INTRODUCTION. However, there is a challenging situation that exists when the heat is removed from the

Transcription

1 1 CHAPTER I INTRODUCTION One f the challenges the industry facing tday is the rate f heat dissipatin frm the micr electrnics systems. The higher and the fast heat remved, the better it is. Hwever, there is a challenging situatin that exists when the heat is remved frm the electrnic cmpnents. Generally, the heat sink is attached directly t the heat generating device s that heat is remved fast and device can be always kept cl. Hwever, the requirement in such case is the selectin f material fr the heat sink. The cefficient f thermal expansin (CTE) f the heat sink material must be clse enugh t that f the semicnductr device. Mst f the electrnic devices like micrprcessrs are made ut f silicn r its allys. Hence the CTE f heat sink materials t be chsen shuld be clse enugh t that f silicn. This enables bth the materials t expand by similar rates s that ne can eliminate the fracture in the devices arising due t the differential thermal expansins. Hwever, by chsing the heat sink materials with lw CTE, there is a pssibility that thse materials can have the lw thermal cnductivities s that heat is nt remved at a faster rate. This is cnflicting situatin where if ne needs higher heat dissipatin, it will result in device fractures r if ne needs t prtect the electrnic device, it will be at the cst f higher heat dissipatin. In generally highly cnducting materials like cpper r aluminum are chsen fr manufacturing the heat sinks. Their CTE are very high cmpared t that f silicn devices. One ppular technique is t use the interfacial materials in the gap f heat sink and electrnic device. The interfacial materials cmpensate fr the differential thermal expansin s that the stresses induced at the interface are reduced. There is als ne

2 2 drawback here again, as the interfacial materials act as a thermal barrier between the heat surce and the heat sinks, thereby reducing the heat transfer rate. The advantage prvided by the high cnducting materials is lst when the thermal interfacial materials are used. One can use interfacial materials which has gt high thermal cnductivity t increase the heat dissipatin rate. But, there is a pssibility that the high cnducting pastes can shrt the built in electrnic circuitry f the micrprcessr r IC at the surface. Hence it is required t develp slutins fr 1. Increasing the heat transfer by develping new materials which has gd thermal cnductivity and gd CTE. 2. Increasing the heat transfer by develping new cntact resistance mdels with reductin in the pssibility f shrting f the circuitry s that benefits f high cnducting greases can be made use f. Metal-matrix cmpsites are ppular in the applicatins f space vehicles, air craft s, semicnductr substrates, rad vehicles, etc. Amng the MMC s, the metal matrix cmpsites based n aluminum are being used in vast majrity, while a large number f usage need the matrix prperties f super-allys and high strength materials. Similar t ther cmpsites, aluminum based MMC s have a grup f materials whse mechanical, physical and thermal prperties can be altered t suit nes needs. The mechanical prperties that can be tailred include stiffness, strength, density, and thermal prperties are thermal cnductivity, specific heat and cefficient f thermal expansin. The physical prperty that can be designed t suit the design requirement is density. These prperties can be varied by varying the factrs like

3 3 Type f material chsen fr reinfrcement quantity and frm f the reinfrcement material Lcatin f the reinfrcement Manufacturing methd Alng with the ease f varying the prperties, aluminum based MMC s ffer the benefit f lesser csts cmpared t mst f the ther metal matrix cmpsites. This is in additin t ther excellent prperties which can be used fr the applicatin in different field s f engineering. Aluminum MMCs can be prduced by prductin prcesses like Mlding Pwder cmpactin Reinfrcement manufacturing in situ and Pressing f fils and fibers Prducts with gd quality are required fr applicatins like autmtive cmpnents, brake rtrs, pistns, bicycles, semicnductr substrates, cld frmed shapes, machinery cmpnents and channels, as well asa large variety f multiple structural and semicnductr applicatins. Fr applicatins like cmpnents in jet turbine engines which perate temperatures beynd 1000 C Super-ally cmpsites are used which are reinfrced with materials like fibers made f tungsten are used. Tailrable prperties can be achieved with graphite/cpper cmpsites, and they are useful fr high temperature expsure applicatins in air and prvide excellent mechanical prperties, high electrical and thermal cnductivity as well. Graphite/cpper cmpsites have better

4 4 manufacturability when matched with titanium, and lw density against steel. Supercnductrs having prperties f gd ductility and malleability may be prduced with a Cpper as matrix and supercnducting filaments f nibium-titanium as reinfrcements. Cpper matrix cnstituted with particles f tungsten material r particles f aluminum xide can be used in heat sinks and semicnductr packaging. The research n the develpment f aluminum reinfrced with silicn carbide fibers is under prgress as skin material fr the aernautical applicatins alng with Titanium. Sme f the matrix materials include Stainless steels, Tl steels, and Incnel and they can be reinfrced with particles f titanium carbide material and manufactured int cmpnents that can be used fr applicatins like high-temperature and gd crrsin-resistance. 1.1 ADVANTAGES OF USING MMC s Cmparisn f MMC s with mnlithic materials MMC s have the fllwing advantages ver the mnlithic materials. Better strength and density ratis Better stiffness and density ratis Gd resistance t fatigue Better elevated temperature prperties Gd strength smaller creep rate Lwer CTE Gd resistance t wear

5 5 1.2 Reinfrcements in MMC s Reinfrcements that are used in MMC s can be gruped int five majr categries: Particulates Whiskers Cntinuus fibers Discntinuus fibers, and Wires These reinfrcements are usually made ut f ceramics, except wires which are metals. Brn fibers are manufactured using CVD f this material n a cre made ut f tungsten material. Carbn can als be used as cre material. T reduce the rate f reactins that may take place between brn particles and metals perating especially at high temperatures, materials like SiC r brn carbide can be used as fiber catings. CVD is used t Manufacture silicn carbide mnfilaments using a cre made ut f tungsten. Cntinuus alumina fibers are als available vary widely. The mechanical, thermal and physical prperties and chemical cmpsitins f varius fibers are significantly different. Tw materials as precursrs, namely, petrleum pitch and plyacrilnitrile are used t manufacture Graphite fibers. Graphite fibers can als be manufactured frm cal-based pitch and the research is underway. Graphite fiber materials have a large range f mechanical and are cmmercially available in market. Presently, fibers f discntinuus reinfrcement like alumina and alumina-silica are very ppular. Bth f these materials have been develped initially as insulating materials. In these cmpsites the imprtant whisker material is Sic. Brn carbide and

6 6 SiC which are used as whisker materials are prduced in the industries which manufacture similar abrasive materials. Similarly, the particulate is prduced as a biprduct when the whiskers f the respective materials are manufactured. Adding the metal wire can strengthen the metal matrices. The examples f the metal wires are mlybdenum, tungsten, titanium and beryllium. The applicatin f the metal wires in strengthening f the metal matrices is ppular especially when dealing with the materials having high strength and gd cnducting prperties. The techniques, used fr reinfrcing the matrix, listed abve, are sme f the very imprtant material presently. Many thers have been tried ver the last few decades, and the research is underway t develp better materials fr the future. 1.3 Materials fr Matrix and Reinfrcements Matrix materials have been large in number and sme f the mst imprtant materials are Al, Ti, Mg, and Cu allys as well as super-allys. Widely used MMC materials are: Al matrix Cntinuus fibers: B, SiC, Al 2 O 3, and Graphite Discntinuus fibers: Al 2 O 3, Al 2 O 3 -SiO 2 Whiskers: SiC Particulates: SiC, B 4 C SiC can be used as particulates, whiskers and cntinuus fiber in Al matrix. Mg matrix Whiskers: SiC Cntinuus fibers: Graphite, Al 2 O 3

7 7 Particulates: SiC, B 4 C SiC can be used bth as particulates and whiskers in Mg matrix. Ti matrix Cntinuus fibers: SiC, Cated Brn Particulates: TiC SiC can be used nly as particulates and cntinuus fibers in Ti matrix Cu matrix Cntinuus fibers: Graphite, SiC Wires: Nb-Ti, Nb-Sn Particulates: SiC, B 4 C (Brn Carbide), TiC. SiC can be used nly as particulates and cntinuus fibers in Cu matrix Super-ally matrices Wires: W (Tungsten) 1.4 Design Characteristics The excellent prperties f metal matrix cmpsites demand their need. One f the significant characteristics f metal matrix cmpsites that are cmmn with ther cmpsite materials is, by prper selectin f matrix materials, cnstituents, and layer alignments, it becmes easy in altering the characteristics f a final part accrding t the demands f a certain design needs. In the range f wider limits, materials with unidirectinal prperties can be manufactured. It is als pssible t manufacture materials with certain prperties dminating in ne directin and ther prperties dminating in

8 8 ther directins. This kind f materials cannt be manufactured that easily using mnlithic materials. Istrpic prperties are the characteristic f the mnlithic materials, which means, the prperties d nt change with the directin. Certain prperties can be imparted t the mnlithic materials using prcesses like frging, drawing and rlling. The stress behavir f these materials is generally elastic and becmes plastic after certain defrmatin. These metals are gd in prving enugh mechanical prperties when used in structural applicatins. The characteristics f metal matrix cmpsites change significantly depending upn its manufacturing prcess as well as matrix and reinfrcement material s frm and size. Factrs that affect their prperties are: Characteristics f the reinfrcement materials Shape f reinfrcement, and gemetric alignment Percentage vlume f reinfrcement materials Characteristics f matrix materials Interfacial characteristics f reinfrcement-matrix Residual stresses Pssible degradatin f the reinfrcement. Similar t mnlithic metals, particulate-reinfrced MMC s tend t be istrpic. The ductility and fracture tughness gets reduced due t presence f brittle reinfrcements and metal xides. Cntinuing imprvement and decrease sme f these deficiencies. The rientatin f whiskers decides the prperties f materials when reinfrced. Istrpic materials can be prduced by randmly riented whiskers. Hwever, prcesses like extrusin can align whiskers, prducing materials with uni-directinal prperties.

9 9 Metal matrix materials can be imparted the uni-directinal prperties by aligning the fibers in a particular directin. These materials have dminating prperties in the directin f alignment than in a directin perpendicular t the alignment f fibers. Fr applicatins like cmpnents fr stiffeners and struts, unidirectinal metal matrix cmpsites are suitable asalignment f the fibers is designed and manufactured alng a particular directin. Due t this reasn f better advantage, metal matrix cmpsites are preferred ver the mnliths. Because the elasticity prperties, yield strength and ultimate strength f matrices f the metal matrix cmpsites are imprtant, cntributin the prcess f careful reinfrcing the fibers is very imprtant. The fiber reinfrced metal matrix cmpsites behave nn-linearly due t the reasn that the matrix underges defrmatin when stressed. Significant difference in CTE between the matrix and the reinfrcement is als anther factr which needs attentin when designing the metal matrix cmpsites. This difference in CTE usually results in the residual stresses when perating at temperatures which significantly high. Als when these metal matrix cmpsites are cled frm high perating temperate, it induces stresses which may ultimately lead the material twards yielding. The stresses can als result frm mechanical lads. The fiber reinfrced metal matrix cmpsites are nt nly nn-linear in behavir but als they are brittle, which means that these materials d nt have enugh ductility, malleability and tughness. Due t lack f these prperties, these materials have limited applicatin when designed as jints. Hence special care is taken when design the jints using these materials. Several methds are already available t handle such needs.

10 Manufacturing Methds Selectin f manufacturing methd is key in the design f metal matrix cmpsites as well as mnlithic materials. There is significant research has already happened in this regard. There is a pssibility f tremendus increase in the perfrmance f the present techniques as well as evlutin f new techniques in the cming years. Existing methds and techniques may be classified int tw types, namely, primary manufacturing methd and secndary manufacturing methd. First type f manufacturing is used t manufacture the metal matrix cmpsites using its cnstituent elements. The resulting frm and cmpsitin may be as near as pssible t final cnfiguratin. Als the resulting material may require enugh prcessing additinally which is called as secndary fabricatin, like machining, cld r ht frming techniques. The methd that may be used in this fabricatin methd is chsen based n the frm and size f the reinfrcement material. There is a pssibility f reactins between the matrix and reinfrcement elements. These reactins tend t ccur mre prminently at elevated temperatures during the primary and secndary prcesses. The elevated temperatures ccur during the melting and frming f the materials. Due t this reasn, there is a limitatin f the type f materials chsen fr the cnstituents. Smetimes it becmes necessary t use cated particles f the reinfrcement materials t prevent the pssible reactins with the material f the matrix. One example is, cating f a B 4 C (brn carbide) n the B (brn) fibers which are used as reinfrcement particles in the matrix f Ti (titanium). There is als a

11 11 pssibility f the reactin between the material used as a cat and the matrix material. This is again is mre prminent when temperatures are high and the duratin f the service. Fr sme f the materials like B and SiC, the size f the fiber is large enugh; hence these fibers can be embedded int the matrix by ht pressing them as a layer between the layers f matrix. This creates a tape kind f material. In this prcess, the matrix material cnfrms t the shape f the fibers and fills the gaps that are present in the layers between the cnstituents. The bnding ccurs between the cnstituents thrugh the diffusin prcess. By fllwing a similar methd, laminates can als be prduced. In case f laminates, the fibers are riented in a particular directin which gives enhanced strength and stiffness. The laminates can als be prduced by fllwing the same prcess again but by using the mnlayer tapes. The fibers can be cnverted int the tapes with the spray f the metallic plasma fllwed by the ht pressing technique. Multiple shapes can be manufactured by pressing the tapes and layers in a die. Applicatin f this type f manufacturing can be fund in the manufacture f the parts used in space craft applicatins. These parts are made by wrapping the mnlayer fils n a mandrel and by subjecting the specimens t ht pressing. Due t this, a bnding ccurs between the layers thrugh diffusin prcess. The mandrel is remved after the frmatin f the part. Mandrel nly prvides supprt t the fils and layers until the part is frmed. Anther technique which can be used in the manufacture f MMC s is by injecting the metal in liquid frm int the already prduced assembly, als knwn as perfrm, f the fibers. During the injectin and subsequent penetratin f the liquid metal, the fibers underg burning. This methd f manufacturing can be perfrmed either

12 12 under vacuum r under pressure. Materials like Ceramic r Organic can be used as a supprt material t create the prefrm f the fibers. The vid rati is least in the frmed part when the pressure is applied during the penetratin than wrking under vacuum. This kind technique is knwn as squeeze casting. The metal matrix cmpsite can be prduced with superir prperties when the casting technique is used instead f any ther traditinal manufacturing methd. Other advantages f casting methd is it is lw in cst f prductin and can be easily manufactured as number fabricatrs available are large cmpared ther methds f manufacturing. Mrever, cast metal matrix cmpsites gives better dimensinal stability. Injectin and penetratin technique is very widely used in the manufacture f Al- Graphite and Mg-Graphite metal matrix cmpsites. Graphite is the material used as fiber reinfrcement and is first passed thrugh the ht furnace. With this the yarn f graphite is burn ff and then it is subjected t CVD t apply cating n the fibers. The cating materials are generally B r Ti, t enhance the wettability f the fibers with the matrix. The yarn after having cated passed thrugh the mlten metal present either in the frm r spray r bath. This prcess prduces a frm knwn as wires and ther shapes like laminates and plates can als be prduced by fllwing the methds explained abve. 1.6 Thermal Cntact Resistances n Heat Transfer applicatins Cntact ccurs nly at a few discrete pints when tw rugh surfaces brught in cntact with each ther, while ther places will nt have any cntact due t the nnplanarity issues. The nn-cntacting areas are pssible t have vacuum r may be filled with sme medium such as air, water etc. This will have an impact n cnductin f heat

13 13 between the tw surfaces. The resulting heat transfer resistance at the interface f tw cntacting materials is called as Thermal Cntact Resistance (TCR). The Thermal Cntact Resistance is defined as the rati f temperature drp at the cntact interface t the average heat flux acrss the junctin. Thermal jint resistance and thermal bundary resistance are the alternative terms used fr Thermal Cntact Resistance. Thermal Cntact Cnductin is the reciprcal f Thermal Cntact Resistance. Actually, all these terminlgies are used when dealing with heat transfer f the cntact interface. The prblem f TCR is encuntered in many applicatins in aircraft industry, nuclear industry, crygenics, and micr-electrnics and s n. Since last few decades, the pwer f electrnic devices has increased many times while their sizes have reduced. Hence thermal management has becme mre challenging with cntinuusly reducing sizes f features in the electrnic chips. Intel Crpratin had frecasted that the prgress in field f thermal slutins culd nt meet the demand fr increasing cmputing and cmmunicatin needs in Even nw, the prblem still persists. Imprving the heat transferability f the interface is very imprtant fr reducing the size f the chips as TCR is fund t be a limiting factr. In aerspace applicatin, such as lw temperature, vacuum and micr-gravity envirnments, heat transfer takes place prminently with cnductin and radiatin and little r nthing with cnvectin. Als, heat transfer ccurs nly by cntact cnductin in many interfaces n satellites. Fr preserving the reliability and perating lives f electrnic elements, they need t be made t perate within a limited temperature range. TCR research deals with basic theries and engineering applicatins. In fact, the aim f the research is t cntrl (decrease r increase) the TCR t meet applicatin requirements. Keeping in view f reliability aspect f electrnic cmpnents, their junctin temperature must be kept relatively lw. Fr imprving the rate f heat remval,

14 14 clers are used. Fr enhancing the thermal cntact cnductivity, researchers have prpsed many methds. The mst widely used methd fr enhancing thermal cntact cnductance cnsists f filling the interstitial gap with interstitial materials. Cmmnly used interstitial materials are thermal grease, thermal tapes, phase change materials, thin fils etc. Cnsiderable research has been carried ut n TCR prblems invlving direct applicatins and applicable TCR database has been created. But it is equally imprtant t develp theretical mdels, crrelatin equatins and numerical methds t predict TCR. Such TCR predictin mdels help in designing the mating parts, deciding n the surface finish, cntact pressure and fr chsing the interstitial materials. Hwever fr develping TCR mdels, basic research needs t be dne with reference t surface gemetry, mechanics f cntact areas and thermal mechanisms. Develping theretical predictin mdels helps in getting a deeper understanding f physical mechanism f TCR and als helps in prviding slutins t real life engineering prblems. 1.7 HEAT TRANSFER AT THE INTERFACE The heat transfer thrugh the interface between tw slids can take place by three different mdels. I. Cnductin thrugh the cntact spts II. III. Radiatin thrugh the gap between the slids and Cnductin thrugh gas r fluid that may fill the gap These three mechanisms f heat transfer are generally treated separately, and the thermal cnductin f the jint in the summatin f the cntact, radiatin and gap cnductance. Further, experimental studies have shwn that TCR between tw cntact surfaces depends n the cntact pressure, the temperature difference between the surfaces, surface tpgraphy f cntact surface, the defrmatin f cntact spt, the physical and thermal characteristics f the material junctin etc. Hence slutin t TCR

15 15 prblem requires multidisciplinary apprach including gemetry, material, mechanics and heat transfer. Based n the interdisciplinary theries, the researchers have prpsed many predictin mdels. Figure 1.1 (Ref N51, 52 and 53) shws the pictrial representatin f the cmmn dmain f thermal cntact resistance with respect t ther areas f research TCR Mdels based n a Single Cntact Spt THERMAL CONTACT RESISTANCE CONSTRICTION RESISTANCE CONTACT MECHANICS GEOMETRY THERMAL MECHANICS THERMAL ELASTOPLASTICITY Fig 1.1: TCR with respect ther areas f research Thermal cnductin f a single cntact spt culd be cnsidered as the lgical starting pint fr analyzing TCR prblems. Further, the ttal thermal cntact resistance f tw cntact spts is cnsidered as the sum f resistances fr all sides f the cntact spts. If we assume that the heat enters r leaves an istrpic half space thrugh a single cntacted spt, the flux lines spread as far apart as the cntact area, then the thermal resistance is called spreading resistance. The inverse f spreading resistance is knwn as cnstrictin resistance. Figure 2 (Ref N 54, 55, 56 and 57) depicts the way the number f cntacts can be increased by changing the surface f cntact. Many researchers have develped mathematical mdels based n single cntact spt.

16 16 Fig. 1.2: Surfaces f cntact and the pints f cntact The abve mentined TCR mdels fr a single spt cntact are based n the Furier's law. But at micrstructure level (r mlecular scale) it is fund that the heat cnductin deviates significantly frm the predictins f Furier's law. This is due t bundary and interface scattering. The heat energy is carried by electrns and phtns scattering at the interface between the slids. The mvement f thermal particles (electrns & phtns) is crrelated t the temperature in a crygenic state and hence the thermal cnductivity f the cntact spt culd be affected. Fr very small cntact spts, the scale effect f thermal cnductivity is remarkable. In all these cases, heat cnductin n cntact spts is cnsidered as the mst imprtant mde f energy transprt. The imprtant issue fr generally rugh cntact surface is t btain the dimensins and distributins f all the cntact spts at the interface fr calculating the actual cntact surface area TCR Mdels based n Surface Rughness Apart frm TCR, ther areas such as triblgy, abrasin, cntact resistance etc. require study f surface tpgraphy. The tpgraphy f machined surface cnsists f 3

17 17 categries f errrs namely waviness, rughness and gemetrical frm. Actual cntact takes place nly at certain discrete spts when tw rugh surfaces are brught int cntact. If ne can btain the size f the cntact spt and its distributin, by using the abve mathematical mdels, the TCR can be predicted fr a single cntact spt. If an interstitial material exists between the cntact gaps the heat transfer capability gets mdified t such an extent. Hence TCR mdel fr a rugh surface in a vacuum is very useful. (Ref N 78 and 129) Y s 1 m =dy /dx X s 2 m =dy /dx Y Y s m =dy / dx Y X s = s 2 + s m= m 1 + m2 Fig. 1.3: The rt mean square value f the surface rughness TCR Mdels based n defrmatin f Cntact Spts Almst all predictin mdels invariably assume defrmatin f all cntact spts at the cntact interface between tw slids. The studies f macrscpic material mechanics have frmulated three mechanical mdels which culd directly affect the TCR, namely plastic cntact mdel, elastic cntact mdel and elastic-plastic cntact mdel. In this research wrk, effrt has been made t imprve the heat transfer at the in the thermal management system by develping new metal matrix cmpsites fr certain applicatins and new TCR mdels are develped t enhance the heat transfer at the

18 18 interface.the traditinal mdels use the imprvement techniques t enhance the heat transfer at the surfaces f the cntact by cntrlling the surface rughness, surface waviness and gemetry. Macrscpically, these techniques increase the area f surface cntact. Micrscpically it increases the number cntact pints there by increasing the heat transfer. Hwever, in the present wrk, the mdels with n direct surface t surface cntact is cnsidered as the filler material is used between the surfaces f cntact. Surface rughness and surface waviness need nt be cntrlled very accurately. Hwever, the gemetric tlerating f the surfaces like paralality and flatness t be maintained perfectly t avid uneven gap between the cntacting surfaces. These tw gemetric tlerances paralality and flatness is required be maintained accurately fr the traditinal mdels als.