte and its potential varied then the porous layer will tend to expand or contract whereas the solid layer will tend to maintain its dimensions This will result in shear stress at the interface between the two layers and in a bending of the foil quite analogous to the effect of the differential thermal expansion used in bimetal thermometers A similar arrangement has also been used to produce carbon nanotube actuators 15 To make the bilayer foils a 2 mm thick sheet of silver gold alloy was cold welded to a 0 5 mm thick sheet of pure gold by rolling After reducing the thickness of the stack to 30 m by further rolling the resulting foil was annealed for stress relief and strips 35 40 mm long and 2 mm wide cut from it Dealloying resulted in a composite foil consisting of a 6 m thick layer of solid Au covered with 24 m of porous Au Two foils were immersed in 1 M HClO4 and wired as the working and the counter electrodes respectively Figures 3A and 3B show a schematic drawing and a photograph of the experimental setup Both foils undergo a Nano Lett Vol 4 No 5 2004 Figure 3 Illustration of the operation of the composite foils A schematic cross section through an electrochemical cell comprising two identical foils that serve interchangeably as working electrode and counter electrode B D photographs of an electrochemical cell with two bimetallic stripes nanoporous gold on gold similar to the schematic in A The electrolyte is 1 M perchloric acid The lower scale of the ruler is calibrated in mm C D Two enlarged views of the cell in B showing the tip of one of the foils with two different voltages applied between the two foils 1 V C and 1 V D It is seen that when the voltage is inversed the tip moves by ca 3 mm The arrows serve as reference markers emphasizing the tip displacement reversible bending as the voltage is changed Figures 3C D show enlarged views of the tip of one of the foils before and after inverting the applied voltage When the potential difference between the electrodes is switched from 1 V to 1 V the tip moves by as much as 3 mm Thus compared to cantilever bending experiments using planar surfaces the displacement resulting from surface stress changes has increased from few nanometers to the millimeter regime that is by about a factor of 106 A video clip showing the actuator operation is displayed as Supporting Information For the first time the effects potential induced changes of the interface stress which had previously required sophisticated experimental equipment have become visible to the naked eye For actuator applications the response time is important Figure 4A shows the time dependent L during a series of 795 Figure 4 A Length change L of the sample in 50 mM sulfuric acid versus time measured in the dilatometer during a series of potential jumps from 0 2 to 1 V and back dashed line potential B Frequency dependence of the amplitude during potential jumps rectangular wave in sulfuric acid Large squares Amplitude of the charge curve Small circles Amplitude of the length change as measured in the dilatometer The dilatometers maximum sampling rate of 10 s 1 limits the experimental strain amplitude at high frequency potential jumps from 0 2 V to 1 V and back The half times of the jumps in current and strain are 220 and 270 ms Because of the limited sampling rate 10 s 1 of the dilatometer the time constant obtained from the charging curves is considered more accurate The strain amplitude at a frequency of 0 3 Hz is almost identical to that during slower switching Figure 4B which is consistent with the response time given above The bilayer foils react similarly fast despite the drag of the electrolyte The intrinsic time scale is given by the time constant of the charging current which was determined as 25 ms considerably faster than in the thicker dilatometer samples This agrees qualitatively with the expectation that the drift of ions into the pores will be accelerated as the path is shortened The large mechanical response induced by changes in the surface stress predestines porous gold as an active component in sensors especially if its surface is modified by adsorption e g of molecules functionalized by thiol groups These can be chosen to react selectively with specific molecules for instance antibodies the reaction changes the surface stress e g by steric repulsion of the product and sensors detecting these changes have been proposed and tested 8 25 27 Their sensitivity may be significantly enhanced by using nano porous layers instead of planar surfaces In addition to its performance as a simple actor producing reversible strain controlled by an applied voltage the device shown in Figure 3 can also be regarded as a primitive voltmeter If the tip displacement was observed with an optical microscope as in Lippmann s device it would be suited to measure small voltage differences Thus Lipp mann s 19th century voltmeter based on changes of the surface tension of liquid mercury interface has found a modern equivalent based on changes in the surface stress of a solid metal Acknowledgment Stimulating discussions with H Gle iter and support by DFG Center for Functional Nanostruc tures are gratefully acknowledged Supporting Information Available Experimental de tails two additional Figures S1 S2 and a video showing the movement of the bilayer foils This material is available free of charge via the Internet at http pubs acs org References 1 Lipkowski J Schmickler W Kolb D M Parsons R J Elec troanal Chem 1998 452 193 197 2 Schmickler W Leiva E J Electroanal Chem 1998 453 61 67 3 Lin K F Beck T R J Electrochem Soc 1976 123 1145 1151 4 Haiss W Sass J K J Electronanl Chem 1995 386 267 270 5 Ibach H Surf Sci Rep 1997 29 195 263 Ibach H Surf Sci Rep 1999 35 71 73 6 Raiteri R Butt H J J Phys Chem 1995 99 15728 15732 7 Miyatani T Fujihira M J Appl Phys 1997 81 7099 7115 8 Ibach H Bach C E Giesen M Grossmann A Surf Sci 1997 375 107 119 9 Raiteri R Butt H J Grattarola M Electrochim Acta 2000 46 157 163 10 Friesen C Dimitrov N Cammarata R C Sieradzki K Langmuir 2001 17 807 815 11 Cammarata R C Trimble T M Srolovitz D J J Mater Res 2000 15 2468 2474 12 Weissmu ller J Viswanath R N Kramer D Zimmer P Wu rschum R Gleiter H Science 2003 300 312 315 13 Weissmu ller J Cahn J W Acta Mater 1997 45 1899 1906 14 Baughman R H Science 2003 300 268 269 15 Baughman R H Cui C Zakhidov A A Iqbal Z Barisci J N Spinks G M Wallace G G Mazzoldi A De Rossi D Rinzler A G Jaschinski O Roth S Kertesz M Science 1999 284 1340 1344 16 Gu G Schmid M Chiu P W Minett A Fraysse J Kim G T Roth S Kozlov M Mun oz E Baughman R H Nature Mater 2003 2 316 319 17 Baughman R H Synth Met 1996 78 339 353 18 Erlebacher J Aziz M J Karma A Dimitrov N Sieradzki K Nature 2001 410 450 453 19 Kolb D M Prog Surf Sci 1996 51 109 173 20 Kramer D Thesis University of Ulm Germany 2000 21 Haiss W Nichols R J Sass J K Charle K P J Electroanal Chem 1998 452 199 202 22 Nichols R J Nouar T Lucas C A Haiss W Hofer W A Surf Sci 2002 513 263 271 23 Mukerjee S Srinivasan S Soriaga M P McBreen J J Electrochem Soc 1995 142 1409 1422 24 Gleiter H Weissmu ller J Wollersheim O Wu rschum R Acta Mater 2001 49 737 745 25 Berger R Delamarche E Lang H P Gerber Ch Gimzewski J K Meyer E Gu ntherodt H J Science 1997 276 2021 2024 26 Chen G Y Thundat T Wachter E A Warmack R J J Appl Phys 1995 77 3618 3622 27 Fritz J Baller M K Lang H P Strunz T Meyer E Gu ntherodt H J Delamarche E Gerber Ch Gimzewski J K Langmuir 2000 16 9694 9696 NL049927D 796 Nano Lett Vol 4 No 5 2004 本文研究了一種雙層復合材料箔的制備 其中一層為固體金 另一層為納米多孔金 當箔浸入電解質(zhì)水溶液中 并改變?nèi)芤旱碾娀瘜W勢 可以觀察到數(shù)毫米大小的端部位移 這一現(xiàn)象表明納米多孔金屬可以作為作動器中的應 激組件 還首次揭露了金屬 電解液界面上表面應力 f 的改變將引起宏觀應變 該應變的大小比用于測量表面應力 f 的傳統(tǒng)懸臂梁彎曲實驗中所能達到的振幅要高幾個數(shù)量級 早在 19 世紀 人們就已觀察到 在電勢改變時液態(tài)汞電極將會產(chǎn)生形狀改變 1872 年 Gabriel Lippmann 加布 里埃爾 李普曼 外國人的名字不必翻譯 發(fā)明了毛細靜電計 可通過觀察一個水銀彎曲面的位移來測得微小的 電壓改變 李普曼方程建立了液態(tài)電極的表面張力和電極電勢的相關關系 而且對固體電極也可以獲得令人滿意 的近似結(jié)果 1 然而 固體的表面應力 f 并不近似等于其表面張力 并表現(xiàn)出對電勢的不同依賴性 1 一般而 言依賴性更強 2 進一步講 由于固體具有一定的剛度 其表面依賴于電勢的位置和形狀改變遠小于液體電極 高敏延展計 3 用于監(jiān)測應變 在過去的十年間 利用原子力顯微鏡技術已可測量表面應力的改變 將薄金屬片置 于懸臂梁上作為電極 然后采取激光束偏轉(zhuǎn)分析技術 就可以得到由表面應力改變引起的懸臂梁末端位移 可達 到納米級 如參 4 5 10 然而由于目前僅能在實驗室環(huán)境內(nèi)依托精密儀器檢測到固體的表面應力 所以這一發(fā) 現(xiàn)還只被視作一種缺少實用價值的 獨特 現(xiàn)象 即使在界面導致應力較大的薄膜生長方面 其重要性也有待進 一步研究 11 最近 在具有毫米量級尺寸的納米多孔鉑立方體上觀測到了由于表面應力引起的長度變化 達到 1 5 m 這說明 提高表體比 能夠放大毛細管效應 12 而多孔納米結(jié)構的表體比非常大 由于塊體材料中平衡表面應力所需的 體積應力會隨 變化 而 不依賴于微觀結(jié)構的幾何特性 所以納米多孔金屬的表面應力變化將引起可觀的機 械功密度及大的體積改變 12 因此 這類金屬被認為在作動器制造方面前景誘人 14 然而 將多孔金屬集成在設 備上需要精確且可重復化的多空金屬成型工藝 并且能與設備上傳遞位移和荷載的部分緊密聯(lián)結(jié) 目前為止 使 用納米粉末一體化壓實技術還不能達到這些要求 因為雖然一體化納米粉末抗壓能力優(yōu)良 但其抗剪能力很差 本文中展示了利用固熔塊體脫合金法制備的納米多孔金屬不僅具有與一體化納米粉末壓實相近的大應變幅值 并 可與固體金屬箔有效粘結(jié) 制造出一種復合懸臂梁作動器 通電后 多孔金屬發(fā)生延展或收縮 產(chǎn)生雙軸平面應 力分量 從而引起箔的大幅度彎曲 這樣一來 界面導致應力所產(chǎn)生的效應被放大至肉眼可見的撓度 梁末端移 動了 3mm 比之前使用光滑平面的懸臂梁彎曲實驗提高了 106 倍 這說明納米多孔金屬中表面應力的變化可被用 于懸臂梁彎曲實驗 類似于最近報導的碳納米管 15 釩氧化物納米纖維 16 和導電高分子 17 眾所周知 將固溶物中占比較小的組分通過脫合金法選擇性溶解 能夠獲得納米多孔結(jié)構 18 脫合金法作為一種 制備納米多孔固體的技術 其優(yōu)點在于應用時不受設備應激部件形狀的限制 包括刻蝕技術成型的微型組件 本文所使用的試樣是通過對 Ag75Au25 母合金片進行脫合金腐蝕制備的 詳見 方法 一章 圖 1 為納米多孔 金微觀結(jié)構的掃描電子顯微鏡照片 系帶尺寸約為 20nm 使用配備有原位電化學電池的商業(yè)膨脹計 對尺寸為 1 2 1 2 1mm3 的多孔金長方體進行了研究 圖 2A 是浸入 0 05mol L 硫酸的納米多孔金樣本的循環(huán)伏安圖 電流 電勢曲線 曲線數(shù)據(jù)原位記錄于膨脹計電池中 電勢極 限對應于氫開始析出 約 0 25 V 和金開始氧化 高于 1V 圖 2A 是多晶金表面典型的伏安圖 整個電勢值域 內(nèi) 電流值幾乎保持恒定 表示了一種連續(xù)式電容性雙層充放電現(xiàn)象 與 SO4 離子僅能與金發(fā)生微弱的反應的已 知結(jié)論相一致 19 圖 2B 為電勢在 0 26V 至 1 05V 之間循環(huán)時 浸在 0 05mol L 硫酸溶液中的試樣長度改變量 L 隨時間的變化曲 線 長度隨電壓作周期性 往復變化 并伴隨微量的不可逆疊加收縮 當建立長度改變量 L 的可逆部分隨電 勢變化的曲線 圖 2C 時 可以清晰地得出如下結(jié)論 通過調(diào)控電勢便能重復化地改變樣本長度 約有 0 1V 或 0 2 m 的微量滯后 其中 將零勢能點 pzc 設為 0 25V 與參 20 對比 并對圖 2A 中的電流進 行積分 就可以計算出電量 應變 電量曲線如圖 2E 所示 無論在正電荷還是負電荷環(huán)境下 曲線都具有高度的 可逆性和線性性質(zhì) 并在零勢能點附近出現(xiàn)了斜率變化 使用掃描隧道顯微鏡 可在金 111 的表面觀察到相似 的線性相關性 21 22 但沒有給出圖 2E 中零勢能點附近的中斷 表面應力的電勢依賴性能在多大程度上反映被吸附物與表面之間的結(jié)合細節(jié) 是有待于討論的問題 見參 10 及其參考文獻 使用高氯酸作為電解液 我們發(fā)現(xiàn)了與圖 2 定性一致的實驗結(jié)果 見支持信息 由于 ClO4 離子吸附力比 SO4 還弱 這一發(fā)現(xiàn)與如下概念一致 電勢誘導產(chǎn)生的應變本質(zhì)上并不要求在特定吸附中形成化學 鍵 這也意味著表面應力的變化反映了金屬表面空間電荷層中結(jié)合鍵的修正 2 12 另外兩個深入觀察結(jié)果也支持 了這一概念 i 負電位下 金將收縮 而碳納米管卻相反地展現(xiàn)出延展效應 15 這表明表面應力變化與固體 成鍵本質(zhì)密切相關 ii 原位 X 射線吸收近邊結(jié)構譜數(shù)據(jù)顯示 對鉑 電解質(zhì)交界面通電時 鉑納米粒子的 d 帶 占有情況會發(fā)生顯著變化 證實了固體的表面電子結(jié)構可以改變 23 如果我們的試樣中表面應力的變化和表面誘 導應變確實是由金屬中結(jié)合鍵的修正引起的 那么該結(jié)果將支持一個更一般層面的概念 24 通過控制金屬表面空 間電荷層中的凈電荷 可以調(diào)整物質(zhì)表面的電子態(tài)密度從而改變其表面局部特性 對于有著大表體比的納米材料 來說 這將導致一系列整體性能變化 并為調(diào)控所有基于電子態(tài)密度的材料性能提供了一種新的方法 用雙層箔可以放大表面應力作用 每層箔都由一層多孔金和一層固體金結(jié)合而成 橫截面如圖 3A 所示 將箔浸 入電解液 并改變其電勢 多孔金層