(6.5)--Role of mechanical strain on the机械工程材料机械工程材料.pdf
![资源得分’ title=](/images/score_1.gif)
![资源得分’ title=](/images/score_1.gif)
![资源得分’ title=](/images/score_1.gif)
![资源得分’ title=](/images/score_1.gif)
![资源得分’ title=](/images/score_05.gif)
《(6.5)--Role of mechanical strain on the机械工程材料机械工程材料.pdf》由会员分享,可在线阅读,更多相关《(6.5)--Role of mechanical strain on the机械工程材料机械工程材料.pdf(9页珍藏版)》请在得力文库 - 分享文档赚钱的网站上搜索。
1、Role of mechanical strain on thermal conductivity of nanoscalealuminum filmsH.-F.Lee,S.Kumar,M.A.HaqueDepartment of Mechanical and Nuclear Engineering,The Pennsylvania State University,University Park,PA 16802,USAReceived 21 April 2010;received in revised form 15 August 2010;accepted 18 August 2010A
2、vailable online 28 September 2010AbstractThin film components of conventional and flexible solid-state devices experience mechanical strain during fabrication and operation.At the bulk scale,small values of strain do not affect thermal conductivity,but this may not true for grain sizes comparable wi
3、th theelectron and phonon mean free paths and for higher volume fraction of grain boundaries.To investigate this hypothesis,thermaland electrical conductivity of nominally 125-nm-thick aluminum films(average grain size 50 nm)were measured as functions of tensilethermo-mechanical strain,using a modif
4、ied version of the 3-x technique.Experimental results show pronounced strainthermal conduc-tivity coupling,with?50%reduction in thermal conductivity at?0.25%strain.The analysis shows that mechanical strain decreases themean free path of the thermal conduction electrons,primarily through enhanced sca
5、ttering at the moving grain boundaries.This con-clusion is supported by similar effects of mechanical loading observed on the electrical conduction in the nanoscale aluminum specimens.?2010 Acta Materialia Inc.Published by Elsevier Ltd.All rights reserved.Keywords:Nanocrystalline microstructure;Disl
6、ocations;Grain boundaries;Thermal conductivity;Electrical resistivity/conductivity1.IntroductionHeat conduction in thin film components is receivingincreasing attention as the length scale of the micro-elec-tronic,opto-electronic and micro-electro-mechanical sys-tems continues to shrink 1,2.The powe
7、r density of thesedevices scales as L?1,where L is the length scale,whichmakes thermal conductivity of thin films a critical concernregarding device performance and reliability 3.High ther-mal conductivity is desired for micro-electronic devices forefficient heat removal,whereas the opposite is true
8、 forthermo-electric energy conversion devices.In both theseclasses of applications,very small mechanical or thermo-mechanical strain is experienced by the devices duringoperation and fabrication.Classical models for electricaland thermal conduction in solids 4,5 ignore such smallstrain values and ra
9、ther focus on the strain rate as a moreinfluential parameter.As a matter of fact,the static strainterm does not even appear in the equations,as shownbelow 6.k_T;ii qc_T 3k 2laT0_ e;ii?qh1where e is the strain,h is a generation term,k is thermalconductivity,T is temperature,q,c and a are density,spe-
10、cific heat and expansion coefficient,respectively,and k andl are the Lame s constants.The time and spatial deriva-tives are presented by the dot and i indices,respectively.It is commonly accepted that thermal conductivity,likeother fundamental physical properties,may not be a con-stant at the nanosc
11、ale.More recent research on nanoscalesolids shows significant effects of specimen dimension 79,grain size 10,11,microstructure 12,temperature 13and deposition processes 14 on thermal conductivity.For metallic thin films,in particular,a strong size effecthas been reported for film thickness or grain
12、size smallerthan or comparable with the mean free path of electrons1517.However,all these studies treat defects or micro-structures as static scattering agents,which is not true1359-6454/$36.00?2010 Acta Materialia Inc.Published by Elsevier Ltd.All rights reserved.doi:10.1016/j.actamat.2010.08.024Co
13、rresponding author.Tel.:+1 814 8654248;fax:+1 814 8659693.E-mail address:mah37psu.edu(M.A.Haque) online at Acta Materialia 58(2010)66196627for a material undergoing mechanical loading.Becauseclassical thermo-elastic laws,such as Eq.(1),do not predictany thermal coupling with strain,there are only a
14、few stud-ies that consider the effect of mechanical deformation onthermal conduction 18,19.In these studies,the strainsare intrinsic or residual in nature and not externally variedfor a systematic study.Also,these studies consider verylarge grains,which do not show any considerable couplingbetween m
15、echanical deformation and thermal conductiv-ity.However,with continued trend of miniaturization,the issue of coupling becomes practically relevant for aplethora of applications involving thin films that experi-ence deformation.For example,the heat dissipation den-sity of modern micro-electronic devi
16、ces is high enough todevelop thermo-mechanical strain and significantly affectthe integrity of the thin film components 20.Also,variousfabrication processing temperatures induce thermal misfitstrain in the device layers during fabrication.Assumingcontinuum-based constitutive laws,these strains(typic
17、ally0.1%)appeartobeinconsequential.However,asexplained in the next few paragraphs,this assumption mustbe investigated at the nanoscale.Mechanical deformation mechanisms are expected toinfluence thermal conduction,because thermal energy car-riers(electrons and phonons)are primarily scattered by thede
18、formation mechanisms.For example,electrons carryheat in metals and are scattered by dislocations and grainboundaries.However,bulk metals do not show any smallstrainthermal conductivity coupling because dislocations(the dominant mechanism from bulk to micro scales)areknown to scatter electrons apprec
19、iably only 100 K.However,two-dimensional microstructures(such as grain boundaries)scatter electrons more strongly,but play only an insignificant role(compared with disloca-tions)on the deformation at the bulk scale.Hence thestrainthermal conductivity coupling is non-existent at lar-ger length scales
20、.As the specimen thickness or grain size approaches thenanoscale,it becomes increasingly difficult for the grainsto accommodate dislocations inside the grains.For alumi-num,dislocations may cease to exist in grains as large as60 nm 34,35.Therefore,mechanical deformation mecha-nisms for grain sizes s
21、maller than?50 nm are fundamen-tally different from the bulk ones.This is because theabsence of dislocations is accompanied by a very high vol-ume fraction of the grain boundaries 31.In the absence ofdislocations in the grain interior,the grain boundaries(dis-ordered atoms)predominantly accommodate
22、the appliedstrain 80.To do this,the grain boundaries must slide,rotate or emit dislocations that also move along the grainboundaries as strain is applied 32.Since static grainboundaries strongly scatter electrons,their motion willonly pronounce the scattering events.It is therefore postu-lated that
23、the motion of grain boundaries is the primarymechanism behind strain-dependent scattering of thermalconduction electrons.This mechanism is shown schemati-cally in Fig.6.The effect of strain is therefore primarilythrough increase in the reflection coefficient,since the dis-locations generated from th
24、e grain boundaries make itincreasingly difficult for the conduction electrons to passthrough their strain fields.Also,for nano-crystalline metalsKb?D,and the reflection coefficient dominates the effec-tive mean free path.To verify the absence of dislocations in the grain interiorsand pronounced grai
25、n boundary activities,in situ mechani-cal testing of 125-nm-thick aluminum thin films was per-formedinsidethetransmissionelectronmicroscope.Details of the specimen preparation and experimental setupare given elsewhere 35.Fig.7 shows two snapshots duringthe mechanical deformation,where dislocations a
- 配套讲稿:
如PPT文件的首页显示word图标,表示该PPT已包含配套word讲稿。双击word图标可打开word文档。
- 特殊限制:
部分文档作品中含有的国旗、国徽等图片,仅作为作品整体效果示例展示,禁止商用。设计者仅对作品中独创性部分享有著作权。
- 关 键 词:
- 6.5-Role of mechanical strain on the机械工程材料机械工程材料 6.5 Role the 机械工程 材料
![提示](https://www.deliwenku.com/images/bang_tan.gif)
链接地址:https://www.deliwenku.com/p-96599400.html
限制150内