材料課件材料科學(xué)概論

1、introduction to materials science and engineeringguan rui-fang材料與新技術(shù)革命材料與新技術(shù)革命材料與國(guó)防現(xiàn)代化材料與國(guó)防現(xiàn)代化this book is divided into four major sectionsi.the fundamentalsii. the structural materialsiii. the electronic and magnetic materialsiv. materials in engineering designsciencematerialsengineering nature of

2、materials properties microscopic or submicroscopic structure engineering designchapter i materials for engineering1.1 types of materials1.2 from structure to properties1.3 selection of materials1.4 materials science and engineering alloy 5alci atomic-scale-architecture brittle 5britl ceramic si5rami

3、k composite 5kcmpezit, -zait concrete 5kcnkri:t conductor crystalline 5kristelain adj. devitrified di:5vitrifai ductility dqk5tiliti electronic and magnetic material element 5eliment fiberglass 5faibelb:s glass glass-ceramic insulator materials science and engineering materials selection metallic mi

4、crocircuitry 5maikrejs:kit microscopic-scale architecture noncrystalline nonmetallic 5ncnmi5talikkey word oxide periodicpieri5cdik table plastic polyethylene 7pcli5ewili:n polymer property refractory semiconductor silica硅石，硅石，sio2 silicate硅酸鹽硅酸鹽 silicon硅元素硅元素 steel structural material woodkey word a

5、 . metals metallic bond b . ceramics (glass) ionic ai5rnik and covalent keu5veilent bond c . polymers covalent and van der waals bond d . composites e . semiconductorswhat materials are available to me ?five categoriesa metals超塑性的,超塑性材料(制)的1 . i.e.latin， the written abbreviation for id est ,也就是，即。 讀

6、作：that is2 . state-of-the-art技術(shù)發(fā)展水平，現(xiàn)代化的。3 . superplastic. characteristics(metallic 金屬的，金屬性的金屬的，金屬性的) it is strong and can be readily formed into practical shapes . ductile 5dqktail permanent deformability metallic luster electric conductor metals have no brittleness and are safe for many applicatio

7、ns.structural steel gasket5askit墊圈,襯墊springwirevalvevalvb ceramics si5ramiksahomogeneousmixtureoftwoormoresubstances,whichmaybesolids,liquids,gases,oracombinationofthese.1 . refractory ri5frakteridifficulttomeltorwork;resistanttoheat.2 . fracture toughness斷裂韌度3 . crystalline5kristelainbeing,relating

8、to,orcomposedofcrystalorcrystals.4 . devitrify di:5vitrifaitocause(aglassymaterial)tobecomecrystallineandbrittle.5 . solution the ceramics is derived from greek word “keramic” which means an art of potter .include : glass ; enamels i5namel ; refractories ; cements ; limes ; plasters(plaster of paris

9、) ; cermets5se:met金屬陶瓷金屬陶瓷, 含陶合金含陶合金; electronic ceramics ; high and low tension electrical insulators ; structural clay products ; nuclear ceramics ; whitewares ; abrasives e5breisiv .rubysapphire5safaie why shouldnt al2o3 be chosen for certain applications where metallic aluminum has been used ?1、

10、al2o3 have two advantages:first ,chemicallystable;aloxidizedsecond ,highermeltingpoint(20200c)al(6600c)refractoryuseinindustrialfurnaceconstructionquestions1. what materials are available to you in this book?2. what fundamental characteristics metals have?3. please shade metallic elements in periodi

11、c table and give some examples of alloys.4. please list two principal advantages of al2o3 relative to metallic al.5. whats the most limiting property of ceramics.themostlimitingpropertyofceramicsbrittlenessrecentdevelopmentinceramictechnologyareexpandingtheutilityofceramicsforstructuralapplicationno

12、tbyeliminatingtheirinherentbrittleness,butbyincreasingtheirstrengthtosufficientlyhighlevels(greaterthan700mpa)andincreasingtheirresistancetofracture.2、al2o3mgosio2si3n4at least one metallic element +one of five nonmetal element (c, n, o, p and s)carbide 5kb:baidnitride 5naitraidoxide 5cksaidphosphid

13、e 5fcsfaidsulphide. 5sqlfaidoxide ceramicsnonoxide ceramics3、onemetallicelementplusoneoffivenonmetallicelements(c,n,o,p,ors)4、characteristics:refractoryhighhardnesshighstrengthhighbrittleness the similar structural feature of metals and ceramics: crystalline: their constituent atoms are stacked toge

14、ther in a regular, repeating pattern. the distinction: ceramics can be made in a noncrystalline form . ski5matiksio2 (approximately 72%) na2o caoglass: brittleness similar with crystalline ceramictransmit visible lightchemical inertness glasses are important engineering materialsglass-ceramics is a

15、third categorylithium aluminosilicatescertain glass compositions can be fully devitified by an appropriate thermal treatmentlow-thermal expansion coefficientscomplex formshigh mechanical strengthvitreous 5vitriesglassy statesubsequentcrystallizationc polymers(plastics)exempli gratia. for example.1 .

16、 ethylene a colorless, flammable gas, c2h4, derived from natural gas and petroleum and used as a source of many organic compounds, in welding 5weldin焊接and cutting metals, and as an anesthetic 7anis5wetik麻醉劑.2 . polyethylene a polymerized ethylene resin, used especially for containers, kitchenware, a

17、nd tubing, or in the form of films and sheets for packaging.3 . e.g. any of numerous organic compounds, such as benzene and methane, that contain only carbon and hydrogen.4 . acrylics e5kriliks丙烯酸樹脂5 . nylon 5nailen any of a family of high-strength, resilient synthetic polymers, the molecules of whi

18、ch contain the recurring amide group conh.6 . hydrocarbon these man-made materials are a branch of organic chemistry . 1. poly +mer: mer in a polymer 1. poly +mer: mer in a polymer is a single hydrocarbon is a single hydrocarbon molecule such as ethylene(cmolecule such as ethylene(c2 2h h4 4). ). lo

19、ng chain olecules: long chain olecules: polymers are long-chain polymers are long-chain molecules composed of many molecules composed of many mers bond together.mers bond together.ch2ch2nch2ch2nnch2ch2cch3ch3ch3 1) no brittleness2. characteristics: 2) lower melting point3) high strength along its ch

20、ain and low strength between its chains 4) lightweight low-cost5) higher chemical reactivity6) man-made materialssubstitution for traditional structural metalsqustions1.please illustrate two type of elements that composed ceramic compounds in periodic table.2.what are the major compositions of glass

21、? why glasses are important engineering materials?3.please list two categories and give some examples of ceramics according their compositions.4.please list three categories and give some examples of ceramics according their atomic structrual feature.5.what are polymers? what are characteristics of

22、polymers? please give some examples of polymers.sio2 (approximately 72%) na2o caoglass: brittleness similar with crystalline ceramictransmit visible lightchemical inertness glasses are important engineering materials according to their compositions, ceramics can be divided into two catogories. one i

23、s oxide ceramics such as aluminum oxide (al2o3), magnesium oxide (mgo), and silica (sio2), the other one is nonoxide ceramics, silicon nitride is an important nonoxide ceramic. according to their atomic structual feature, there are three types of ceramics as crystalline ceramics, noncrystalline cera

24、mics and the third category, glass-ceramics. silicon nitride is one of crystalline ceramics, glass is a kind of noncrystalline ceramics and glass-ceramics is a relatively recent material, that is, certain glass compositions (such as lithium aluminosilicates ) can be fully devitified by an appropriat

25、e thermal treatment. polymers are long-chain molecules polymers are long-chain molecules composed of many mers bond composed of many mers bond together. together. polyethylene, acrylics and nylon are polyethylene, acrylics and nylon are all typical polymers.all typical polymers.polymers are no britt

26、leness, high polymers are no brittleness, high strength along its chain and low strength along its chain and low strength between its chains, strength between its chains, lightweight, low-cost, lower melting lightweight, low-cost, lower melting point and higher chemical reactivity point and higher c

27、hemical reactivity compared with ceramics, at last compared with ceramics, at last polymers are man-made materials.polymers are man-made materials.shellwoodd composite (fiber reinforce plastics) possessing hereditaryhi5rediteri rank in a political system or social class derived from a feudalistic 9f

28、ju:delistik stage of a countrys development. inactive or inert.inactive or inert.1 . albeit c:l5bi:it page 12, line 5even though; although; notwithstanding.2 . noble page 12, para. 2, line 2 combination of individual materials from the previous categories. characteristics: the best of both worlds it

29、 carry along the superior properties of each component, producing a product that is superior to either of the components separately.example: fiberglass ,wood ,concrete .s supplementary introduction semiconductors a piece of equipment that produces a powerful narrow beam of light that can be used in

30、medical operations , to cut metals , or to make patterns of light for entertainment . 1 .arsenide 5b:senaid砷化物 a compound of arsenic with a more electropositive element.2 .laser sophisticated page 13，line 6 from below se5fistikeitid adj.1.詭辯的, 久經(jīng)世故的2.very complex or complicated: 非常復(fù)雜精密或尖端的 最新最尖端的技術(shù)

31、the latest and most sophisticated technology. semiconductors are relatively invisible compared with polymers , but have a comparable social impact. semiconductors are the basis of modern electronics.a relatively small group of elements and compounds has an important electrical property。three semicon

32、ducting elements (si (silicon),ge (germanium dve:5meiniem ) , sn(tin). compounds: gaas gallium5aliem arsenide, cds cadmiumsulfide 5kadmiem 5sqlfaid ceramics compound : zno precise control of chemical purity allows precise control of electronic properties. characteristics: semiconduction, in which th

33、ey are neither good electrical conductors nor good electrical insulators 5insjuleite , instead, their ability to conduct electricity is intermediate 7inte5mi:djet. atomic-scale-architecturecompositedevitrifieddi:5vitrifai 使不透明使不透明ductilityelectronic and magnetic materialfiberglassmaterials selection

34、microcircuitry 5maikrejs:kit電子電子微電路微電路microscopic-scale architecturerefractorystructural materialqustions1、please define composites, and then demonstrate the characteristics of good composites by some examples. 2、please enumerate ( i5nju:mereit v.列舉) semiconducting elements and compounds.compositeis

35、materialcomposedofamicroscopic-scalecombinationofindividualmaterialsfromthecategoriesofmetals,ceramics(andglasses),andpolymer.e.g.fiberglassisakindofcompositesystemcomposedofpolymericmatrixreinforcedwithglassfibers.characteristicoffiberglassprovidesthebestofbothworlds:itcarryalongthesuperiorproperti

36、esofeachcomponent,producingaproductthatissuperiortoeitherofthecomponentsseparately.thehighstrengthofthesmall-diameterglassfibersiscombinedwiththeductilityofthepolymermatrixtoproduceastrongmaterialcapableofwithstandingthenormalloadingrequiredofastructuralmaterial. therearethreesemiconductingelements(

37、si,geandsn)fromcolumnivaandaclusterofelementsoftheperiodictableimmediatelyadjacenttocolumniva,whichformscompoundsthataresemiconducting.examplesincludegalliumarsenide(gaas),whichisusedasahigh-temperaturerectifier(5rektifaie整流器)andalasermaterial,andcadmiumsulfide(cds)whichisusedasarelativelylow-costso

38、larcellforconversionofsolarenergytousefulelectricalenergy.1.2 from structure to properties the unit of luminous flux flqks流量, 通量 in the international system, equal to the amount of light given out through a solid angle by a source of one candela kan5di:le intensity radiating equally in all direction

39、s. 1 .hexagonal heksaenelhavingsixsides.2 .lumen5lju:min縮寫 lm物理學(xué) 流明（光通量單位）：國(guó)際度量系統(tǒng)中的光通量單位，等于一燭光的均勻點(diǎn)光源在單位主體角內(nèi)發(fā)出的光通量.equivalent i5kwivelent adj. 相等的, 相當(dāng)?shù)? 同意義的n. 等價(jià)物, 相等物avenue 5avinju: n. 林蔭道, 大街, 方法, 途徑, 路refraction ri5frakfenn. 折光, 折射determination of the refractive characteristics of the eye. 屈光度測(cè)定：

40、對(duì)眼睛折射特性的測(cè)定 translucent tranz5lju:sntadj. 半透明的, 透明的 opaque eu5peikn. 不透明物adj.不透明的, 不傳熱的, 遲鈍的 transparent trans5pzerentadj. 透明的, 顯然的, 明晰的 densification 7densifi5keifen致密化 to understand the properties of engineering materials it is necessary to understand their structure .atomic-scale architecture10000

41、000 timesmicroscopic-scale architecture1000timesexample 1: al , mg al alloys are ductile mg alloys are brittle al structure follows a cubic packing arrangement and mg a hexagonal one .example 2: transparent ceramicstraditional ceramics contained a substantial amount of residual porosity .the porosit

42、y leads to loss of visible light transmission .only about 0.3% porosity can cause al2o3 to be translucent , and 3% porosity can cause the material to be completely opaque .the elimination of porosity : adding a small amount of impurity . theeliminationofporosityresultedfromarelativelysimpleinvention

43、thatinvolvedaddingasmallamountofimpurity(0.1wt%mgo),whichcausedthehigh-temperaturedensificationprocessfortheal2o3powdertogotocompletion. poros microstructure in polycrystalline al2o3figure 1polycrystalline faceted sapphires and rubiesfigure3polisheddiscswithseveraldopants(thickness:0.8mm)the two exa

44、mples just cited show typical and important demonstrations of how properties of engineering materials follow directly from structure.qustions 1.aluminumandmagnesiumarebothmetal,whyaluminumisrelativelyductilewhilemagnesiumisrelativelybrittle?pleaseexplainthedramaticeffectthatstructurehasonpropertieso

45、ntheatomicscale. 2.whytraditionalceramicssuchasaluminumoxideareopaquewhereasrecentlyopticallytransparentaluminumoxideceramicsareproduced?whatarethedifferencesbetweentheirmicroscopic-scalearchitectures?1.3 selection of materials anyofagroupofverystronglightweight,syntheticfibersusedinmakingradialtire

46、s,bulletproofofvests,etc.1 . etc. latinthewrittenabbreviationforetcetera,usedafteralisttoshowthattherearemanyothersimilarthingsorpeoplethatyoucouldhaveadded.andsoforth,andsoon,andtheothers2 . astmamericansocietyfortestingandmaterials.3 . aramid apolymercontainingrepeatedamidegroups,asinvariouskindso

47、fnylon. 4 . aromatic 7areu5matikof,relatingto,orcontainingoneormoresix-carbonringscharacteristicofthebenzene5benzi:n, ben5zi:nseriesandrelatedorganicgroups.5 . polyamide appreciatione7pri:fi5eifenn.ajudgmentoropinion,especiallyafavorableone。評(píng)價(jià)：判斷或意見，尤指有益的。 2.materialsselectionisthefinal,practicaldec

48、isionintheengineeringdesignprocessandcandeterminethatdesignsultimatesuccessorfailure. 3.solid-stateadj.使用電晶體的,不用真空管的 4.indefinitein5definitadj.不確定的,語不定的(indefinitearticle) 5.althoughsomestructuralceramicscanwithstandtheanticipatedserviceload,serviceload負(fù)載 6.criterionkrai5tierienn.(批評(píng)判斷的)標(biāo)準(zhǔn),準(zhǔn)據(jù),規(guī)范crit

49、ical5kritikeladj.評(píng)論的,鑒定的,批評(píng)的,危急的,臨界的 7.theaddedcostoffabricatingthesemoresophisticatedmaterialsystemsisjustifiedonlyifaspecialadvantageresults.fabricate5fabrikeitvt.制作,構(gòu)成,捏造,偽造,虛構(gòu)justifytodemonstrateorprovetobejust,right,orvalid:論證或證明正當(dāng)、正確或有效 8.optimal5cptimeladj.最佳的,最理想的 9.propertycomparisonkem5par

50、isn 10.junction5dvqnkfenn.連接,接合,交叉點(diǎn),匯合處 10.spherical5sferikeladj.球的,球形的;cylindricalsi5lindrik(e)l圓柱的 11.integral5intireladj.essentialornecessaryforcompleteness;constituent:必須的：構(gòu)成整體所必須的；應(yīng)有的 12.inthepastquartercentury,thetermthathascometolabelthegeneralbranchofengineeringconcernedwithmaterialsismateri

51、als science and engineering.thislabelisaccurateinthatthisfieldisatrueblendoffundamental,scientificstudiesandpracticalengineering.ithasgrowntoincludecontributionsfrommanytraditionalfields,includingmetallurgy,ceramicengineering,polymerchemistry,solid-statephysics,andphysicalchemistry. label5leiblvttoi

52、dentifyordesignatewithalabel;describeorclassify:把歸類，描述：用標(biāo)簽分別或命名；描繪或歸類 contribution7kcntri5bju:fenn.捐獻(xiàn),貢獻(xiàn),投稿 metallurgyme5taledvin.冶金,冶金術(shù) 13.optimum5cptimemn.最適宜adj.最適宜的which material do i now select for a particular application ? two separate decisions: first: onemustdecidewhichtypeofmaterialisappro

53、priate. second: thebestspecificmaterialwithinthatcategorymustbefound. preferable5preferebl adj.更可取的, 更好的, 更優(yōu)越的a . competition among the five types of materials b . selecting an optimal metal propertycomparisonsmustbemade 2.cost1.superiormechanicalpropertiesc. selecting a metal substitute1.4 material

54、s science and engineering first : science second : materials finally : engineering 大學(xué)課件出品 版權(quán)歸原作者所有 聯(lián)系qq ：910670854 如侵權(quán)，請(qǐng)告知，吾即刪 更多精品文檔請(qǐng)?jiān)L問我的個(gè)人主頁 http:/ 附贈(zèng)人生心語附贈(zèng)人生心語 我們都老得太快 卻聰明得太遲 把錢省下來，等待退休后再去享受 結(jié)果退休后，因?yàn)槟昙o(jì)大，身體差，行動(dòng)不方便，哪里也去不成。錢存下來等養(yǎng)老，結(jié)果孩子長(zhǎng)大了，要出國(guó)留學(xué)，要?jiǎng)?chuàng)業(yè)做生意，要花錢娶老婆，自己的退休金都被拗走了。 當(dāng)自己有足夠的能力善待自己時(shí)，就立刻去做，老年人有時(shí)候是

55、無法做中年人或是青少年人可以做的事，年紀(jì)和健康就是一大因素。小孩子從小就告訴他，養(yǎng)你到高中，大學(xué)以后就要自立更生，要留學(xué)，創(chuàng)業(yè)，娶老婆，自己想辦法，自己要留多一點(diǎn)錢，不要為了小孩子而活我們都老得太快卻聰明得太遲，我的學(xué)長(zhǎng)去年喪妻。這突如其來的事故，實(shí)在叫人難以接受，但是死亡的到來不總是如此。學(xué)長(zhǎng)說他太太最希望他能送鮮花給他，但是他覺得太浪費(fèi)，總推說等到下次再買，結(jié)果卻是在她死后，用鮮花布置她的靈堂。這不是太蠢愚了嗎？！ 等到.、等到.，似乎我們所有的生命，都用在等待。 等到我大學(xué)畢業(yè)以后，我就會(huì)如何如何我們對(duì)自己說 等到我買房子以后！ 等我最小的孩子結(jié)婚之后！ 等我把這筆生意談成之后！ 等到我

56、死了以后 人人都很愿意犧牲當(dāng)下，去換取未知的等待；犧牲今生今世的辛苦錢，去購(gòu)買后世的安逸 在臺(tái)灣只要往有山的道路上走一走，就隨處都可看到農(nóng)舍變精舍，山坡地變靈塔，無非也是為了等到死后，能圖個(gè)保障，不必再受苦。許多人認(rèn)為必須等到某時(shí)或某事完成之后再采取行動(dòng)。明天我就開始運(yùn)動(dòng)，明天我就會(huì)對(duì)他好一點(diǎn)，下星期我們就找時(shí)間出去走走；退休后，我們就要好好享受一下。 然而，生活總是一直變動(dòng)，環(huán)境總是不可預(yù)知，現(xiàn)實(shí)生活中，各種突發(fā)狀況總是層出不窮。身為一個(gè)醫(yī)生，我所見過的死人，比一般人要來得多。這些人早上醒來時(shí)，原本預(yù)期過的是另一個(gè)平凡無奇的日子，沒想到一個(gè)意料之外的事；交通意外、腦溢血、心臟病發(fā)作等等。剎那

57、間生命的巨輪傾覆離軌，突然闖進(jìn)一片黑暗之中。那么我們要如何面對(duì)生命呢？我們毋需等到生活完美無瑕，也毋需等到一切都平穩(wěn)，想做什么，現(xiàn)在就可以開始做起。 一個(gè)人永遠(yuǎn)也無法預(yù)料未來，所以不要延緩想過的生活，不要吝于表達(dá)心中的話， 因?yàn)樯辉谝凰查g。 記??！ 給活人送一朵鮮花，強(qiáng)過給死人送貴重的花圈，每個(gè)人的生命都有盡頭，許多人經(jīng)常在生命即將結(jié)束時(shí)，才發(fā)現(xiàn)自己還有很多事沒有做，有許多話來不及說，這實(shí)在是人生最大的遺憾。 別讓自己徒留為時(shí)已晚的空余恨。逝者不可追，來者猶未卜，最珍貴、最需要實(shí)時(shí)掌握的當(dāng)下，往往在這兩者蹉跎間，轉(zhuǎn)眼錯(cuò)失。 人生短暫飄忽，包得有一首小詩這樣寫： 高天與原地，悠悠人生路； 行

58、行向何方，轉(zhuǎn)眼即長(zhǎng)暮。 正是道盡了人生如寄，轉(zhuǎn)眼即逝的惶恐。 有許多事，在你還不懂得珍惜之前已成舊事；有許多人，在你還來不及用心之前 已成舊人。 遺憾的事一再發(fā)生，但過后再追悔早知道如何如何是沒有用的，那時(shí)候已經(jīng)過去，你追念的人也已走過了你。 一句瑞典格言說：我們老得太快，卻聰明得太遲。 不管你是否察覺，生命都一直在前進(jìn)。 人生并未售來回票，失去的便永遠(yuǎn)不再得到。 將希望寄予等到方便的時(shí)間才享受 我們不知失去了多少可能的幸福 不要再等待有一天你可以松口氣，或是麻煩都過去了。 生命中大部分的美好事物都是短暫易逝的， 享受它們、品嘗它們， 善待你周圍的每一個(gè)人， 別把時(shí)間浪費(fèi)在等待所有難題的完滿結(jié)

59、局上。 -找回迷失的生命 -死亡也許是免費(fèi)的 但是，卻要付出生命的代價(jià)。 -勸大家一句話：把握當(dāng)下，莫等待。 做一個(gè)終生學(xué)習(xí)的人，離開學(xué)校并不意味著學(xué)習(xí)就結(jié)束了。學(xué)習(xí)可以成為一種生活方式，幫助你發(fā)揮最大的潛能。我們從未停止學(xué)習(xí)，總會(huì)有新的，有趣的東西等待我們?nèi)グl(fā)現(xiàn)。學(xué)習(xí)新的技能可能讓人感到有一點(diǎn)恐懼，但每當(dāng)我們?cè)趥€(gè)人學(xué)習(xí)上停滯不前時(shí)，我們都需要去學(xué)習(xí)新的東西。積極地尋求支援和建議，突破停滯期。參加一些培訓(xùn)，進(jìn)修，夜校任何新的興趣都將會(huì)有助于發(fā)展你的優(yōu)勢(shì)。多看，多聽，讓你的頭腦保持活躍。活到老，學(xué)到老。 令自己感到沮喪的秘訣就是用空閑時(shí)間去煩惱自己是否快樂。所以不要費(fèi)事去想它！摩拳擦掌干起來吧。

60、你將熱血沸騰，你會(huì)頭腦清醒。很快，在你身體中的這種高漲的積極人生觀將把煩惱從你的頭腦中趕出去。行動(dòng)起來，忙碌起來。這是世界上最便宜的一種藥，也是最好的一種。 在困境中尋找成功的希望逆境是一所最好的學(xué)校。每一次失敗，每一次打擊，每一次損失，都蘊(yùn)育著成功的萌芽，都教會(huì)我在下一次有更出色的表現(xiàn)。我再也不會(huì)逃避現(xiàn)實(shí)，也不會(huì)拒絕從以往的錯(cuò)誤中獲取經(jīng)驗(yàn)，我不再因此而促成自己的失敗。因?yàn)槲抑溃瑢氂癫唤?jīng)磨礪就不能發(fā)光，沒有，我也不能完善自我?，F(xiàn)在我知道，靈魂倍受煎熬的時(shí)刻，也正是生命中最多選擇與機(jī)會(huì)的時(shí)刻。任何事情的成敗取決于我在尋求幫助時(shí)是抬起頭還是低下頭。無論何時(shí)，當(dāng)我被可怕的失敗擊倒，在最初的陣痛過去