WO2017062697A2 - 2-dimensional thermal conductive materials and their use - Google Patents

Abstract

The development and manufacture of thermal interface materials including, among other forms, greases, pastes, gels, adhesives, pads, sheets, solders and phase change materials, with good through-piano thermal conductivity for thermal interface applications. The good through-plane thermal conductivity is achieved, through the formation of a conductive network by the use of thermal conductive material- coated fillers, combinations of thermal conductive material-coated fillers and uncoated fillers.

Description

that ..&t≠- da crihed in this: ^ ii ^k :

This application is a utility application form, and claims priority from ibS. Provisional patent a plicatio sor i¾l onisoer ^i iS^,^'· 57 fil d October , 2015,

OF W, w mi

This invention deals with the development and manufacture of graphene based he mal interface jaatsrials including,, among other, greases, pastes, gels, adhesives, pads, sheets, solders and phase change materials,; with good through-plane thermal conductivity for ae si s l i n er face applicat ons,: The good^"through- lan th :rmal conductivity is achieved through the formation of a conductive network by the use of the grephane and graphene"-coated. f l l era.

B¾CK 5 O¾f»B OF X VENTIOS

Therm l interface materials (TIM) a e used to minimize the contact therma resistance between a heat source and a heat sink, It is widely applied in electronic and other industries where heat removal from chips or processors is critical since operation of inrearahod circuits at elevated temperatores is a major cause: of failure for electronic devisee . Such thermal management is becoming jsors^' and more important wit the rapidly increasing functions and hence oc e >· densities of advanced electronics. Generated heat needs to be transferred or dissipated: to a heat sink in order to maintain an appropriate operating temperature.

When two solid: surfaces, such as a heat source and a heat sink, are put to ether:, ho var, th real contact area between them is limited due to the fact that the two surfaces are oof completely flat, only a very aiaall portion of the apparent surface area is actually in contact. As a result, thermal transfer between these m ting surfaces is very limited as well, causing a notable temperature difference at the interface. Th major role of TlKa is to fill the gap between the two mating surfaces and increase the heat transfer between them. The i major reqoi reman s fo a IM material areo i go thermal conduct iv.it y, easily deformed y a mail ress to till the v id.^' be wee the contacting surfaces, good n t ing and affinity w th the w contacting sur ces, ability to form a layer with minimal thickness, mechanically stable, not easy to leak out, good therm l cycle life, and, easy to apply.

Traditional TIMs include greases, pads, gels, adhesives, solders, and p ase change materials, etc. Hosr. of them are made of a polymer or silicone matrix loaded with thermall conductive filler particles .

Thermal greases are a form of thick pasts com sed of^" thermally conductive filler dispersed in silicone or hydrocarbon oil. The filler can be me allic, ceramic, or carbonaceous materials. Metal-based the m l greases ofte employ silver., co pe , ¾¾? alu mim articlesi They usually have good thermal conductivi y, b¾t ma suffer high cost.

^dditldnalTy^ they are electrically^'conductive: w iffs may limit their applications without an additional electrically insulating material to go with them_* Ceramic-based thermal pastes typically use conductive ceramic particles, such as beryllium oxide, aluminum nitride, aluminum oxide, sine o¾ide. and silica as the filler. They usually have good thermal conductivity and low cost. Carbon-based oe rod greases are relatively new. Good fi llets include carbon n notube (CiiT) and carbon nano fibers (CPf} , In general, thermal greases have high thermal conductivity, thin bond line thickness {BLT) with minimal pressure* low viscosity to fill the voids between mating surfaces, and no need to be cured. However, thermal grease is

susceptible to grease pomp-out and iosssy to apply. The pxa -out is typically caused by mismatched coefficients of thermal expansion (CTE) of the m ting surfaces, which could forc the TIM to flow out of the interface by alternatel squeezing and releasing the system during thermal cycling.

Thermal pads are a group of TIMs in the form of pad. They typically consist of an eiastoms.t matrix such as silicone rubber and thermally conductive fillers such as boron nitride, alumina, or tine oxide. The material is often made nto a soft pad that can be conforaable t the mating sur ces ¾pon compression. ate e sil

handled and^·a li d, less s sc ible to ussp-out, and can serves as a vibration damper. Their major drawbacks Include the n ed for high contact press e nd lower thermal conductivity a d higher osts than thermal greases.

Thermal gels typically consist of silicone (or olefin) polymers with low cross-link density loaded ¾ith thermally conductive filler, either ceramic or metallic:. The silicone has low jaoduiws o

elasticity; good wetting characteristics and high thermal stability. The m terials are li e greases bnt can he cured. They have relatively de ent thermal conduc ivity, good we ti g c aracteris ics , easy to conform, to mating surfaces, and are lass susceptible to pump out.

H wever, they need to be cured and y delaminate during thermal

Thermal adhesivea ar a type of thermall edndixctlve glue normally consistin of at adhesiv resin and a thermally conductive filler, An example is silver particles dispersed in a cured epoxy matrix. Such TI s eliminate the need for mechanical attachment of permanent pressure and are easy to apply. They are not susceptible to pump-out and can conform to the rsating surfaces. However, they need to be cured and there is a risk of del min&tion during usage,

A_. phas change material (PCM} is a substance with a high heat of fusion whic is capable of s oring or releasing a large amount of energy upon melting or solidifying. The phase change thermal interface materials are typically made of suspended particles of high thermal conductivity and a base material. Exam les include conductive metal oxide particles dispersed in an organic matrix such as fully refined paraffin, a polymer, a co-polymer, or a mixture of the three, At room temperatures, they are similar to thermal pads, hen heated to a certain temperature, normally >50^S,C, they change to semi-soiids o liquids to fill the void between mating surf ces. They solidify again when the temperature drops below the transition temperature,. PCM is less susceptible to pump-out and its application is easier than grease. It also does not need to cure and there is no deia ination concern . he :n;;co: d awba ks ere their iov/e.r: condueti ei i as com arad to that of grease and a ressur is e i ed as vail .

The the mal conductivity of commercially available Ms is around 5 W/mK which is considerably low than those of the typical ma ing surfaces. s a result, there has been a growing interest in searching for better TIMs, especially sore effective fillers. Advanced carbon- based nano materials such as carbon nanot be, graphene, and graphene naoo a tele t s are projaising candidates du to thei high intrinsic thermal conductivities- For szamole th thermal conductivi y of single-wall carbon nanotube petit) is in the range a 3000-5000 /m at room tem rat e er as that of graphene is even higher. While Cff has received considerable attention for IM applications , it has yet to be commercially successful due to both performance and

manu acturihg cos dasb et laxly wcrb foensed^: on dispersing δΗΤ randoml an the results hav been lees satis actory. Reeenti i attention has shifted to the vertical alighssent of -V T and edact J on of boundary resistance at the interface between ChlT and two mating surfaces. But, it eill be a challenge for such a technology to be used for mass application due to its high processing costs.

Recen l ,, craphene became a neu focus for advanced thermal mana e ent solutions due to its high thermal conductivity. Graphene has a unique thermal propert : it has an extre el hi.gh in-plana thermal conductivit but the through-piane conductivity of graphe e is at least two orders of magnitude loser. The high in-plane thermal conductivity results frors the covalent so- banding between carbon atoms whereas the poor through-plane thermal conductivity is mainly due to we k van dec aals coupling in that direction- The thermal

conductivity of a suspended monolayer grapheme was reported to be about 5000 fcVmK ¾hen measured, by an optical method from shift in Raman G band, it' s for this reason attempts have been made by m n

investigators to incorporate g a h me ox graphene nano l telets in various materials or forms fo thennal applications including sheet products for thermal dissipation and. graphene-based pastes/'adhesives for thermal interface heat transfer. Far example, the inv ors herein have d velo d a graphen -bas she product 1Kb¹ Leaf 8 that can be sed £©r s readi g eats from a heat source to a heat sink. The material has & high in-plane hermal conductivity of >S00 /rah and a low through-plane conductivity of 5 i/ K . This m terial utilises the 2-diisen ionai and anisotropic features of graphen n&nopiateiets so that heat is dissipated

laterally away rom heat source instead of transferring through to other parts of electronic devices_* For som other applications,, howeve , heat n eds to be transferred across two mating surfaces of a heat source and a heat sinh. for e am e, Figure i s ows an

application of thermal interface material in a LED lighting device. Currently, silver-based solder paste is used to transfer beat away from LED chip to a heat sink. There are several disadvantages with tbis^: t ¾r¾¾i jenagbian soi tiorr- hirst t : pate needs to be: cured at : -eppera u e that can easily cause damage bo the: chips. Second, once cu ed, it is ve:y difficult to be tahan apart. ¾h n one chi fails_r the entire unit has to be replaced. Third, silver-based paste is expensive, hs a result,, it is desired to replace the solder paste with a thin grease, gel, or tape with a high through-plane thermal conductivity and low cost- Seme other applications requi e the TIM to be in the form of an adhesive, ad, phase change materials, and the like, ft was anear this circumstance tha following inventions »s re conceived_*

Graphene and graphene-baaed raateri ls have been used in ther al interface materials as a filler as found in ^02035/103^35^

US2014/32802I, and US2014/120399. Due to their 2-diisensional nature, however, graphene sheets or gra ene nanopiatelets tend to align or orient parallel to the thermal interface, especially under pressure. ¾s a result, the effect in enhancing the thru-plane thermal

conductivity is substantially diminished. Therefore, it is imperative to establish a thermal pathway that can effectively conduct heat in the through plane direction. The instant invention has unique

distinctions over the prior art.

WO20X5/103435, deals with a rseth d of aligning graphene flakes perpendicular to th mating substrate using magnetic f nctionaiiKstios and magnetic fields , This e ui s x siv specialized equ i. .^enr. to generate the mag t c f lds, A ditionally, the gra hene alignment may decrease o e time, in a fluid sys em, once the .magnetic field Is no longer pplied. In the instant Invention, qraphe-ne, graph® © n nopiatelets_f or t:chat thermally conductive materials such as boron nitride platelet can be coated or anchored on the surface of fillers. The partial qraphene platelet alignment perpendicular to the mating subs ates is an inherent property κχ ¾ I ade with grspheae or another coated tille , and nil! remain stable. Such a TIM can be processed using standard I dust y equi ent and methods and still get the benefit of aligned grap ene and other thermally conductive platelets; .

US2014/ 120359 describes the thermal benefits of adding graphene to ¾^■■ matri to nest s a Til, b t mk s no men ion of ibe problem ··..·?. platelet alignment encountered is: the t in bond lines nana In

practical TIM applications. ut instant Invention addresses that alignment roblem_*

S2G13/ 22126S describea a thermal paste using graphene latele a in conjunction with other filler materials to create a 3D conductive ne w k. Ho eeer, by coating the graphene platelets onto the other fillers without significantly amaging their structure the instant i ventio achieves similar thermal conductivity improvement ith greatly reduced viscosit_f resulting in a superior product for handling and thermal resistance.

U.S. 7,866,813 describes a TIM material with filier particles coated w t high thermal conductivity coatings. The coatings are met ls and the use of a graphite or a sheet materiel to coat the fillers is not contemplated.

U.S. 2014/0255?$ describes a process: tot coating particles with, graphene. Use of such particles for neat: transfe such as in a: rib! is not co tesisplateb. BRIEF BESCEIPfXO OF fHI DK&!lii&S

figure I is an exem la y application illustration of the mal interface ssaterial shoeing a pad an an ¾I board 1, LED chip 2, thermal interface mat al 3, silicon , board S, the mal

interface material 6, md heat sink 7.

figure 2 is an illustration of nanoplatelet coated filler particle showing the graphene coating 8 and the filler particles 9 arsd

Figure 3 is an illustration of a .thermal in e face ma erial ssade with thermally conductive nanoplatsleta and nanoplateiet c a ed fillers shoving the resin matrix 10, the gra me coated filler particle 11, the graphene sheet or grap ene nanoplateiet 2,

Figure 4 is a microp otograph of g a ene nanoplateiet coated alhmina fillers for t etisa! intetfaen mat ialss,

Figpre S is grap of thatma i conductivity of loMna o the pr;.o: art compared to coated alumna of the idatant la^enfida.

Figure 6 is a graph of thermal resistance of alusana of he prior art compared to coated alumina of the instant inverc:: ;m.

Figure ? is a graph of thermal conductivity showing dry coated alumina v rsus w coated alumina.

Figure 8 is a graph showing thermal conductivity for alumina "A" versus iom.ina treated according to this invention^BB" and coated al i^ftC*_f and coated alumina and nanoplateiet blend^tt'-" .

Figure 3 is a graph showing thermal resistance of dry coated alumina versus wet coated alumina ,

Figure 10 is a graph showing th. rs¾¾l resistance of alumina and wet coated aluraina_*

Figure II is a graph s oeing thermal conductivity of alumina and ml coated alumina.

TE iwmmic®

hnn, in one bodiiii t of this invention, there is a t erma interface material containing a jaaterial selected from the group consisting of fillers,, graphene coated fillers, and, fixture of fillers and graphene coated fillers. In another embodiment, there is a method o providing a thermal interface composite, the method, c m sing - rovidi a irst substrate- that is a heat sink and providing a second substrate tha is a heat sou ce, and placing- a thermal interface ma erial as described herein between the first substrate and the second substrate.

There is a further embodiment_f which is a composite structure comprising a solid heat source; a solid heat sink, and, a thermal int rface mat al as described hersln contained, between the solid heat source and the solid heat sink.

In the instant invention, a through-plane thermal pathw y can be c ie ed through two approaches:

1. Use of nanoplatelet-ooated fille particles, F r e am le, ceramic particles can be coated by a highly conductive nanoplatelet material such- s grapheme nanopletele and boron oitride platelet:, :¾ coating: helps ensure a vertical heat conducting pathway with a

amount of graphenei nanop!atelet as ltibh. Figures 2 illustrates this conce t .

2, Use of fillers with different sizes and morphologies For exam le, in one embodiment, nano-piatelefc coated spherical filler particles are used together with graphene nanopiatelets to foras a 3-D conductive network better than using spherical particles alone. The additional graphene naneplatelets serve to better bridge the fillers with improved contact due to the 2~e and flexible feature of graphene naneplatelets as illustrated in Figure 3. For example, the contact between two spheres is theoretically a single point contact.

Introduction of flexible and fi&ke-Iihs graphene nanopiafelets can significantly increase the contact area of conductive fillers.

In one embodiment, this invention comprises a TIM grease made with graphene -nanopiatei t coated alumina Sillers. mina fillers were coated with graphene-nanoplatelet by a process using a. mechanical billing machine, Th coating ocess was designee to effectively attach graphene naneplatelets onto alumina filler without

significantly pulverising the graphene naneplatelet or producing amorphous carbon coating. The coated alumina filler is shown in Fig. 4. The IM Hiade with graphene coat¾d alumina filler showed

S sigiiifleant ingrease in thartssX conduct. xvit y fig 5} and decrea e in hermal resis vity {Fig_* 6} as compared with bare al mi a

In another emodimen , graphene coating of fillers is achieved by a wet met od, Graphene nanopiatelet;; and alumina are mi¾ed together in a solution of appropriate organic solvent, where they are dispersed and agitated by ultrasonic mixing for 5 minates. T « sol en is t an evaporated, leaving behind a homogeneous powder. The powder is dispersed into silicone oil in. order to create a thermal grease, The resulting therc-ai grease shows enhanced thermal, conduc i city compared to a grease made with an equal loading of unmodified alumina, as shown in F gures ? and 8.

In vet another embodiment, g a h me nanopiatelets ar added to a TIM grease together with graphene-ooated alui¾ina tillera. The

additional graphene nanopiateiet serve o better bridge the fillets one to the_:2™S and flexible features of graphena nancpiatelets , h flex ble and lake-like oraphen hasoplafelefs can signxfioaru:Iy increase the contact area of conductive fillers as illustrated in Figure .

The erm graphene as used in this invention shall include graphene nanopiatelets from fully exfoliated graphite to particles with thicknesses of less than lOOnm and/or number of layers less than .100, and preferably with thicknesses of less than 2Qnm and/or number of layers less than €0.

EXAMPLES

Exampl 1: Milling

Graphene nanoplatelets and alumina were added together into a canister with milling madia, and ball milled for id minutes. The resulting homogeneous powder was dispersed into silicone oil in order to create thermal grease.. The resulting thermal grease showed

substantially increased thermal conductivity and lower thermal resistance compared to grease made with an. equal loading of unmodified lumn * The g e se ul shewed egpai therm ! conduc i i and lower h r l resistance and viscosity c s & ad to gros.se mads »y a simple mi ture of the same graphene nanopXatelet and. alumina fi tu e.

Example 2: Solution P cessing

Graphene nanopl taiets and alumin were mixed together in a solution of appropriate organic solvent, whe e Lhoy w®r® dispersed and agitated by ultrasonic mixing for 5 minutes .

The solvent was evaporated., leaving behind, a homogeneous powder- The powder «as dispersed into silicone oil in order to areata a thermal grease. The resulting thermal grease showed enhanced thermal conductivity compared to a grease made with an equal loading of ij.nmadifi.ed alumin .

Example 3t

r¾a hegetn ao X e:let coated alumina pon r w¾s prepared os described in e mple 1, his powder w s d spe sed into silicone oil together ¾d.th ¾Bp¾0cesB«# ¾ra Se¾e- Ι¾«ό|>1¾β·1®¾ ond r, ?he resulting the-ntssl grease showed superior thermal conductivity compared to thermal, greases prepared with an eqnai filler content of unmodified aXumina, a mixture of unmodified aiamina and unprocessed graphene nanoplatelef powder with the same graphene nanopiateiet to alumina ratio, o graphene nanopiateiet coated alumina with the same graphene nanop latelet to alumina rati.e.. Shown in table I .

0 I¾.8 9.27

α. I 1¾.?02 0,198 3

Tofcal Sim le Mass (g)

Τ!;_;:Λ Conductivity iWniK}

20 2.22

20 3, 5

2 3.61

20 .7S

2 4.2

Claims

hat is claimed i¾i i. ¾h^'sr a in erfa e Μ½rial c.or & .-a . m t r a:!

from h© g oup eonsisting .of-:

a. fillers,

fa. coated fillers_f wherein th coating is selected from the group oonsisting -of i. grapheno, and,

ii , Boron nitride, a d,

c, raixtures of a. and b.

The t e siii intsr£e¾¾ m erial as claimed in dials; 1 wherein the: thermal in erface material is in a form selected, om the gr up consisting oft

a. g e se,

b. gel,

c. adhesiv ,

d. paste,

e.. solder,

f. pad, and,

g. se change material.

The thermal interface m teri l as claimed in claim 2 wherein the thermal interface material is in a iota selected from the group consisting of:

, grease,

b. gel,

0. adh sive,

d. paste,

e. solder,

f. d, and,

O phase change material.

The thermal interface material as claimed in claim 1 wherein the filler is selected from he group consisting of y

a- a ceramic,

b.. a metal,.

c. polyme y

d. carbonaceous SMteriaiSi,

e. composite materials,, and,

f. mixtures of any of a. - d.

The thermal interface ma erial as claimed in claim 4 wherein the ceramic filler is selected from the group consisting of:

«. » oxide,

b. a: carbide,

c. a boride, and,

d. nitride.

6. The tn.e ?:·:··■:;] interface material as claimed in claim 5 wherein the ceramic filler is sel cted f o the g ew consisting of a, alumina,

b, zinc oxi<te,

·::: . silica,

d. baron nitride;,

e. silicon car ide_r

f. aluminum nitride,

g. tin oxide?

. s-sagnesium oxide

i. it nium OKicie, and,

1, er liinxa oxide.

7, The thermal interface material as claimed in claim i therein the rcset lXi: :fiiier^: is s. m& l allol,

S, ffie thermal interface m t-ri l as claiased in cl^:ai¾ he ei t e etallic filler is selected from the group on is i g of:

&·. copper_f

b. aitmumim,

c. nickel,

<L silv , and,

e, gold,

3. The thermal interface jsateriai as claimed is. cl im v?he;:e the polymeric filler is selected from a groo consisting of:

a„ thermoplastic polymers,

b. therrsoset polymers, and,

c. elastomers,

10. The thermal interface material in claim 9 wherein the

polymeric filler is selected from the group consisting of

a, polyoiefin,

b. poly&mide,

c pol imide_r

d . ny1on,

e - polyester,

f,- polystyrene,

f. polyacrylate, ? p 1 yv x ny 1 c .ori de ,

h. tiuoropolyssera,

i. polyvinyl acetates,

polybatadienas,

k. oiychl.o oprene,

L polyuratb&n s, d,

m. copolymers or any of a . - as

11, The thermal interfac ;nateoal s claimed: is 4 wherein the carbonaceous filler is selected om the group consisting o a. graphite,

b. carbon black,

c. carbon fi er,

o amorphous carbon, and.,

e. dissiion ,

12. The tber el interface mat ial as elalased is claim 1 wherein grap erse Coating la c iev d by a method selected f ros:a the group consisting of;

a, mechanical milling,

fo, slurry coating,

a . s ay drying_.,

d. chemical vapor deposition,

e_:. physical va or deposition, a d,

r . graph! hi n ion,

lis. The thermal interface material as ::: laimad in claim 12 wberain the graphene source is selected from the grou consisting of:

a_* graphsne natoplateiat,

b_* graphite,

c. carbon black,

d, activated carbon, and,

a. pitch,

14, Tbe thermal 1 ^ er !iace ma e ial as claimed la claim 1 wherein, in additi.cn, ther is present an additional filler,

15, The thermal interface material as claimed in claim 1 wherein the additional tiller is graphene n&noplatel ts. Ι ·^:> . RB thermal interface material sa claimed in claim IS t erei the graphene nanopi telels have a thickness: bel vr 100 am. 17, The t ermal interface material as claimed in claim 15 wherein, the o/raphene nanoplateiets have a thickness .below SO m. IB, The thermal interface material as claimed in claj.m 15 wherein the eraphene nanoplateiets have a thickness beicei 25 nm. 1$. The thermal, interface material as claimed in claim 15 wherein the graphene eanapl teleta have a sice below 5SD ms,

20. The thermal interface material as claimed in claim 15 wherein the graphene nanoplateiets have a siz below IQQ am.

21. The thermal interface ma erial as claimed in claim IS wherein the graphene nanoplatsiets have a else elow ID am,

22. ¾ thermal interface material comprised of a filler coated with a thermally conductive materaal,

23. The thermal interfac material as claimed in claim 22 wherein the thermal if conductive ii¾ateriaX is 2-d:.mensie;;al material <

24. The thermal, interface material as claimed in claim 23 wherein the 2-dimensional m te ial is grapheme.

25. The thermal interface material as claimed in claim 23 wherein the 2-di ens on l material is grapheae nanopiateiets.

26. The thermal interface material as claimed in claim 23 wherein, the -dimensional material is boron nitride platelets.

27. Ά a-ethod of providing a thermal interface composite,, said method co rising:

A. providing a fi at substrate that is a heat sink;

B. providing a second substrate that is a heat source;

C. placinp: a thermal interface material as claimed in claim X between said first substrate and said second substra e . IB . A method of pro idi- j a thermal interface coH- ijs ta, said method comprising :

A. providing a first: s bs ata, that: la a heat sink;

B. providing a second subs r e that is a heat source;

C. placing a thermal interface material as claimed in claim 22 hetwean said fi sr substrate and said second sabstrs e .

2 » A composite structure

a solid heat scar e;

a solid hsat sink

a thermal interface material as claimed la claim 1 con ined be wo en said solid he xo rce d said solid heat sink..

.30. A compcaite stradtiire con riaino :

a solid heat source;

a solid heat sink;

a thermal interface material as claimed in claim 22 contained, between said solid heat source and said solid heat sink.