İSTANBUL TECHNI CAL UNI VERSI TY INSTI TUTE OF SCI ENCE AND TECHNOLOGY
MES OP HAS E PITCH DERI VED
GRAP HI TI C CARBON FOA M
MSc. Thesi s by Ayşenur GÜL, BSc.
De part me nt : Advanced Technol ogi es i n Engi neeri ng
Progra mme: Mat eri al Sci ence and Engi neeri ng
İSTANBUL TECHNI CAL UNI VERSI TY INSTI TUTE OF SCI ENCE AND TECHNOLOGY
MSc. Thesi s by Ayşenur GÜL, BSc.
706021009
Dat e of sub missi on : 27 Dece mber 2004
Dat e of defence exa mi nati on: 26 January 2005
Supervi sor ( Chai r man) : Prof. Dr. M. Ferhat YARDI M Prof. Dr. Ekre m EKİ NCİ Me mbers of t he Exa mi ni ng Co mmi ttee Prof. Dr. Hüs nü ATAKÜL
Prof. Dr. Must af a ÜRGEN Prof. Dr. Sezai SARAÇ
J ANUARY 2005
MES OP HAS E PITCH DERI VED
GRAP HI TI C CARBON FOA M
İSTANBUL TEKNİ K ÜNİ VERSİ TESİ FEN Bİ Lİ MLERİ ENSTİ TÜS Ü
MEZOFAZ Zİ FT BAZLI
GRAFİ Tİ K KARBON KÖPÜK
YÜKSEK Lİ SANS TEZİ Ki mya Müh. Ayşenur GÜL
706021009
OCAK 2005
Tezi n Enstitüye Veril diği Tari h : 27 Aralı k 2004 Tezi n Savunul duğu Tari h : 26 Ocak 2005
Tez Danı ş manı : Prof. Dr. M. Ferhat YARDI M
Prof. Dr. Ekre m EKİ NCİ
Di ğer Jüri Üyel eri Prof. Dr. Hüs nü ATAKÜL
Prof. Dr. Must af a ÜRGEN Prof. Dr. Sezai SARAÇ
PREFACE
Duri ng my r esearch, I have had a gr eat deal of hel p i n t er ms of i nfor mati on, i nvesti gati on and gui dance fr o m my dearest super vi sors Pr of. Dr. M. Fer hat YARDI M and Pr of. Dr . Ekr e m EKĠ NCĠ. I a m gr at ef ul t o t he m f or t heir hel p, super vi si ons, and for t heir pati ence.
I a m grat ef ul t o St at e Pl anni ng Or gani zati on for financi al support duri ng my MSc. I woul d li ke t o expr ess many t hanks t o Pr of. Dr. Must afa ÜRGEN, who i s coor di nat or of Mat eri al Sci ence and Engi neeri ng Pr ogr a m, f or hi s advi ce, encour age ment and support duri ng t hi s st udy.
I shoul d t hanks t o t echni ci an Hüseyi n SEZER f or hi s assi st duri ng Heli u m pycno met r y measure ment and r esearch assi st ant Tuncay TURUTOĞLU f or hi s hel p duri ng SE M anal ysi s.
It i s a gr eat pl easure f or me t o t hank Pr of. Dr. As ao OYA who i s t he head of De part ment of Nano- Mat eri al Syst e ms i n Gun ma Uni versit y f or hi s spirit ual support, pr eci ous gui dance, and gener osit y. He accept ed me as one of me mbers of his Car bon Gr oup and he gave me a gr eat courage t o co mpl ete my t hesi s wit h success.
I al so woul d li ke t o t hank Ass oc. Pr of. Jun-i chi OZAKI, Dr. Soshi SHI RASI, Dr. Ni rav PATEL f or t heir gener osit y duri ng my r esearch i n t he l aborat ori es of Gun ma Uni versit y.
I al so woul d li ke t o s peci al t hank t o my dearest br ot her Bar bar os GÜL and ot her me mber of my fa mil y who have gi ven me endl ess support all my life l ong.
Ma ny t hanks ar e al so due t o my dear est fri ends; Yasi n ARSLANOĞLU, Bi r gül DE MĠ RKOL, Me h met Özgür SEYDĠ BEYOĞLU, Andeli p ERDOĞAN, Taner BOSTANCI, Ser kan ÇAĞLI, Di de m OMAY, Nagehan GENCAY, Fili z KARADAĞ, Ayhan EKġĠ LĠ OĞLU and Mel da SĠ PAHĠ f or t heir i nval uabl e support s.
CONTENTS Page No LI ST OF TABLES vi LI ST OF FI GURES vii SUMMARY viii ÖZET i x 1. I NTRODUCTI ON 1
2. GENERAL I NFORMATI ON ABOUT CARBON AND CARBON FOAM 2
2. 1. Carbon 2
2. 2. Bondi ng i n Carbon mat eri al s 3
2. 3. Cryst al Struct ures of Carbon 3
2. 4. Hist ori cal Overvi ew of Carbon Mat eri al s 5
2. 5. Order and Di sorder i n Carbon Mat eri al s 6
2. 5. 1. Mor e Or dered Struct ures 6
2. 5. 2. Less Or dered Str uct ures 6
2. 6. Carbon For ms 7
2. 6. 1. Gr aphiti c and Non- Gr aphiti c Car bons 7
2. 6. 2. Gr aphiti zabl e and non Gr aphiti zabl e Car bons 7
2. 7. Carbon Foa m 8
2. 7. 1. Hist or y of Car bon Foa m 9
2. 7. 2. Preparati on and Charact eristi c of Mes ophase Pit ch Deri ved Gr aphiti zed Carbon Foa m 10
2. 7. 2. 1. Mesophase Pit ch 10
2. 7. 2. 2. Foa mi ng Mechani s m 11
2. 7. 2. 3. St abili zati on of Car bon Foa m 13
2. 7. 2. 4. Car boni zati on of Car bon Foa m 13
2. 7. 2. 5. Gr aphiti zati on of Car bon Foa m 13
2. 7. 2. 6. The rol e of structur e on t he t her mal Conducti vit y of Gr aphit e Foa m 14
2. 8. Appli cati on Areas of Carbon Foa m 15
2. 8. 1. Radi at ors 16
2. 8. 2. Personal Cooli ng De vi ces 17
2. 8. 3. Co mput er Chi p Cooli ng 17
3. EXPERI MENTAL 19
3. 1. Precursor for Carbon Foa m Producti on 19
3. 2. Experi me nt al Procedure 22
3. 3. Bl endi ng Pol y mer 24
3. 4. Characteri zati on Techni ques for Carbon Mat eri al s 26
3. 4. 1. Ther mal Diffusi vity Measure ment 26
3. 4. 2. X- Ray Diffract o metry 26
3. 4. 3. Scanni ng El ectron Mi cr oscopy 27
3. 3. 4. Heliu m Pycno met r y 27
4. RESULTS AND DI SCUSSI ONS 29
4. 1. The Effect of Te mperat ure on t he Foa m Struct ure 29
4. 2. The Effect of Pol yme r Addi ti ves on t he Foa m St ruct ure 31
4. 3. The Effect of Pressure on t he Properti es of Graphi ti zed
Carbon Foa m 35
5. CONCLUSI ONS 43
6. RECOMMEDATI ONS AND FUTURE WORK 44
REFERENCES 45
CI RRI CULU M VI TAE 50
LI ST OF TABLES
Page No
Tabl e 2. 1. Ther mal pr operti es of pitch deri ved car bon f oa ms co mpar ed t o
ot her t her mal manage ment mat eri als... 8
Tabl e 3. 1. Typi cal pr operti es of AR pit ch ……….. . 20
Tabl e 3. 2. Heati ng regi me of st abilizati on pr ocess... 23
Tabl e 3. 3. Heati ng regi me of car boni zati on pr ocess... 23
Tabl e 3. 4. Heati ng regi me of graphi tizati on pr ocess... 24
Tabl e 3. 5. St r uct ure of THF, P MMA and PS... 25
Tabl e 4. 1. Co mpari son of X- Ray diffracti on results and t he degree of gr aphiti zati on of car bon foa m sa mpl es pr oduced at different pr essures, vari ous car bon fi bers and foa m... 38
Tabl e 4. 2 Ther mal conducti vit y of car bon foa ms pr oduced and different ki nd of car bon f oa ms ……….. ... 41
LI ST OF FI GURES Page No Fi gure 2. 1 Fi gure 2. 2 Fi gure 2. 3 Fi gure 2. 4 Fi gure 2. 5 Fi gure 2. 6 Fi gure 2. 7 Fi gure 2. 8 Fi gure 2. 9 Fi gure 2. 10 Fi gure 2. 11 Fi gure 2. 12 Fi gure 2. 13 Fi gure 2. 14 Fi gure 2. 15 Fi gure 2. 16 Fi gure 2. 17 Fi gure 3. 1 Fi gure 3. 2 Fi gure 3. 3 Fi gure 4. 1 Fi gure 4. 2 Fi gure 4. 3 Fi gure 4. 4 Fi gure 4. 5
: Ener gy l evel s graph of car bon at o m... : Struct ure of di a mond... : Gr aphit e struct ure... : Full erene Mol ecul e... : A di agra m i ndi cati ng t he gr owt h of car bon mat erials... : Marsh- Griffit hs model of car boni zati on/ graphitizati on
pr ocess...
: Sche meti c represent ati on of nongr aphiti zabl e (l eft) and
gr aphiti zabl e (ri ght) car bon...
: Typi cal RVC f oa m... : Struct ure of mesophase pit ch (a) AR mes ophase pit ch (b) A
t ypi cal petrol eu m mesophase pit ch...
: Bubbl es gr owt h under appli ed pressure... : The t er mi nol ogy used for SEM phot ographs of car bon foa m.... : The mechani s m of graphiti zati on... : Sche mati c di agra ms sho wi ng t he i nt eracti on of phonon
wit h an i mpurit y (a) Low Te mper at ure, (b) Hi gh te mper at ure..
: Effi ci ency of radi at ors... : Pi ct ure of modul ar radiat or wit h heat di ssi pati on capacit y of
33 KW...
: Gr aphit e foa m cool er... : Evapor ati ve cooli ng syste m mount ed on chi p package... : Mes ophase Pit ch... : Vari ati on of vi scosit y of AR pit ch as a functi on of t e mper at ure : Eval uati on of AR pit ches by t he pr ocessi ng wi ndo w... : SEM phot o mi cr ogr aphs of foa m pr oduced at different
te mper at ures (all sa mpl es car boni zed at 1050 oC) (a) T=300 oC, (b) T=350 oC, (c) T= 450 oC...
: SEM i mages of foa m wit h 10 % and 30 %P MMA additi ve and
t he foa m pr oduced wit hout additi ve (all sa mpl es are
gr aphiti zed at 3000 oC)...
: SEM i mages of foa m wit h 20 % and 30 %PS additi ve and t he
foa m pr oduced wit hout additi ve (all sa mpl es are graphiti zed at 3000 oC)...
: SEM phot ographs of carbon f oa m pr oduced at vari ous
pr essure experi ment s (a) P= 58 bar, (b) 68 bar, (c) 78 bar...
: Ther mal conducti vit y of foa ms accor di ng t o t he te mper at ure...
2 3 4 4 5 6 7 9 11 12 12 14 15 16 17 17 18 19 21 21 30 32 34 36 40
MES OP HAS E PI TCH DERİ VED GRAP HI TI C CARBON FOA M SUMMARY
The r ecent devel op ment of ne w t echnol ogy devi ces e mer ge t he r equire ment of t he li ght wei ght and mor e effi ci ent t her mal manage ment mat eri als. The mai n concept of t hese t her mal manage ment mat eri als appli cati ons are hi gh t her mal conduct i vit y, l ow wei ght, l ow coeffi ci ent of t her mal expansi on, hi gh specifi c strengt h and l ow cost. Conventi onal t her mal ma nage ment mat eri als have cent ered on al u mi nu m and copper due t o its hi gh t her mal conducti vit y ( 180 W/ m. K f or al u mi ni u m and 400 W/ m. K f or copper). When t he wei ght i s t aken i nt o account, t he s pecifi c t her mal conducti vit y (t her mal conducti vit y di vi ded by s pecifi c gr avit y) i s onl y 54 and 45 W/ m. K respecti vel y.
Car bon f oa m mat erials are used si nce 1960’s. Exi sti ng car bon f oa ms are us ed as t her mal i nsul at or or struct ural rei nf orce ment until 2000. The opti mizati on i n pr oducti on t echni ques of car bon f oa m r esult ed i n hi gh t her mal conduct i vit y l ow-densit y mes ophase pit ch deri ved gr aphiti c f oa m. The pit ch deri ved gr aphitic f oa m i s used i n t her mal manageme nt devi ces such as radiat or, heat exchanger.
The goal of t hi s st udy i s t o pr oduce gr aphiti c car bon f oa m. Mes ophase pit ch i s used for pr ecursor. Pit ch i s heat ed above t he soft eni ng poi nt i n aut ocl ave and t hen pr essure i s appli ed. Gr een car bon f oa m i s obtai ned. Then, it i s st abili zed and car boni zed. Foll owi ng t hese i niti al st ages, carbon f oa m i s gr aphiti zed up t o te mper at ure of 3000 oC. Gr aphiti zed car bon f oa ms ar e charact eri zed wi t h t he scanni ng el ectron mi cr oscopy, x-ray di ffract o metry, and heli um pycno metr y. Al so t her mal diffusi vit y of t he sa mpl es are measured by l aser fl ash apparat us.
The str uct ures and pr operti es of t he pr oduced gr aphiti zed car bon f oa ms are obt ai ned wi t h r espect t o t he par a met ers i nvol ved and f urt her usi ng t he charact eri zation r esults. The effect of f oa mi ng pressure, pr ocess t e mper at ure and pol y mer additives on t he struct ure are i nvesti gat ed.
As a r esult of t hese experi ment s it i s f ound t hat; t he str uct ure of t he carbon f oa ms obt ai ned at t e mper at ures hi gher t han 300 oC, sho wed det eri orati on i n t he cell, li ga ment s, and j uncti ons. Pol y met hyl met hyacr yl ate ( P MMA) and Pol yst yrene ( PS) additi ves r esult ed i n smoot her car bon f oa m t opogr aphy. Thi s struct ural change acco mpli shed wi t h l oss of f oa mi ng qualit y f or t he case of PS additi on co mpar ed t o P MMA. Hi gher operati ng pr essure ended wi t h ho mogeneous, i nt erconnect ed, bett er ali gned str uct ure. Gr aphitized f oa ms obt ai ned at 68 bar have bett er f ormat i on i n ter ms of i nt erl ayer spacing, st ack hei ght, cr yst al si ze and gr aphiti zati on degr ee t han ot her obt ai ned foa ms.
MEZOFAZ Zİ FT BAZLI GRAFİ Tİ K KARBON KÖPÜK ÖZET
Teknol oji ni n il erl e mesi yle ağırlı ğı düĢük ve yüksek veri mli ı sıl yöneti m ci hazl arı na duyul an i hti yaç art mıĢtır. Bu ı sıl yöneti m uygul a mal arı ndaki a maç yüksek ı sıl ilet kenli ğe sahi p düĢük ağırlı klı, ısıl genl eĢ me katsayısı düĢük, dayanı mı yüksek ve üreti m mali yeti düĢük t utma ktır.
Gel eneksel yönt e mde kull anıl an ı sı yöneti mi yüksek il et kenli ğe sahi p bakı r ve al ümi nyu m üzeri nde t oplan mıĢtır ( al ümi nyu mun ısıl ilet kenli ği 180 W/ m. K, bakırı n ısıl ilet kenli ği 400 W/m. K ). Ağırlı k hesaba katıl dı ğı nda spesifi k ı sıl ilet kenli ği n (ısıl il et kenli k/ yoğunl uk) çok düĢ ük ol duğu görül ür. Al ü mi nyu mun s pesifi k ı sıl ilet kenli ği 54 W/ m. K i ken bakır içi n bu değer 45 W/ m. K dır.
Kar bon köpüğü 1960’ dan beri kull anıl makt adır. Mevcut kar bon köpükleri 2000 yılı na kadar ı sı i zol asyonunda veya yapı güçlendiricisi ol arak kull anıl makt aydı. Kar bon köpüğü üreti m t ekni kl eri ni n daha uygun hal e getiril mesi yl e yüksek ı sıl ilet kenli ğe sahi p, düĢük yoğunl ukl u mezofaz zift bazlı grafiti k köpük el de edil di. Zift bazlı grafiti k köpük radyat ör, ısı değiĢtiricisi gi bi pek çok ci hazda kullanılma kt adır. Bu çalıĢ manı n a macı grafiti k kar bon köpüğü üret mektir. Mezofaz zifti n ha mmadde ol arak kull anıl dı ğı bu çalıĢ mada, zift eri me sıcaklı ğı nı n üst ünde bir sı caklı ğa ot okl avda ı sıtılır ve yüksek bası nca t abi t ut ul ur. El de edil en ha m kar bon köpüğü st abilizasyon ve kar boni zasyon aĢa mal arı ndan geçer. Daha sonra 3000 o
C de gr afiti ze edilir. Gr afiti k köpük t ara malı el ektron mi kr oskobu, x-ı Ģı nı diffrakt omet r esi ve hel yu m pi kno met resi kul lanıl arak kar akt eri ze edilmi Ģtir. Ayrı ca l azer fl aĢ ci hazı il e ısıl diffusi vitesi öl çül müĢt ür.
Kar akt erizasyon sonuçl arı na göre i se deği Ģi k proses Ģartları nda üretilen kar bon köpüğünün yapı sı belirlen miĢ ol ur. Bu çalıĢ mada deği Ģi k bası nçl ar, sı caklı kl ar ve kat kı maddel eri ni n köpük yapısı na et kisi i ncel enmi Ģtir.
Yapıl an deney sonuçl arında, 300 oC den yüksek sı calı kl arda üretilen köpükl eri nde
hücre, li ga ment, bağl antı nokt al arı açısı ndan gerile me gözl enmiĢtir. Poli metil met akril at ( P MMA) ve Poli stiren ( PS) il avel eri yl e kar bon köpüğünün yüzeyi ni n daha pür üzsüz ol duğu gözl enmiĢtir. PS kat kılı kar bon köpüğünde ( P MMA kat kılı kar bon köpüğüne oranl a) gözenek ve hücre yapı sı nı n bozul duğu gözlenmiĢtir. Art an bası nçl a ho moj en, gözenekli ve düzenli bi r yapı ya sahi p kar bon köpüğü el de edil miĢtir. 68 bar bası nçta el de edil en kar bon köpüğü, t abakal ar arası mesafe, yı ğı n yoğunl uğu ve yüksekli ği açısı ndan di ğer bası nçl arda el de edil en köpükl ere göre daha i yi sonuç ver miĢtir.
1. I NTRODUCTI ON
On t he fi el ds of devel opme nt of advanced mat erial s t echnol ogy has been dri ven by t he r equire ment f or i mpr oved mor e effi ci ent and li ght wei ght t her mal ma nage ment mat eri als.
Cont e mpor ar y t her mal ma nage ment mat eri als have cent ered on al u mi nu m and copper due t o t heir hi gh t her mal conducti vit y. Howe ver, i n advanced devi ce wei ght is si gnifi cant concer n. Ther ef ore r esearch of light er wei ght t her mal manage ment mat eri als are t he i nt erest of many researchers.
Car bon f oa ms have been st udi ed si nce 1960‟s [1]. Mes ophase pit ch- based car bon foa ms ar e pr ovi ng t o be popul ar i n t he fi el d of carbon mat eri als. Its popul arit y i s due t o pr oduci ng hi gh pore struct ure, hi gh t her mal conducti vit y and l ow densit y. Mes ophase pit ch deri ved gr aphiti c car bon f oa ms can be consi dered as an i nt erconnect ed net wor k of gr aphiti c li ga ment s. Ther ef ore gr aphiti c car bon f oa ms have hi gh t her mal conducti viti es al ong t he li game nt s of t he f oa m. Their t her mal conducti viti es are fi ve times hi gher t han copper and si x ti mes great er t han al u mi nu m. In t hi s st udy, mes ophase pit ch based gr aphiti c carbon f oa m wi ll be pr oduced. The effect of different pr ocess para met ers ( pressure, t e mper at ure and additive) on t he pr operti es of car bon f oa m wi ll be i nvesti gat ed by t he several charact eri zati on met hods.
E N E R G Y 2s 2px 2py 2pz 1s
2. GENERAL I NFORMATI ON ABOUT CARBON AND CARBON FOA M
2. 1. Carbon
Car bon i s t he most i mport ant el e ment f or all li ving or gani s m on t he Earth, because all or gani c co mpounds are co mposed fr o m carbon net wor ks. Car bon mat eri als, whi ch consi st of mai nl y car bon at o ms, have been used si nce pr ehi st ori c times i n t he for m of charcoal. [2]
Car bon has an at o mi c wei ght of 12 and it i s t he si xt h el e ment i n t he periodi c t abl e. Thr ee i sot opes are mostly kno wn: 12C, 13C, 14C. Car bon- 12 and car bon- 13 ar e st abl e isot opes. They do not spont aneousl y change t heir struct ure and di si nt egr at e.12C account s f or ar ound 99 % of t he nat urall y occurring car bon and i s used as reference defi niti on of at o mi c ma ss. 13C i s used as pr obe i n nucl ear magneti c resonance because of its magneti c mo me nt (spi n=1/ 2). 14C is radi oacti ve and has l ong half life of 5730 years and i s used ext ensi vel y i n t he dating of ar chaeol ogi cal artifact s and as a „label‟ i n t he st udy of organi c reacti on mechani sms. [2]
The confi gurati on of carbon at o m i s sho wn i n Fi gur e 2. 1. Car bon has f our el ectr ons i n its val ance s hell ( out er s hell). Si nce t he ener gy shell can hol d ei ght el ectr ons, each car bon at o m can s hare el ectrons wi t h up t o f our di fferent at o ms. It makes advant ages t o co mbi ne wit h different ki nd of at o ms as well as it self. [3]
2. 2 Bondi ng i n carbon mat eri al
Bondi ng i n car bon co mpounds i s expl ai ned by t wo pri nci pl e r egi mes as descri bed bel ow:
1. bonds: di a mond or al i phati c t ype. Thi s r esult i n chai n of car bon at oms such as pol yol efi ne‟s, or t hree di mensi onal struct ures whi ch are ri gi d and isotropi c.
2. A mi xt ure of and bonds: gr aphit e ar omati c t ype. Thi s r esults i n pr edo mi nantl y l ayered struct ures wit h hi gh degree of ani sotropy.
The maj orit y of car bonaceous mat eri als cont ai n exa mpl es of bot h bondi ng r egi mes wi t h an i mmense range of co mpl exit y [3].
2. 3 Cryst al struct ures of carbon
Car bon at o ms can have t hree different hybri d orbit al, sp3, sp2, and s p. Thi s vari et y ma kes car bon at o m t o have different f or ms t herefor e t here ar e many ki nds of car bon all otropes. Di a mond, gr aphit e and f ull erene ar e mostl y kno wn. C- C bonds usi ng s p3 and s p2 hybri d or bit al was kno wn i n t he constructi on of di a mond and gr aphit e respecti vel y. Full erene is construct ed by co mbi ni ng sp and sp2 hybri d or bital [5] Di a mond i s t he one of t he har dest mat eri al and has no col or. Its struct ure consi st s of regul ar t hree di mensi onal net wor ks of sp3 bonds wi t h l ong r ange peri odi cal repetiti on. Most di a mond cr yst al bel ongs t o cubic syst e m. Di a mond i s used as an el ectri cal i nsul at or because of its fi xed bondi ng el ectron wi t hi n t he di a mond l atti ce. It is used as i ndustri al cutting t ool s due t o its har dness ( Fi gure 2. 2) [5, 6].
Gr aphit e struct ure (see Fi gur e 2. 3) has sp2 and bonds. It represent s t he hexagonal cr yst al syst e m wi t h t he regul arit y of ABAB. There i s a bond bet ween t he l ayers and t he bonds bet ween t he st ack. The s mall a mount of mat eri al i s st acked accor di ng t o t he ABCABC whi ch i s kno wn as rho mbohedr al f or m. Thi s mat eri al account s f or l ess t han 10% of t he gr aphit e. Gr aphit e i s a good conduct or of heat and el ectri cit y. Si nce t he ener gy f or sli di ng l ayers over one anot her i s l ow, gr aphit e i s ver y soft mat eri al. Thus, it used as a l ubri cant [3].
Fi gure 2. 3 Gr aphit e struct ure [7]
Full erene (see Fi gure 2. 4) was di scovered i n 1985. It i s t he cage mol ecul e of car bon. The nat ure of bondi ng cl ose t o t he sp2 bond but it i sn‟t cl ear. The hybridi zati on i s modi fi cati on of t he s p3 hybri di zati on and sp2 hybri di zati on. It means t he si g ma () or bital don‟t di spl ay charact er, and t he pi () or bital don‟t displ ay charact er pur el y. Full erene is t he one of t he well kno wn superconducti ve mat eri al [5].
Fi gure 2. 4 Full erene Molecul e [8]
We a k Van der Waal forces bet ween layers
2. 4 Hist ori cal Overvi e w of Carbon Mat eri al s
Car bon, i n t he f or m of charcoal, i s an el e ment of pr ehi st ori c di scover y and was fa mili ar t o many anci ent ci vili zati ons. A hi st orical perspecti ve of car bon and it s all otropes are sho wn i n Fi gure 2. 5.
Pr e- 1880
La mp Bl ack ( writi ng)
Char coal s (gunpo wder, medi ci ne, deodor ant s) Nat ur al graphit e ( writi ng mat eri al)
1880- 1940
Acti vat ed car bons Car bon bl acks
Coal coki ng (coal-t ar pit ch) Del ayed coki ng
Synt heti c graphit e and di a mond 1940- 1999
Car bon fi bers ( PAN) Car bon fi bers (pit ch- based) Car bon fi bers ( micr opor ous) Car bon / resi n co mposit es Car bon / car bon co mposit es Speci alit y acti vat ed car bons Car bon as a cat al yst support Car bon wi skers / fila ment s Pr ost heti cs
Int ercal ati on co mpounds Gr aphit e / oxi de refract ori es Pyr ol yti c car bon
Glassy car bon
Mesocar bon micr obeads Dia mond fil ms
Dia mond-li ke fil ms El asti c car bon Full erenes Nanot ubes Nanor ods
Hi gh t her mal conducti vit y graphi te foa ms
2. 5 Order and Di sorder i n Carbon Mat eri al s
Car bon mat eri al can be cl assifi ed wit h r espect t o t he pr oporti on of t he or der ed and di sor dered struct ure. The pr oporti ons of t he m are r el at ed wi t h t he pr operties of t he mat eri al.
2. 5. 1 More Ordered Struct ures
Or der ed struct ure can be i magi ni ng by gr aphit e l attice. Small vol u mes exhi bit perfect gr aphit e cr yst al struct ure. As vol u me i ncreases, t he pr esence of defect s, di st orti ons and het er oat o m destroy t he r egul arit y and pr oduce di sor dered mat eri al. Layers can be sli de over anot her l ayer by littl e a mount of ener gy. Al so t wi sti ng makes t he str uct ure parall el and equi di st ant l ayers, but wit h r ando m ori ent ati on. These are call ed „t urbostratic‟ car bons. [ 3, 9] The di a monds li ke parts of t he most car bon mat eri als have s hort er range or der t han gr aphiti c r egi ons, alt hough l ar ge pr oportions of t he di sor dered part s are ali phati c or der. Def ect s and irregul ariti es can be observed i n t he l ong range or ders [3].
2. 5. 2 Less Ordered Struct ures
Car bon mat eri als can be cl assifi ed by t he i sotropy and ani sotropy. Ani sotr opi c car bons have or dered and gr aphiti c struct ure. Isotropi c car bons have r ando ml y arranged mat eri als. The or der can be i ncreased by f urt her heat treat ment. The cr yst al struct ure changi ng by heat treat ment is sho wn i n Fi gure 2. 6 [ 3].
2. 6 Carbon For ms
2. 6. 1 Graphi ti c and Non- Graphi ti c carbons
Gr aphiti c car bons are all vari eti es of mat eri al consisti ng of t he el e ment carbon i n t he all otropi c f or m of gr aphi te, irrespecti ve of t he pr esence of str uct ural defects. Nat ur al gr aphit e i s a mi neral consisti ng of car bon r egar dless of cr yst alli ne perfecti on. So me nat ural graphit e‟s show a hi gh degree of perfecti on but most are mi ned i n t he f or m of flake graphit e‟s cont ai ning ot her mi neral matt er. Synt hetic graphit e i s defi ned as a mat eri al consi sti ng mai nly of gr aphiti c car bon, which has been obt ai ned by means of a gr aphiti zati on heat treat ment of a non gr aphi tic car bon or by che mi cal vapor depositi on ( CVD) from hydr ocarbons at te mperat ures above 1800º C [ 10].
Non- graphiti c car bons are all vari eti es of subst ances consi sti ng mai nl y of t he el e ment car bon wi t h t wo- di mensi onal l ong r ange or der of t he car bon at o ms i n pl anar hexagonal net wor ks, but wit hout any measur able cr yst all ographi c order i n t he c- directi on. Many non-graphiti c car bons can be convert ed t o gr aphit e by gr aphiti zati on heat treatme nt t o above 2200 º C.
2. 6. 2 Graphi ti zabl e and non Graphi ti zabl e Carbons
Non- graphiti zabl e car bons (see Fi gure 2. 7) can not be t ransf or med i nto gr aphiti c carbon sol el y by heat t reat ment at t e mperatures of 3000º C or above under at mos pheri c or l ower pr essures. Non Gr aphiti zable car bons are pr oduced fro m wood, nut shells and non f usi ng coal s. Duri ng heat treat ment, t heir macr o mol ecul ar str uct ure does not change. Fusi on can not t ake pl ace onl y s mall mol ecul es l eave fr o m t he struct ures, and at t he sa me ti me mor e cr oss li nki ng struct ure occurs [ 10].
Gr aphiti zabl e car bons can pass fl ui d st age duri ng t he heat treat ment. Mol ecul es can gr ow and l ar ge ar o mati c mol ecul es can be f or med. Thus t hey ali gn wi t h each ot her so graphiti c struct ure can be devel oped [ 10].
Fi gure 2. 7 Sche mati c repr esent ati on of nongr aphitizabl e (l eft) and graphitizabl e
2. 7 Carbon Foa m
Car bon f oa m has been pr oduced fr o m di fferent ki nd of pr ecurs ors si nce 1960‟s such as s ynt heti c mes ophase pit ches [ 11, 12], coal t ar and petr ol eu m pit ch [ 13], coal [ 14], pol yacr yl onitril e ( PAN) [ 15], pol yuret hane [ 16], vi nyli di ne chl ori de
pol y mer [ 17], phenoli c pol y mer [ 18] and pyroli zabl e or gani c co mpound s uch as sugar or cell ul ose [ 19].
The car bon f oa m i s an alt er nati ve mat eri al t o traditi onal mat eri al i n ma ny
appli cati ons due t o its uni que pr operti es. For exa mpl e, car bon f oam i s used as i nsul at or and conduct or mat eri al accor di ng t o t he operati ng t e mper at ure. Car bon
foa m has l ow t her mal conducti vit y when it i s pr oduced bel ow t he t emper at ure of 1000 oC. I n contrast, car bon f oa m hi gh t her mal conducti vit y when it i s pr oduced
above t he t e mper at ure of 2500 oC. Car bon f oa m conduct s heat si x ti mes f ast er t han
copper and f our ti mes f ast er t han al u mi num [ 20]. The t her mal pr operti es of t he car bon f oa m co mpared t o ot her t her mal ma nage ment mat eri al are ill ustrat ed i n Tabl e2. 1.
Tabl e 2. 1 Ther mal pr operti es of pit ch deri ved carbon f oa ms co mpar ed t o ot her
t her mal manage ment mat eri als [21]
Mat eri al Specifi c gr avit y Ther mal Conducti vit y
Specifi c t her mal conducti vit y* In- pl ane Out -
of-pl ane In- pl ane
Out - of-pl ane ( W/ m. K) ( W/ m. K) ( W/ m. K) ( W/ m. K)
ARA24 deri ved foa m- D 0. 57 149 149 261 261
Conoco- deri ved foa m- D 0. 59 134 134 227 227
Typi cal 2- Dcar bon car bon 1. 88 250 20 132 10. 6
E WC- 300/ Cyanat e Est er 1. 72 109 1 63 0. 6
Copper 8. 9 400 400 45 45
Al u mi nu m 2. 77 150 150 54 54
Al u mi nu m honeyco mb 0. 19 - ~10 - 52
Al u mi nu m f oa m 0. 5 12 12 24 24
Car bon f oa m has nearl y 90 % open por e struct ure. The densit y of car bon f oa m i s rangi ng fr o m 0. 20 g/ c m3 t o 0. 6 g/ c m3. Car bon f oa m has hi gh por ous str uct ur e and unif or m pore si ze di stri buti on (average bet ween 10 and 500 micr ons) [ 13].
Car bon f oa m s ho ws hi ghl y or dered gr aphit e properti es when it i s gr aphiti zed. It exhi bits average i nt erl ayer spaci ng as l ow as t hat of perfect graphit e [ 21]. For i nst ance, car bon f oa ms pr oduced by Kl ett et al .[22] has a 0. 336 n m i nt er l ayer spaci ng and 203. 3 n m coherent l engt h ( La) and 442 n m st acki ng hei ght (Lc).
2. 7. 1 Hist ory of Carbon Foa m
Wal t er For d [ 1] first devel oped car bon f oa m i n t he l at e 1960‟s. Thi s i niti al car bon foa m was pr oduced by car boni zi ng t her mosetti ng pol y mer f oa ms t o obt ai n reti cul at ed vitreous (gl assy) car bon f oa m whi ch is shown Fi gure 2. 8.
Fi gure 2. 8 A Typi cal RVC f oa m [ 21]
Googi n et al. [ 26] pr oduced car bon f oa m by pol y meri zati on of f urf ur yl al cohol and ur et hane t o get parti all y cur ed ur et hane f oa m i n 1967. It was t he first pr ocess t o contr olli ng t he struct ure and mat eri al pr operti es of car bon f oa m.
Researches f ocused on vari et y of appli cati ons of car bon f oa m. Car bon f oa m was used as el ectrode, i nsulat or, filt er, cat al yst bed and cat al yst support. In additi on, car bon f oa m was used as t he t e mpl at e f or many of t he met al. I n 1970‟s and 1980‟s, alt er nati ve pr ecursors and pr ocessi ng conditi ons wer e expl ored f or pr oduci ng car bon foa m and modi f yi ng its properti es [16, 17, 23, 24].
In 1976, Ral ey et. al. [ 17] used vi nyli di ne chl ori de pol y mers wit h a mmoni a t o deri ve car bon f oa m. Thi s car bon f oa m had been used as a cat al yst support and filter mat eri al for gases, such as ci garette s moke and li qui ds.
In 1981, Bonzo m et. al. [ 13] devel oped car bon foa m fr o m petr ol eu m and coal t ar pit ch. The car bon foa m was used for t her mal and sound i nsul ati ng.
In 1988, Hopper [ 18] di ssol ved pul veri zed sodi um chl ori de parti cl es and phenoli c pol y meri c resi n i nt o t etrahydr of uran ( THF) as a precursor of car bon foa m.
In t he earl y 1990‟s mesophase pit ch deri ved car bon f oa m was di scovered [25]. Thi s wor k was f ocused on t he devel opi ng a hi ghl y struct ural li ght wei ght mat eri al whi ch exhi bits ver y hi gh specific t her mal conducti vit y.
In 1997, Kl ett, J. [ 22, 27-29] at t he Oak Ri dge Nat i onal Labor at or y ( ORNL) r eport ed t he first graphiti c f oa ms wi t h bul k t her mal conducti viti es up t o 180 W/ m. K. They used napht hal ene deri ved mes ophase pit ch as a pr ecursor. Due t o hi gh bul k t her mal conducti vit y, t he gr aphiti c car bon f oa m i s a pot enti al mat eri al as t her mal ma nage ment mat eri als.
2. 7. 2 Preparati on and Characteri sti c of Mesophase Pitch Deri ved Graphi ti zed Carbon Foa m
2. 7. 2. 1 Mes ophase Pitch
The mes ophase pheno menon i s di scovered i n 1965 by Br ooks and Tayl or [ 30]. Mes ophase means s phere and mosai c subst ances t hat f or m bef ore t he s ol i difi cati on. Whe n hydr ocar bon i s heat ed under i nert at mosphere, it condenses t o l ar ge pl anar mol ecul es. As t he mol ecul es gr ow, t hey nucl eate and gr ow a li qui d cr yst al phase, call ed t he mes ophase. The li qui d cr yst al phase consi st s of st acki ng pl anar mol ecul es t hat are foot pri nt of t he graphiti c pell ets [30].
The mes ophase pit ch based car bon f oa m was produced at t he Wri ght Patterson Ai r Force Base Mat erials Lab i n 1990‟s [ 31]. It is a synt hetic napht hal ene derived pit ch, whi ch is 100 % ani sotropic mesophase.
Mes ophase pit ch i s derived fr o m vari ous pr ecursors such as petr ol eu m and coal t ar and ot her synt heti c pr ecursors. Mes ophase pit ches deri ved fr o m s ynt heti c precursors, such as Mit subi shi AR pit ch whi ch i s pr epared by t he cat al yti c pol y merizati on of
napht hal ene usi ng HF- BF3 cat al yst [ 32, 33], have mor e ho mogeneous compositi ons
co mpar ed t o mes ophase pit ch deri ved fr o m petr oleu m and coal t ar pit ch [ 34]. Fi gur e 2. 9 sche mati call y sho ws t he pr esent ati on of t he str uct ures of Mit subi shi AR and t ypi cal petrol eu m mes ophase pit ches [21].
Synt heti c Mes ophase pi tch i s pr eferabl e pr ecursor f or hi gh t her mal conducti vit y car bon f oa m mat eri als. Whe n a s ynt heti c mes ophase pit ch i s used, t he do mai ns ar e stret ched al ong t he cell wall s of t he f oa m str uctur e and t hereby pr oduce a hi ghl y ali gned graphiti c struct ure parall el t o t he cell walls [11]
(a) (b)
Fi gure 2. 9 Struct ure of mes ophase pit ch (a) AR mes ophase pit ch (b) A t ypi cal
petrol eu m mesophase [21]
2. 7. 2. 2 Foa mi ng Mechani s m
Whi l e mes ophase pit ch i s heat ed, mesophase mol ecul es begi n t o enl ar ge and weaker
hydr ogen bonds decompose. Vol atil es coal esce i nt o l ar ger bubbl es whil e t he pr eferred ori ent ati on occurs i n t he pit ch. Mesophase mol ecul es begi n t o ali gn on t he surr oundi ng of t he bubbl es. Hi gh mol ecul e wei ght co mpounds i mpede
t he fl ow of bubbl es t o t he surface. At t he same ti me, t here i s additional stress on t he pit ch due t o t he appl yi ng pr essure. Bubbles enl ar ge i n t he depr essuri zati on
st ep (see Fi gure 2. 10). Aft er t he pr essure r el ease, t hey r apt ure (li ke boili ng wat er) and i nt erconnect ed pores occur
Fi gure 2. 10 Bubbl es gr owt h under appli ed pressure [35]
The r el at ed t er mi nol ogy is i ndi cat ed i n t he Fi gure 2. 11 f or a bett er underst andi ng of t he eval uati on of f oa m. As it can be seen fr om t he fi gure, cell has a s pheri cal geo met r y wi t h open por es. The nu mber of t he pores generall y vari es bet ween one and four. These por es are i nterconnect ed t o ot her cells. I nt erl ayer bet ween t he cells i s defi ned as li ga ment s. Thi s i s t he part wher e r od-li ke mes ophase mol ecul es ali gned and l ayered whi ch causes gr aphiti c struct ure at t he f urt her st ep of heat t reat ment . Juncti ons ar e l ocat ed bet ween t hree or mor e cel ls. Non uni f or m el ongat ed r egi on i nsi de t he cell are defi ned as cracks. The cracks affect t he strengt h of t he mat eri al.
Fi gure 2. 11 The t er mi nol ogy used for SEM phot ogr aphs of car bon foa m
Li ga ment s Juncti on Open Por e Cell Wall Cr ack Al i gn ment of li ga ment s
2. 7. 2. 3 St abili zati on of Car bon Foa m
St abili zati on i s descri bed as pr o moti on of cross li nki ng bet ween mol ecul es and t he r e moval of s maller mol ecul es by deco mpositi on. St abili zati on i ncludes bot h oxygen di ffusi on and oxi dati ve r eacti ons. Oxygen di ffuses t o r eacti on sit es and pr oduct gases must diffuse out [36, 37].
Ther mo gr avi met ri c anal yses of t he st abili zati on pr ocess i ndi cat e t hat co mpeti ng che mi cal pr ocesses i nvol ve bot h wei ght gai n and wei ght l oss. We i ght gai n pr edo mi nat es at l ower oxi dati on t e mper at ures and s hort er peri ods of ti me. Ho wever,
wei ght l oss pr edo mi nates at hi gher oxi dati on t e mper at ures and l onger peri ods of ti me. The wei ght gai n peri od i s acco mpani ed by t he pr oducti on of H2, H2O,
wher eas t he wei ght l oss peri od i s acco mpani ed by t he pr oducti on of CH4, CO
and CO2 [38, 39].
Rel ati ve r at es of oxygen di ff usi on and oxi dati on r eacti ons depend on r eacti vit y of pr ecursor, t e mper at ure, sa mpl e t hi ckness and r eacti vit y of oxi dant s. St abili zati on reacti ons begi n wit h t he ali phati c si de chai ns. Then, ar o mati zati on and cross li nki ng are f oll owed. St abili zati on i nfl uences t he perfor mance of r esulti ng pr oduct. Insuffi ci ent st abili zati on all ows def or mati on aft er car boni zati on. Excessi ve st abili zati on causes deco mpositi on of i ntroduced oxygen gr oup and mor e def ect s [40- 42].
2. 7. 2. 4 Carboni zati on of Carbon Foa m
Car boni zati on i s a pyr ol ysis pr ocess t o i ncrease cont ent of t he el e ment car bon at i nert at mos phere. The pr ecursor i s heat ed sl owl y i n an i nert envir on ment. The or gani c mat eri al i s deco mposed i nt o a car bon r esi due. Vol atil e co mpounds diffuse out struct ure. Wei ght l oss occurs duri ng t he car boni zati on st age [ 43, 44]. Sever al reacti ons s uch as dehydrogenati on, condensati on and i so meri zati on t ake place at t he sa me ti me i n t he car boni zati on pr ocess. Lo wer heati ng r at es and l onger soaki ng ti mes reduce car boni zati on yi eld duri ng car boni zati on [43- 45].
2. 7. 2. 5 Graphi ti zati on of Carbon Foa m
Gr aphiti zati on i s t he transf or mati on of non gr aphiti c car bon i nt o gr aphiti c car bon by means of heat treatme nt at t e mper at ures bet ween 1700 oC and 3000 oC [ 46].
spaci ng decreases fr o m 3. 44 Ao ( a mor phous car bon i nt erl ayer spacing) t o t he 3. 35 Ao ( graphiti c car bon i nt erl ayer spaci ng). Gr aphiti zati on i nvol ves di spl ace ment
and r earrange ment of pl anes and s mall gr oups of pl anes t o achi eve t hree di me nsi onal or deri ng [47].
The mechani s m i s schemati call y expl ai ned i n Fi gure 2. 12. Basi c struct ural unit ( BSU), i s a parall el st ack of t wo or f our l ayer pl anes each cont ai ni ng l ess t han 10- 20
ar o mati c ri ngs. At st age 1, up t o 1000 oC heat treat ment t e mper at ure ( HTT), t he car bon cont ai ns fl at BSU wi t h a hi gh degree of di sori ent ati on. Bet ween 1000 and 1500 oC (stage 2), t he BSU gr ow t hi cker and col u mnar arrays l ook li ke st uck of coi ns. I n st age 3, ar ound HTT= 1500- 2000 oC, t he di sori ent ati on bet ween t he col u mns of BS U decreases, car bon l ayers pl anes can f or m by coal escence of adj acent BSU. At t he fi nal st age, above 2000 oC, perfect car bon l ayer pl anes are pr oduced. For mati on and gr owt h of gr aphit e
cr yst allites occur [5].
Fi gure 2. 12 The mechanis m of graphiti zati on [ 5]
2. 7. 2. 6 The Rol e of St ruct ure on t he Ther mal Conducti vit y of Graphi te Foa m
Heat i s transferred i n t he soli d by t wo mechani s ms. These ar e el ectrons and phonons. The met hod f or assessing t he mechani s m of t her mal conducti vit y depends on cal cul ati on of t he Wi ed mann- Fr anz rati o as function of t e mper at ure:
4 3 2 1 1000 1500 2000 2500 3000 oC St ages (1) fl at BSU
(2) dist ort ed col u mns
(3) dist ort ed l ayers
e Temperatur y Resistivit Elecrical ty Conductivi Thermal Ratio Franz -Wiedman (2. 1)
Ther mal conducti vit y i s contr oll ed by t he el ectrons near t he Wied mann- Franz r ati o. Car bon based mat eri als have ver y di sti ncti ve r ati os fr o m Wied mann- Fr anz r ati os; t heref ore l atti ce vi brations ( phonons) do mi nat e t her mal conducti vit y [ 48, 49]. Phonons are t he t her mal waves t hat transfer heat ener gy f or m at t he one end of t he latti ce t o ot her end li ke Me xi can wave [ 50].
Heat transfer i s f ast t hr ough t he gr aphit e l atti ce due t o ver y stiff nat ure of coval ent bonds. When phonons r each def ect s and cur vat ures i n t he str uct ure, t he vi br ati on of at o ms i s i nt err upt ed and t he phonon i s scatt ered. Al so heat transfer i s affect ed by t he phonon - phonon i nt eractions and i mpuriti es [51]
At l ow t e mper at ures, t here are f e w phonons so t hat phonon i nt eracti ng i s l o w. The ener gy of phonon i s s mall and t heref ore t he wave l engt h i s l ar ge. As a consequence,
t he phonons ar e not easil y scatt ered by i mpuriti es and i mperfecti ons (see Fi gur e 2. 13). I n contrast, t he pr obabilit y of t he i nt eracti on bet ween phonons
i ncreases at hi gh t e mperat ures. Al so wave l engt h i s short whi ch means t hat t he phonons i nt eract strongl y wi t h i mpuriti es ( see Fi gure 2. 12). Mean free pat h of t he phonons i ncreases as t he t e mper at ure decreases. Consequentl y, t her mal conducti vit y reduced t o zer o as t he t emper at ure i ncreases. [50]
(a) (b)
Fi gure 2. 13 Sche mati c diagra ms sho wi ng t he i nt eracti on of phonon wit h an
i mpurit y (a) Lo w Te mper at ure, (b) High t e mper at ure [50]
2. 8 Appli cati on Areas of Carbon Foa m
Heat exchangers and heat si nks are used f or t hermal manage ment. Heat exchangers transfer heat ener gy fr om one ar ea t o anot her, and heat si nks di ssi pat e heat i nt o t he air. Conventi onall y, al umi nu m and copper heat exchangers and heat si nks ar e
superi or f or most of t he areas. Car bon f oa m ar e t he alt er nati ve of t he m by t he havi ng li ght wei ght and hi gh t her mal conducti vit y [ 52, 53, 54].
2. 8. 1 Radi at ors
Car bon f oa m cor e r adi ator r e moves mor e heat al u mi nu m cor e r adi at or at t he sa me si ze (see Fi gur e 2. 14). Car bon f oa m can be used for t aki ng a way heat fr om t he f uel cell. Theref ore, car has l i ght wei ght and s mall size. If t he si ze of t he car is reduced, t he car will not have t o push as much air i n front of t he moti on. It hel ps mor e effi ci ent f uel usage and ma kes car li ght er. Car will go f ast er and poll ut e at mos pher e less due t o aer odyna mi c desi gn. For exa mpl e, a r adi at or (see Fi gur e 2. 15) was desi gned f or a r aci ng car t hat can di ssi pat e up t o 33 k W i n a vol u me 30 % l ess t han a t ypi cal r adi at or. Thi s r educed si ze wi ll all ow f or bett er aer odyna mi cs, resulti ng i n mor e t han a 4 mph gai n on t he s uper speed ways. In heavy vehi cl es, t he effect of t he radi at or i n t he fr ont creates si gnifi cant dr ag, accounti ng f or up t o 12 % of t he f uel use at hi gh way s peeds. Cl earl y, a s mall er r adi at or will all ow t he r edesi gn of t he fr ont cab and dra mati call y i mpr ove effi ci ency [52].
Air Flow of 12 m/s 0 0,05 0,1 0,15 0,2 0,25 0,00 0,20 0,40 0,60 0,80 Water Flow ( kg/s ) T he rm al C oe ff ie nt ( k W / o C ) AluminiumCore CarbonCore
Fi gure 2. 15 Pi ct ure of modul ar radi at or wit h heat di ssi pati on capacit y of 33 k W [ 52]
2. 8. 2 Personal Cooli ng De vi ces
A personal cooli ng s yste m bei ng devel oped by r esearchers at t he Depart ment of Ener gy‟s Oak Ri dge Nati onal Laborat ory (ORNL) The syst e m i s devel oped for fi ght er pil ot s, raci ng car dri vers and fire fi ght ers. These devi ces r emove heat for m t he body and hel p to t ake cool ed air t o breathe [ 53].
Fi gure 2. 16 Gr aphit e foam cool er [52] 2. 8. 3 Co mput er Chi p Cooli ng
Current co mput er chi ps are pl aced pri nt ed si de up i n a cera mi c package wi t h “over-t he-t op” wire bonds. Unf ort unatel y, t hi s desi gn i s not s uit ed for si gnifi cant heat di ssipati on t hr ough t he t op of t he package t o a st andar d heat si nk as t here i s an i nsul ati ng air gap bet ween t he sili con chi p and t he cer a mi c
package [54]
In cooperati on wi t h t he Nati onal Securit y Agency ( NSA) r esearchers at ORNL have st udi ed on ne w desi gn f or cooli ng el ectroni c co mput er chi ps whi ch is call ed “fli p-chi p”. I n t hi s desi gn, t he sili con chi p i s i nvert ed wit h t he s moot h
back of t he pri nt ed chi p ori ent ed t owar ds t he t op of t he package. A heat spr eader is att ached di rectl y t o t he sili con chi p and i mmer sed i n an evapor ative cooli ng fl ui d (see Fi gur e 2. 16). The li mit ati ons of t hi s desi gn ar e t he s urface ar ea and t her mal conducti vit y of t he spreader mount ed t o t he back of t he chi p. [54]
The s preaders utili zed by t he NS A ar e pol ycr ystalli ne di a mond waf ers wit h t her mal
conducti viti es up t o 1600 W/ m. K ( mor e t han 4 ti mes t hat of copper). Ho wever, as a r esult of t he li mit ed s urface ar ea of t he di a mond s pr eader, t he maxi mu m po wer densit y achi eved wi t hout over heati ng t he s yst e m was 28 W/ c m2 [54].
When t he di a mond spreader was repl aced wit h ORNL‟ s graphit e f oa m, a po wer densit y of 100 W/c m2 was att ai ned wit hout overheati ng t he s yst e m. ORNL‟ s f oa m has hi gher power densit y mor e t han %350 of t he current desi gn. [54]
Fi gure 2. 17 Evapor ati ve cooli ng syst e m mount ed on chi p package [52]
Li d Evapor ati ve Cha mber Heat Si nk Gr aphit e Foa m bonded t o chi p Sili con chi p
Act ual Foa m bonded t o chi p
3. EXPERI MENTAL
In t hi s r esearch, 100 % Mit subi shi AR napht hal ene based s ynt heti c mes ophase pit ch was used t o pr oduce gr aphiti c f oa m. Al l f oa m sa mpl es were st abili zed
at 310 oC, and car boni zed at 1050 oC and t hen gr aphiti zed at 3000 oC. In or der t o underst and t he f unda ment al charact eristi c of t he f oa m st r uct ure and gr aphiti c mor phol ogy, sa mpl es wer e exa mi ned by usi ng a scanni ng el ectr on
mi cr oscopy ( SE M), x-ray diffract o met r y, and heli u m pycno met r y. In additi on, t her mal diffusi vit y is measured by l aser fl ash diffusi metr y.
3. 1 Precursor for Carbon Foa m Producti on
Mes ophase pit ch ( Fi gure 3. 1) was pr ovi ded by Mit subi shi Gas Che mi cal Cor p., Tokyo, Japan. The pit ch; na mel y Mit subi shi AR mes ophase pit ch was pr oduced by cat al yti c pol y meri zati on of napht hal ene wi t h t he ai d of HF- BF3 cat al yst. Napht hal ene
is ar o mati c hydr ocar bon, s o it i s call ed as a synt heti c mes ophase pit ch. Properti es of AR mes ophase pit ch are gi ven i n Tabl e3. 1
Tabl e 3. 1 Typi cal pr operties of AR pit ch [39]
Physi cal Pr operti es
Appear ance Bl ack pellets (25º C)
Bul k Densit y (g/ c m³) 0. 70 ± 0. 3 ; pass: 0. 69 Specific Gravit y (25º C) 1. 23
Specific Heat (cal/ g ·º C) 0. 65
Soft eni ng Poi nt (º C) 285 ± 5 ; pass: 282. 9 Mes ophase Cont ent ( %) 100
Hydr ogen/ Car bon (at o m/ at o m) 0. 58- 0. 64 Fl ash Poi nt (º C) > 300 As h ( pp m) < 20 Sol ubilit y ( %)
Wat er Sol ubl e 0
Benzene Sol ubl e 35- 44
Pyri di ne Insol ubl e 40- 50
Coki ng Val ue ( %) at 1 hr, 600º C 1 at m 80- 85 30 at m 90- 95
Toxi col ogi cal Inf or mati on
Ac ut e Or al LD60 (rat) > 5000 mg/ kg Ski n Irrit ati on sli ghtl y irrit ati ng Mut ageni cit y
( Sal monell a)
negati ve Mut ageni cit y ( E. coli) negati ve
Whe n usi ng mes ophase pit ch as a pr ecursor f or ma ki ng advanced car bon mat eri al s and car bon/ car bon co mposit es, t he vi scosit y of t he mes ophase i s a ver y i mport ant fact or. If t he vi scosit y i s t oo hi gh, t he mesophase pit ch does not i mpr egnat e well i nt o car bon pr ef or m. The mes ophase pit ch fl ows i n a li qui d st at e above it s s oft eni ng poi nt, mai nt ai ni ng t he mol ecul ar or deri ng i n a certai n t e mper at ure r ange. Viscosit y of t he mes ophase pit ch i s dependent on t e mper at ure and s hear [ 32]. The vi scosit y cur ve of AR pit ch i s sho wn i n Fi gur e 3. 2 and t he processi ng wi ndo w i s present ed i n Fi gure 3. 3
Fi gure 3. 2. Vari ati on of vi scosit y of AR pit ch as a functi on of t e mper at ure [32]
Fi gure 3. 3 Eval uati on of AR pit ches by t he pr ocessi ng wi ndo w [ 32]
4 3 2 1 0 -1 380 360 340 320 300 280 260 Te mper at ure (oC) Log ( Viscosit y) ( Pascal-seconds) 400 350 300 250 Log ( Viscosit y) ( Pascal-seconds) 5 4 3 2 1 0 -1 -2 1. 4 1. 5 1. 6 1. 7 1. 8 1. 9 2. 0 2. 1 Inverse Te mper at ure (1/Tx 103 ( K- 1))
3. 2 Experi me nt al Procedure
Pit ch pell ets are pl aced i n al u mi nu m mol d. The mol d i s i ntroduced i n an aut ocl ave.
Aut ocl ave i s st ai nl ess steel hi gh t e mper at ure- hi gh pr essure verti cal vessel i n f or m of cyli nder. Maxi mu m a ll owabl e wor ki ng t e mperat ure and pr essure ar e 300° C and
200 bars, respecti vel y.
Bef ore experi ment st arted, air i n t he s yst e m was pur ged out wit h nitr ogen at t he pr essure of 5- 10 bars. Thi s s weepi ng pr ocess was r epeat ed at l east t hr ee ti mes
t o ensure t he r e moval of air fr o m t he aut ocl ave. Aft er s weepi ng was co mpl et ed,
Ni tr ogen was vacuu me d fr o m t he s yst e m. Then, aut ocl ave was heat ed above t he soft eni ng poi nt of t he mes ophase pit ch. Aft er achi evi ng r equest ed t emper at ure, 15 mi nut es of soak time was appli ed f or homogeneous t her mal st abilit y. Lat er
nitrogen was appli ed t o gr aduall y buil d up t o pr essure t o t he desired l evel i n a 20 mi nut es peri od and t hen t he pr essure was r el eased t o t he at mos pheric pr essur e
rapi dl y, t hereby pr ovi ding means f or vol atil es re moval whi ch makes t he por ous foa m. Furt her mor e, t he aut ocl ave was cool ed t o ambi ent t e mper at ure where t he f oa m sa mpl e is re moved.
The s ubsequent st ep was st abili zati on of por ous f oa m. The st abili zati on st ep was
reali zed i n a hori zont al t ube f ur nace. The por ous f oa ms wer e i nst all ed at t he cent er of t he f ur nace and st abilized i n t he dr y air at mosphere wit h a fl ow r at e 0. 5 L/ mi n.
For st abili zati on t he sa mpl es wer e heat ed t o 185° C fr o m a mbi ent t e mperat ur e wi t h a heati ng rat e of 1° C / mi n. Aft er a soak ti me of 5 hours at 185° C, t he sa mpl es wer e heat ed t o 275° C wi t h a heati ng r at e of 0. 5° C/ mi n and 5 hour soak ti me was appli ed at t hi s t e mperat ure. Fi nall y, t he sa mpl es wer e heat ed up t o 310° C wi t h a heati ng r at e of 0. 12° C/ mi n t o t er mi nat e t he heati ng r egi me and 2. 5 hour of s oak ti me was appli ed at t hi s t e mper at ure. The syst em was cool ed t o r oo m t e mper at ure
wi t h a cooli ng r at e of 0. 8° C/ mi n. The heati ng regi me of t he st abili zation pr ocess is gi ven i n Tabl e 3. 2.
Tabl e 3. 2 Heati ng regi me of st abili zati on pr ocess
Te mper at ure (° C)
Heati ng or cooli ng rat e (° C / minut es)
Hol di ng Ti me ( Hour) 25 - - 185 1 - 185 - 5 275 0. 5 - 275 - 5 310 0. 12 - 310 - 2. 5 25 0. 8 -
The st abili zed f oa ms wer e car boni zed under nitrogen at mos phere. Duri ng car boni zati on i n t he first st ep t he st abili zed f oa ms wer e heat ed t o 310° C f r o m r oo m te mperat ure wit h a heating rat e of 1° C / mi nut e. A s oak ti me of one hour appli ed at t his t e mper at ure. At t he second st ep heati ng t he sa mpl es wer e heat ed up t o 750° C wi t h a heati ng rat e of 0. 5° C/ mi nut es and one hour soak ti me was i s applied at t hi s te mper at ure. Fi nall y t he sa mpl es wer e heat ed t o 1050° C wi t h a heating r at e of 0. 5° C/ mi nut es and 2. 5 hour soak ti me was applied at t he fi nal t e mperat ure. The car boni zed f oa m sa mpl es wer e cool ed t o r oo m t e mper at ure wit h a cooling r at e of 0. 8° C/ minut es. The carboni zati on procedure are shown i n Tabl e 3. 3
Tabl e 3. 3 Heati ng regi me of car boni zati on pr ocess
Te mper at ure (° C)
Heati ng or cooli ng rat e (° C / minut es)
Hol di ng Ti me ( Hour) 25 - - 310 1 - 310 - 1 750 0, 5 - 750 - 1 1050 0, 5 - 1050 - 2. 5 25 0, 8 -
The car boni zed f oa m sampl es wer e gr aphiti zed under i nert at mos phere. Car boni zed foa ms wer e heat ed i mme di at el y t o 1300 oC. Heating r at e of 0. 3 oC/ mi n wa s appli ed bet ween 1300 oC and 1310 oC. A s oak ti me of 39 mi nut es was applied at t hi s
te mper at ure. Next, t he sa mpl es wer e heat ed t o 2000 oC wi t h heating r at e of 11. 5 oC/ mi n. Foll owi ng t hi s st ep, t he sa mpl es wer e heat ed t o 2800 oC wi t h t he
heati ng r at e of 13. 3 oC/ mi n. At t he l ast st ep t he sampl e ar e heat ed t o 3000 oC wi t h t he heati ng r at e of 3. 3 oC/ mi n and one hour soak ti me i s appli ed at t hi s t e mperat ur e. The sa mpl es wer e cool ed t o 1300 oC wi t h a cooli ng rat e of 15 oC/ mi n. Lat er, sa mpl es wer e cool ed do wn t o r oo m t e mper at ure wit h cooli ng r at e of 5. 3 oC/ mi n. Gr aphiti c car bon f oa ms obt ai ned wer e charact eri zed by several met hods. Tabl e 3. 4 s ho ws gr aphiti zati on of t he pr ocedur e used i n t hi s st udy.
Tabl e 3. 4 Heati ng regi me of graphiti zati on pr ocess
Te mper at ure
(° C) Heati ng or cooli ng rat e (° C / minut es) Hol di ng Ti me ( mi n)
1300 - - 1310 0. 33 - 1310 - 39 2000 11. 50 - 2800 13. 33 - 3000 3. 33 - 3000 - 60 1300 15. 04 - 25 5. 31 -
3. 3 Bl endi ng Pol y mer
Pol y mer bl end t echni que has been wi del y used i n lit erat ure f or i mpr ovi ng s o me pr operti es of car bon fi ber mat eri als [ 55, 56]. I n or der t o i nvesti gat e t he effect of Pol y met hyl met hacr yl at e ( P MMA) and Pol yst yrene ( PS) additi on on t he i nt erconnect ed por e struct ure and cell f or mati on of car bon f oa ms, t hese additi ves ar e bl ended wit h vari ous pr oporti ons t o mesophase pitch.
PS and P MMA wer e purchased fr o m Wa ko Pur e Che mi cal s Co. Mes ophase pit ch and pol y mer wer e bl ended i n t he several wei ght r ati os and di ssolved i n t he tetrahydr of uran ( THF). Excess THF was r e moved fr o m t he mi xt ure by r ot ar y evapor at or.
Aft er re moval of t he s ol vent fr o m t he mi xt ure; sampl e was dri ed i n vacuum oven f or a ni ght. As a r esult pit ch-pol y mer bl end i s obt ai ned. The bl end i s i nst all ed i n a mol d and sa me pr ocedur e (whi ch i s expl ai ned i n secti on 3. 2) was f oll owe d f or t he pr oducti on of car bon foam i s foll owed as expl ai n i n t he secti on of 3. 2.
PS was bl ended wit h pit ch at t he mi xi ng r ati o of 10 %, 20 %, 30 % and PMMA was bl ended wi t h pit ch at t he mi xi ng r ati o of 10 %, 20 %, 30 %. Car bon f oam obt ai ned from 10 % PS r ati os and 20 % P MMA r ati os had l ar ge hol es and i rregul ar struct ure. Ther ef ore t hese sa mpl es wer e not eval uat ed. Di scussi ons and pr esent ations i n t hi s secti on ar e t heref ore on t he f oa ms bl ended wi t h 30 %, 20 % P MMA a nd 10 %, 30 % PS. The struct ure of THF, PS and P MMA mol ecul es are sho wn i n Tabl e 3. 4.
Tabl e 3. 5 Struct ure of THF, P MMA and PS
Mol ecul e THF ( Tetra hydr o Fur an) C4H8O P MMA ( Pol y met hyl
met hacr yl at e)
C5H8O2
PS ( Pol yst yrene)
3. 4 Characteri zati on Techni ques for Carbon Mat eri als
Char act eri zati on of t he struct ure and t ext ure of car bon mat eri als are essenti al and i mport ant f or assi gni ng pr oper utili zati on and underst andi ng t heir struct ures. Bef or e di scussi ng t he gr aphi tic f oa m mat eri al, funda ment al t echni ques f or t he charact eri zati on of t he struct ure and t ext ure of carbon f oa m are expl ai ned bel ow.
3. 4. 1 Ther mal Diff usi vity Meas ure me nt
The t her mal diffusi vit y val ues can be convert ed t o t her mal conducti vit y usi ng t he specifi c heat ( Cp) and densit y (). The cal cul ation of t he t her mal conducti vit y i s
reali zed by usi ng t he follo wi ng equati on:
Cp(3. 1)
The s pecifi c heat, Cp, i s cal cul at ed fr o m t he f oll owi ng equati on t aken fr o m AST M C781 [ 57] whi ch is vali d fro m 300 t o 3000 K.
Cp= 8. 426x10- 18T6- 4, 300x10- 14T5- 1. 42510- 10T4+1. 35310- 06T3- 3. 76510- 03T2
+4. 796T- 428. 1 (3. 2)
In t hi s met hod, a s hort pul se (l ess t han 1 millisecond) of heat i s appli ed t o t he fr ont face of a s peci men usi ng a l aser fl ash, and t he t emper at ure change of t he rear f ace i s meas ured wi t h an i nfrared (I R) det ect or [ 58]. The t her mal conducti vities of t he car bon foa m sa mpl es were measured by The LFA 427 apparat us.
3. 4. 2 X- Ray Diffract o metry
X- r ay diffracti on i s one of t he f unda ment al t echni ques t o charact erize cr yst al struct ure. The t echni que pr ovi des a measure of t he a mount of or dered mat eri al s and gi ve an i ndi cati on of t he si ze of cr yst allites whi ch ma ke up t he or dered struct ure. Whe n a bea m of chr o mat ic x-radi ati on i s direct ed at a cr yst alli ne mat eri al, di ffracti on
of t he x-rays i s observed at vari ous angl es wi t h r espect t o pri mar y bea m. The r el ati onshi p bet ween t he wave l engt h of t he X-r ay bea m (, t he angl e of di ffracti on ( 2, and t he di st ance bet ween each set of at o mi c pl anes of t he cr yst al
latti ce (d), is gi ven by t he Bragg equati on [59, 60]
In equati on 3. 3, n denot es t he or der of diffracti on. Usi ng t hi s equati on, t he i nt er pl anar di st ances of cr yst alli ne mat eri al under st udy can be cal cul at ed. The i nt er pl anar spaci ng depends on t he arrange ment of at o ms i n t he cr yst al unit cell. The i nt ensiti es of t he diffracted r ays are f uncti on of bot h t he diffracti on po wer and t he pl ace ment of t he at oms wi t hi n t he unit cell [59, 60].
The br oadeni ng of t he diffracti on peaks all ows an esti mati on of t he mean parti cl e si ze. The appr oxi mat e cr yst allite si ze (t) can be cal cul at ed fr o m t he a mount of br oadeni ng (, usi ng Scherrer equati on:
t = c./. cos (3. 4) Wher e c i s t he cell di mensi on, i s t he X-r ay wavel engt h and 2 t he scatteri ng angl e and i s t he a mount of broadeni ng due t o sa mpl e [60, 61]. The car bon f oam s a mpl es wer e charact eri zed wit h Ri gaku Multifl ex X-ray Di ffract o met er.
3. 4. 3 Scanni ng El ectron Mi croscopy
Scanni ng el ectron mi croscope ( SE M) has been co mmonl y used t o obser ve t he mor phol ogy and s urface of t he mat eri als. SE M oper at es by f ocusi ng an el ectr on bea m passi ng t hr ough an evacuat ed col u mn on t he speci men s urface using el ectr o ma gneti c l enses. The bea m i s r ast er scanned over t he s urface of t he s peci men i n synchr oni s m wi t h t he bea m of a cat hode r ay t ube ( CRT) di spl ay screen. Inel asti call y scatt ered secondar y el ectrons e mitt ed fr o m t he sa mpl e s urface ar e coll ect ed by sci ntill at or-count er and t he si gnal f or m t hi s used t o modul at e t he bri ght ness of t he i mage on t he CRT. Di fferences i n secondar y e mi ssi on r esult fr o m changes i n s urface t opography. If t he el asti call y ( backscatt ered) el ectrons are coll ect ed, an i mage can be
for med fr o m t he contrast r esulti ng fr o m co mpositi onal differences acr oss t he s urface of t he s peci men [ 61]. I n t hi s st udy, Scanni ng El ectron Micr oscope st udi es are carri ed
out on J EOL J S M- 5100
3. 3. 4 Heli um Pycno met ry
The s kel et on densit y, whi ch i s defi ned as t he str uct ural or soli d densit y, i s det er mi ned wi t h heli u m pycno met r y. I n t hi s met hod, it i s assu med t hat hel i um ent ers t he s mall est por es pr esent, wit hout bei ng adsor bed. The por osit y of t he f oam s a mpl es
are cal cul at ed by t he equati on sho wn bel ow [ 62]: d; bul k densit y, Dr; densit y meas ured usi ng heli um
P= ( 1- d/ Dr). 100 (3. 5) In t hi s st udy, Quant achr ome Ultrapycno met er 1000 is used.
4. RESULTS AND DI SCUSSI ONS
In t hi s secti on t he effect of t e mper at ure, pol y mer additi on and pr essure on pr operti es of car bon f oa m ar e di scussed. I n each case first t he experi ment al results ar e gi ven, t he results are eval uat ed and co mpari sons are made wit h t he avail abl e literat ure dat a.
4. 1 The Effect of Te mperat ure on t he Foa m Struct ure
In t hi s part, t he effect s of operati ng t e mper at ure on f oa m str uct ure are i nvesti gat ed. Thr ee oper ati ng t e mperat ure of 300 oC, 350 oC and 450 oC ar e sel ected f or t he i nvesti gati on of t he effect of t he t e mper at ure on car bon f oa m str uct ure. The choi ce of t hese t e mper at ures was based on pr evi ous st udy r eport ed by Ekşili oğl u [ 63], and goi ng beyond t he li mit of t hi s st udy. Fi gur e 4. 1 gi ves t he SE M i mages of t he car bon foa m sa mpl es obt ai ned at t hree different t e mper at ures.
The SE M phot ogr aphs of t he f oa m pr oduced at 300 oC ar e gi ven i n t he Fi gur e 4. 1a. In t he sa me fi gure; b and c ar e t he SE M phot ogr aphs of t he f oa ms produced at 350 oC and 450 oC respecti vel y.
(a)
(b)
(c)
Fi gure 4. 1 SEM phot o mi cr ographs of foa m pr oduced at different t e mper atur es
(all sa mpl es car boni zed at 1050 oC) (a) T=300 oC, (b) T=350 oC, (c) T= 450 oC
Fr o m t he SE M phot ographs, it i s cl ear t hat t he f oa m pr oduced at 300 ° C has a well for med cell struct ure wi t h evenl y di stri but ed r at her unif or m ci rcul ar por es i nt erli nki ng t he adj acent cells. Li ga ment s, j unctions and wal l s are well for med and surfaces ar e r at her unifor m. At t he t e mper at ure of 350 ° C, t he cell st r uct ure
det eri orat ed s uch t hat t he cell i s di st ort ed fr om s pheri cal geo met r y, li ga ment s, j uncti ons and wall f or mati on weakened and i nterconnecti ng por es ar e r educed i n nu mber, i ncreased i n size and def or med fr o m circul ar geo met r y. At t he hi ghest te mper at ure t he struct ural def or mati on conti nued. It i s not ed i n lit erat ure [ 49, 64- 68] t hat t he ali gn ment of li ga ment s, j uncti ons and cell walls are due t o generat ed stress by t he evol vi ng vol atiles at moder at e t e mper at ures and pr essures. This i s well ill ustrat ed wi t h t he pr ocessi ng envel ope gi ven f or t he mes ophase pit ch used i n t hi s st udy as shown i n Fi gure 3. 3.
As t e mperat ure i s i ncreased from 300 ° C t o 350 and 450 ° C, t he volatile matt er evol uti on and gas mobilit y are i ncreased. At t he sa me ti me, t he vi scosit y of t he pit ch decreased. Medi u m has fe wer pr opensiti es t o wi t hst and t o t he stress generated by t he vol atil es. I n ot her wor ds t he mes ophase pit ch beco mes t oo fl ui d t o r et ai n t he bubbl e shape i mposed by t he volatil es. As a consequence of t hese t he det eri orati ons i n cell, li ga ment, wall and i nt erconnecti ng por es geo met ry and str uct ure has t aken pl ace at t he hi gher t e mper at ures.
The st arti ng poi nt of f oa mi ng i s t he s oft eni ng te mper at ure of t he pit ch [ 63]. I n literat ure operati ng t e mperat ure of t he car bon f oam i s r eport ed t o be ar ound 10- 40 oC above t he s oft eni ng t e mperat ure [ 11, 10]. I n t hi s st udy t he best f oa m f ormat i on was achi eved at t he t e mper atur e of 300 oC. Thi s j udg ment i s based on t he i nvesti gati on of scanni ng el ectron mi croscopy phot ographs which gi ves cl ear i nf or mation on t he for mati on of cells, li ga ment s, j uncti on and general struct ure.
4. 2 The Effect of Pol y mer Addi ti ves on t he Foam St ruct ure
In t hi s part of t he experime nt s, pol y mers li ke Pol y met hyl met hacr yl at e ( PMMA) and PS Pol yst yrene ( PS) ar e used as additi ves at vari ous r ati os i n or der t o i nvesti gat e t he effect of additi ves on carbon f oa m str uct ure and pr operti es. The opti mum pr ocess conditi ons used i n t hese experi ment s are 300 oC, 68 bars and 5 seconds pr essure rel ease ti me.
The SE M phot ographs of f oa ms pr oduced wi t hout additi ve i s illustrat ed i n Fi gure 4. 2a. I n t hi s fi gure, b and c r epresent t he foa ms wi t h 10 % P MMA and 30 %
(a)
(b)
(c)
Fi gure 4. 2 SEM i mages of foa m wit h 10 % and 30 %P MMA additi ve and the f oa m
pr oduced without additi ve (all sa mpl es are graphiti zed at 3000 oC) The nu mber of t he f ormati on of cell and i nt erconnect ed por e i ncreases wi t h t he additi on of t he P MMA. As it can be seen fr o m t he SE M i mages of surface of car bon foa m pr oduced wi t hout pol y mer additi on sho ws st acked pl anes, irregul ar flakes, and fi bers. These t ext ures see m t o be modi fi ed t owar ds a co mpar ati vel y l ess r ough surface. Thi s suggest s t hat t here i s s o me degr ee of surface modi fi cati on as a r esult of
