Cong Nghe Tang Hinh

8/8/2019 Cong Nghe Tang Hinh

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ngha vt l ca hin tng tng hnh

ngha vt l ca hin tng tng hnh (hay "Ch ng nhng ng i ca sng in t")

Tc gi: Trng Vn Tn - ng trn tp ch Vietsciences thng 6/2010

Lu lu ri gp c bi vit th v c lin quan n l thuyt vt l v vt liu tng hnh c thp dng trong k thut qun s trong tng lai,bi vit c nhiu im rt mi v th v (mi tihi kh c mt cht ). Theo mnh n rt c ch cho cc bn no a thch tm ti v ham hiu

bit, xin a bi vit ln cc bn cng tham kho.

Tm tt

Tng hnh l gic m ngn i ca con ngi c th hin qua nhiu truyn thn thoi vnhng tiu thuyt vin tng mi thi i. Tng hnh cng l nim m c ca cc chin lc

gia qun s v nh nghin cu tng hnh hc.Nhu cu che "mt thn" radar ca i phng snsinh ra k thut tng hnh. K thut tng hnh xoay quanh vic ngn chn hay gim thiu nhnglung sng radar phn x t mc tiu b theo di. Mc tiu s tng hnh khi khng cn sngphn x. K thut ny bao gm vic thit k b mt sng b tn x khng quay v ngun quanst, hay cc phng thc ch to vt liu hp th radar gim thiu hoc trit tiu sng phnx. Gn y, khi nim "siu vt liu" vi hiu ng tng hnh l mt ti nghin cu ln hpdn khng t s ch v cng gy ra nhng cuc tranh ci trong cng ng nghin cu khoahc. Hn na th k qua, cc nh khoa hc b ngot, un cong nhng ng i ca sngin t (bao gm c nh sng), thm ch hp th nng lng ca n cho mc ch tng hnh.Trn vn an ninh quc gia, k thut ny l c mt quc phng nhng n li ng trn nntng ca cc quy lut vt l cng khai. Mc ch ca bi vit ny nhm gii thch nhng quy lut

.Bi vit khng lin quan n k nng "tng hnh" ca cc nh o thut.

1. Tng hnh: mt khi nim nhiu th v

"Tng hnh" l mt yu t khng th thiu trong nhng truyn thn thoi hay tiu thuyt vintng t c ch kim, t ng sang Ty. N c xem nh mt php thn thng khi dy stng tng cu c gi v a ngi c khng phn bit tr gi vo mt th gii huyn hoc,khng tng y th v. T nhng nng kiu n h ly ca "Liu trai ch d"hin v sng bnngi tnh th sinh trong m khuya ri bin i lc hng sng li chng hc tr thng nhcung si, n ci nn ca Perseus v chic nhn ca Gyge trong thn thoi Hy Lp m ai mang

vo th s c php mu "h bin", hay chic o chong "tng hnh" ca Harry Potter c lm tnhng vt liu thn b tm thy tn min cc ng th gii, tng hnh tip tc m hoc conngi qua nhiu th h. Quyn tiu thuyt khoa hc vin tng "Invisible Man"(Ngi v hnh)ca H. G. Wells xut bn vo cui th k 19 c tc gi thm vo mt cht "hng v" khoahc, k mt cu chuyn v mt nh khoa hc lm mt lot phn ng sinh ha hc bin nhngphn t ca t bo v c th ca ng ta tr nn trong sut nh khng kh.Nhng s kin trong khoa hc vin tng thng da trn nhng nguyn l khoa hc bit,nhng khi gp phi mt tnh hung m khoa hc cha c cu tr li th tc gi s th hn vo s


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tng tng ring ca mnh v ty tin nh ot hng i tng lai ca khoa hc ct sao chothch hp vi s din bin cu chuyn ca quyn tiu thuyt. Li suy din ny t nhiu c nhnggn b vi cc tri thc khoa hc v thot ra khi ci khung th thin trong cc cu chuynthn thoi.S lin h gia vin tng v khoa hc v vy trong mt chng mc nht nh l mttrao i hai chiu. Cc nh khoa hc cng rt ho hng trc cc s kin vin tng, nhng nh

lut khoa hc l vnh ai b buc t duy ca h phi i vo khun php. C nhiu pht minh canhn loi mt cch v tnh hay c hin hu trong tiu thuyt vin tng trc khi chng lhin thc. Jules Verne vit v my in fax, tu ngm, tu v tr, thm him cung trng c hnmt trm nm trc khi nhng s kin ny thc s hin hu hay c pht minh. Ci thang trileo ln tn my xanh hay vic thuc a ha cc hnh tinh l nhng cht liu vin tngnhng ang c con ngi thc hin.Tng hnh trong o thut l mt phm tr "b mt" ch c nhng ngi hnh ngh trong cuccng giao c vi nhau bo mt cc quy lut hay k xo ngh nghip. Khng t nhng othut gia lm khn gi thn phc v sng st khi h c th bin mt ri xut hin mt nikhc hay lm "tng hnh" nguyn mt toa xe la. C th h vn dng quy lut khoa hc n ginc lin quan n nh sng hay mu sc nh la con mt khn gi. "Tng hnh" trong qun s

cng l mt phm tr b mt nu khng mun ni l cc mt ca nn an ninh quc gia. Nhnghin tng tng hnh c p dng trong qun s khng vt ra ngoi cc quy lut vt l linquan n s tng tc gia nh sng hay ni rng hn sng in t vi vt cht. Nhng quy lutvt l l ti sn chung ca loi ngi khng thuc v ai, hin nhin v r rng. Tuy nhin, cngnh k xo ngh nghip ca o thut gia, trong tng hnh qun s cc "k xo" lm nn vt liutng hnh v hiu nng ca chng l nhng c mt quc gia."Tng hnh" thng c hiu l "bin mt" theo quan im thng thng. Nhng trong khoahc, khi ta "tng hnh" khng c ngha l ta "tan bin" vo mt ci m h... Ta vn lng lngng nhng ngi khng thy ta! Ngy trang bng s ha hp mu sc vi mi trng xungquanh cng c th xem l mt cch tng hnh d l th s. Th d nh vic ha trang mc o eni trong mn m, mc o trng i dt d trn tuyt hay o rn ri khi lun lch trong bi rm(Hnh 1). Thin nhin lm iu ny t ngn xa.Mt s loi ng vt, cc loi c thm ch cntrng c to ha cho kh nng bin i mu sc, hoa vn ngy trang ging vi mi trngxung quanh bo v bn thn hay phc kch con mi. Con tc k hoa c l l nh cao ca victhay i v ha hp mu sc vi cy c.


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Hnh 1:Ngi "tng hnh" bng sn ng cnh bnh xe trc (Credit: Liu Bolin).

Hin tng tng hnh hay hin hnh tht ra l kt qu ca s tng tc gia nh sng (hay sngin t ngha rng) v vt cht. Ta nhn thy c mi vt quanh ta l do s phn x ca nh

sng v khi nhng tia sng phn x p vo mt ta, th gic cho ta s cm nhn mu sc canhng g hin hu trong th gii xung quanh. Nhng khi mn m bung xung hay nh ntrong mt cn phng pht tt th mi vt "tng hnh" v khng cn s phn x ca nh sng. Khinh sng, hay ni rng hn l sng in t, tc ng ln vt cht th c ba trng hp xy ra: (1)phn x (reflection), truyn x (transmission) v hp th (absorption) (Hnh 2).


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Hnh 2: S tng tc gia sng in t (hay nh sng) vi vt cht. (1) Sng ti, (2) Sng phn

x, (3) Sng truyn x v (4) Hp th.Nh vy, nu mun mt vt tng hnh th ta phi lm sao trit tiu c s phn x ca nh sng

hay iu chnh hng phn x ca nh sng i ra xa ngi quan st. Ta c th cm nhn viciu chnh hng phn x nh sng trong cuc sng hng ngy. Khi ta ng trc din trc mttm gng phng, ta s nhn thy ta trong gng.Nhng khi ta nghing tm gng vi mt gc thch hp, ta khng cn thy ta, ta "bin mt" trong gng. Tuy nhin, khi ta c mt cigng hnh cu, d c quay gng theo hng no hay ta di chuyn bt k v tr no lc nocng thy ta hin trong gng. Nh vy, i vi mt cu v tr no nh sng cng c th phnx n ngi quan st, trong khi mt phng ch c mt gc duy nht lm nh sng phn x tr lini ngi quan st l khi nh sng ti ng vo tm gng gc 90 (khi ta ng trc dinthng gc vi tm gng) (Hnh 3). y l mt thng thc nhng li l mt phng php todng c bn cho my bay v tu chin tng hnh. T kt qu ny ta thy ngay nu mt vt thc ni kt mt cch hp l t cc mt phng th vt ny c kh nng lm phn x sng radar ira xa ngi quan st hn vt th hnh cu.


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Hnh 3: Sng ti v sng phn x ca (a) mt phng v (b) mt cu. Ch c sng ti chm vomt phng gc 90 s phn x tr li ngun pht.Radar l cng c rt hiu qu nh v, nhn dng v truy tm my bay v tu b tm xa. Sngradar dng trong qun s v dn s thng l sng vi ba cng c nhng hnh x v bn cht nhnh sng, cng l sng in trng ch khc nhau v tn s (hay bc sng). l loi sng int m ta dng trong l vi ba nu n, hm nng hay dng ni chuyn qua in thoi di ng(Hnh 4). Nhu cu lm gim thiu hay trit tiu sng phn x radar "h bin" a n victrin khai vt liu hp th radar hay cn gi l vt liu tng hnh. Cui cng, mt vt liu ang

lm chn ng cng ng nghin cu khoa hc c tn l "siu vt liu" (metamaterial) m ccnh nghin cu trong lnh vc ny tin rng ci o chong lm bng siu vt liu s cn hiunghim hn ci o chong ca Harry Potter, khin cho ngi mc tng hnh trc "mt thn"radar cng nh mt tht ca ngi trn. C tht hay khng?Chng ta hy i vo nhng phn ktip hiu r cc quy lut vt l lm nn tng trong vic to dng tng hnh, cng nh ccnguyn l ch to vt liu hp th radar v siu vt liu.


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Hnh 4: Cc loi sng in t. Wavelength: bc sng, Frequency: tn s, Radio waves: sng

radio, FM radio and TV: sng FM radio v tivi, Microwaves: vi ba, Infrared: hng ngoi,Optical IR: hng ngoi quang hc, Ultraviolet: t ngoi, Optical UV: t ngoi quang hc, X-radiation: bc x X, Gamma radiation: bc x gamma, Visible spectrum: ph nh sng thy

c, Red: , Green: xanh lc, Blue: xanh lam, Crimson: thm, Yellow: vng, Cyan: xanhcyan, violet: tm (Ngun: Google).

2. Hnh dng vt th v tit din radar

Thit b radar pht ra sng in t cc tn s khc nhau, t megahertz (MHz) n gigahertz(GHz), ty theo nhu cu nh gim st (surveillance), theo di (tracking), nh v hay truy lngmc tiu. Khi sng c pht ra t ngun pht chm vo mt vt th sng bt li do s phn x.Ln sng phn x c ghi nhn bi mt i thu sng v t ngi ta c th nh v v nhndng mc tiu. Trn mn hnh radar trm thu sng, sng phn x cho bit ln ca mc tiu.Thut ng chuyn mn ca ln ny l "tit din radar" (radar cross section).Nu mc tiu l

mt vt th lm t vt liu ging nhau th ng nhin ln ca vt cng to th tit din radarcng to.S ra i ca radar trong Th Chin th 2 mang n nhiu ng dng trong dn s ln qucphng. Trong cc ng dng dn s tit din radar to l iu kin cn thit theo di v gim stmy bay hnh khch hay thng thuyn trn bin khi. Ngc li, trong ng dng qun s trnh sng radar truy lng ca i phng, my bay v cc chin hm phi c tit din radarcng nh cng thun li. T nhu cu ln trnh con mt thn ca phe ch, ngy khai sinh caradar hn 60 nm trc cng l ngy khai sinh ca k thut tng hnh nh mt chiu thc ha


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gii radar. K thut ny bao gm nhiu nghin cu l thuyt v tc ng ca sng in t lntrn b mt vt cht nhm ti u ho cc vt liu hp th radar (radar absorbing materials, RAM)cng nh vic thit k b mt vt th c mt tit din radar cc nh. Khi tit din tin nzero, th ta thc s "h bin" trn mn hnh radar ca i phng!Cc nh tng hnh hc cn c mt tham vng cao hn l to ra mt vt th c tit din radar ln

hoc nh ty lc theo mun ca ngi iu khin. C ngha l mt chic u c hay chin hms c kh nng "tng hnh" hay "hin hnh" trc radar ca i phng. Nghe nh ngi hngtrong truyn c tch c bao php thn thng, nhng y l mc tiu ca nhng nh tng hnh hcnhm n. Nh chin lc gia Tn T ni hn hai ngn nm trc "Vic binh l vic gi di",tht l mt t ph tuyt vi nu cc nh khoa hc qun s c th lm chin u c hay chinhm bit "gi di", hin hnh trc con mt radar ca i phng trong thi bnh v khi lm trntn cng trong thi chin n s i sang t th tng hnh. Vic ny c thc s kh thi khng?Chng ta hy i theo trnh t t d n kh, hiu nhng quy lut thit k vt "tng hnh" t ngin n phc tp.Nhng ai tng c kinh nghim v u t a c u bit ba iu tm nim vng ngc khi muanh t l "a im", "a im" v... "a im"; th trong vic to dng chic my bay v tu

chin tng hnh nhng nh khoa hc v k s thit k cng c ba chin lc quan trng l "hnhdng", "hnh dng" v... "hnh dng"! Vic to dng gim thiu sng phn x radar l iukin tin quyt ca k thut tng hnh v nhng yu t tng hnh khc s c ln lt "h hiphn gii".Th d v ci gng phng v gng hnh cu bn trn cho ta mt khi nim c bnv thit k. Mt phng vi mt nghing thch hp s phn x sng radar ra xa ngi quan st(tc l ngun pht radar) v mt cu phn x tr li ng ngay ngi quan st (Hnh 3). Th dd hiu ny a ra mt kt qu quan trng l ch c bao gi thit k vi mt cu nu mun vtth tng hnh.Vic nh lng tit din radar ca vt th t n gin n phc tp tr thnh mt ngnh c bitca in t hc. Vt th n gin c ngha l nhng hnh dng n gin nh mt phng, hnhcu, hnh tr, hnh nn, hnh khi. Vt th phc tp l my bay, tu b, xe hi, con ngi, thvt, chim mung, thm ch cn trng, lm bi, cy c. S phn hi ca sng radar t mt vt thkhng phi n thun ch l s phn x m cn bao gm c nhiu x (diffraction) nhng nhnhn, ng mp ca vt th. nh lng sng phn hi radar cng rc ri khi hnh th ca mctiu quan st cng phc tp v v vy ngi ta cn c mt m hnh ton hc tinh vi v lp trnh vitnh thch hp tm ra nhng p n. V ci p n mong mun cng rt n gin l lm saotm ra mt thit k to dng ti u cho mt vt th cc to nhng c tit din radar cc nh.Vo thp nin 60 ca th k trc, mt nh khoa hc Nga tr tn l Petr Ufimtsev to ra mtm hnh ton hc tnh ton s phn x, nhiu x v tn x (scattering) ca sng in t t mtvt th c lm nn bi cc mt phng vi cc nghing khc nhau. Thp nin 60 l im caoca chin tranh lnh, chnh ph Lin X lp ra nhiu tng kim sot bo mt nhng ti liunhy cm c lin quan t nhiu n an ninh quc gia. Nhng tht khng ng m hnh ton hcca Ufimtsev c cho php cng b rng ri ti Lin X v ton th gii. S s h xy ra c lv Ufimtsev l mt nh vt l tr tui v danh v m hnh ton ca ng ch c cc "cy a, cy" ca nh cm quyn Lin X nh gi nh l kt qu ca mt nghin cu c bn chung chungkhng c gi tr quc phng ln kinh t.Nh mt bu vt t trn tri ri xung, khi bi bo co c dch t ting Nga sang ting Anhcc chuyn gia ti hng hng khng Lockheed (M) c mt tinh i nhanh tay v ngay ly mhnh ny, ri t trin khai thnh mt lp trnh vi tnh mang tn l Echo 1 tm li gii chom hnh Ufimtsev v tnh ton tit din radar ca chic my bay lm t cc mt phng. Nhng


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tnh ton ny cho thy vt to khng nht thit cho mt tit din radar to nu ta bit sp xp bmt lm gim thiu s phn hi sng radar. Nm 1975, cc chuyn gia Lockheed tm ra mtthit k ti u cho my bay tng hnh c mt tit din radar rt nh v nhng lung sng ti radarb tn x ra xa ngi quan st. Ch vi nm sau, khng qun M cho cho i chin u c F-117Nighthawk (Chim ng m) vi thn my bay c lp ghp vi nhng mt phng ba chiu

(faceted surface) ging nh chic my bay giy origami ca tr con (Hnh 5).

(a)

(b)Hnh 5: (a) F-117 Nighthawk vi cc mt phng origami v (b) B-2 Spirit vi ra cnh trntrnh nhiu x v thn my bay c ti u ha cho hiu qu kh ng lc hc v hiu ng tnghnh (Ngun: Wikipedia).

Khng thiu g nhng li ca lng mn tn vinh ngi hng chin s phi cng tung i cnh st,li nhng chic chin u c hin i trng rt ngu gm tht x tot khng gian, o ln trn


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khung tri xanh "lnh nh ngn my tri m m", mang ti cho ngi em gi hu phng sumng bit bao nim am m ngy ngt... Nhng mt sau ca s lng mn ny ch l nhng skin khoa hc lnh lng v trn tri. Vic o ln ca mt chic my bay siu m nh chinu c cn phi tun th nhng nguyn l ca kh ng lc hc (aerodynamics). Tuy nhin, mtphng origami khng phi l hnh dng em li nhng u th kh ng lc hc cho vic thao tc,

o ln v gia gim vn tc.Cc chuyn gia ti cng ty hng khng Northrop (M) tip tc trin khai lp trnh ca Lockheedtm nhng li gii ton hc cho vic thit k b mt va c li im kh ng lc hc va c thphn tn sng phn x ra ngoi vng kim sot radar ca i phng. Kt qu ca nhng tnhton ny l my bay nm bom tng hnh B-2 Spirit vi i cnh phng c b mt rt to v thnmy bay c hnh dng di thon th tha mn yu cu ca kh ng lc hc v hiu qu tng hnh(Hnh 5). C th ni B-2 l chic my bay nh cao ca vic thit k t n im ti u cho haiyu cu i nghch ny. Khi iu kin kh ng lc hc khng cn l mt i hi quan trng,chng hn nh trong cc chin hm tng hnh, nhng mt phng origami vn l b mt c achung v n d ch to, tin li cho vic lp rp v t tn km trong sn xut (Hnh 6).

Hnh 6: Tu chin tng hnh Sea Shadow (M) vi mt phng origami(Ngun: Wikipedia)

Cc nh tng hnh hc vn cha dng bc vic thit k hnh dng m cn tin thm bc ktip lm tit din radar ca mt vt th cng nh hn.Vn cng l cu chuyn c xoay quanh

vic gim thiu hoc l tng hn l trit tiu lung sng phn x, nhiu x t thn my bay. Khimt vt liu c kh nng hp th mt phn hay ton th nng lng sng ti radar th sng phnx s gim thiu hay hon ton trit tiu. Kim loi v tnh dn in rt cao phn x gn 100 %sng radar. Bi c tnh nh, c tnh bn v d gia cng, nhm l mt vt liu thch hp cho vicch to cc loi my bay, v thp c dng cho tu b. Nh vy, nu khng c mt b mt cth v mt lp sn tng hnh th my bay hay chin hm s c tit din radar rt ln."Pho i bay" B-52 tng gm tht trn bu tri Vit Nam vi li th bom ri thm trong


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nhng nm chin tranh khc lit v trn chic trng vng Vnh, thc ra ch mt chic my baybnh thng rt i "tht th" v khng c trang b nhng phng tin "gi di" tng hnh.Nu nh gi theo quan im tng hnh i hi nhng c tnh ging nh "ninja" m m o ovi nhng hnh x im lm "xut qu nhp thn" th B-52 l k i s! Chic my bay nm bomny c ngi M lit k vo hng my bay chin thut (tactical) l mt vt th bit bay cao

nhng khng bit n hnh, va n o va to xc ch ly sc mnh ngi. B-52 c chiu ngangi cnh l 56 m (my bay tng hnh B-2: 52 m) v chiu di l 50 m (B-2: 21 m). Kch thcca B-52 v B-2 c th coi nh l ngang nhau nhng tit din radar ca B-52 c ln l 100m2 so vi B-2 l 0,1 m2 (1000 ln nh hn). F-117 cng nh hn c tr s l 0,025 m2 (4.000 lnnh hn). Trn mn hnh radar, B-2 hay F-117 c tit din tng ng vi loi chim [1].

3. Vt liu hp th radar

Nh cp trn, khi sng in t chm vo mt vt th s phn x, truyn x v hp th xy ra

(Hnh 2). Mun lm gim phn x, ta cn gia tng s truyn x hay hp th. Ngi v hnh trong quyn

tiu thuyt "Invisible Man"l mt nhn vt c chit sut ging nh khng kh.Tc gi quyn sch c mt

tng ng nghnh nhng mang mt cht mu sc khoa hc. Khi nh sng i qua hai mi trng c chitsut ging nhau th s khng c s phn x m ch c s truyn x 100% v nhn vt ny tr nn trong

sut i vi nh sng; anh ta tng hnh. Sng radar phn ln truyn x qua thy tinh, gm s v plastics.

My bay lm bng thy tinh, gm s hay plastic s tng hnh nh s truyn x, nhng ch to my bay

bng cc vt liu ny y l vic khng tng. Sng radar cng truyn x qua composite cha si thy

tinh.Si thy tinh bn chc c th lm vt liu cho thn my bay tng hnh. Nhng my mc, vt dng v

con ngi trong my bay li l nhng vt phn x. Gim phn x bng cch dng vt liu truyn x l

mt vic hon ton khng thc t. Chn la cn li l hp th nng lng sng radar lm gim thiu

hay trit tiu phn x. S hp th ny bin nng lng sng thnh nhit.

Sng radar thng nm trong vng vi ba (microwave) c tn s t 1 18 GHz ca ph in t (Hnh 4).

tn s ny bc sng c chiu di t milimt n vi centimt. hiu r s phn x, truyn x v hp

th ca sng in t, ta khng cn tng tng mng lung xa vi m hy nhn ci l vi ba khim tn

trong nh bp. L pht sng tn s 2,45 GHz (bc sng: 12,2 cm) l tn sng "anh em" ca radar.

tn s ny nc trong thc n nh tht c, rau ci s hp th nng lng vi ba v bin thnh nhit. Kim

loi l vt liu phn hi vi ba rt tt. Vch l kim loi phn x ton b vi ba vo thc n. Cnh ca l

c lm bng thy tinh quan st bn trong l. Nh cp trn, vi ba truyn x qua thy tinh v

lt ra ngoi. c s an ton, ngi ta dng vch kim loi vi nhng l nh c ng knh vi milimt


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nh hn bc sng vi ba (12,2 cm). N va l bc tng ngn chn s r r vi ba ra ngoi va l ca s

quan st. Th d ny cho thy s phn x t b mt kim loi v s hp th nng lng vi ba ca mt vt

khi c in tnh hay t tnh thch hp. Nc trong thc n c in tnh thch hp cho vic hp th vi ba

tn s 2,45 GHz.

Trong Th Chin th 2, khng qun Anh lm qun i c quc x nhiu phen iu ng v nhng

chin u c loi nh c tn l Mosquito (Con mui). Nhng "Con mui" ny bay nhanh, oanh tc chnh

xc v lm i phng in u v radar phng khng ca c khng nhn thy Mosquito! Nhng trn ra

qun v chin thng ca Mosquito tr thnh huyn thoi ca Th chin th 2 v l nim t ho ca

khng qun Hong gia Anh. Cc chin tch v t ho ny ch da trn mt s kin rt i khim tn v

n gin l thn nhng chin u c "Con mui" ny c ch to bng... vn p (plywood). So vi kim

loi, g l mt vt liu phn x sng radar rt ti v chnh ci "ti t" ny m radar ca qun c khng

pht hin kp thi cc lt tn cng ca "mui".

S thnh cng ngoi d tng ca chin u c Mosquito l mt ngu nhin hn l c . Ngu nhin

chin trng tr thnh iu may mn cho nhn loi. Trn lun im khoa hc ci ngu nhin ny mt

cch gin tip cho thy mt iu hin nhin nhng t ngi ch l phn x ca sng radar rt khc

nhau gia nhm v g. Ngi Anh khng c nh ch to "Mosquito" nh mt loi my bay tng hnh.

Trong lc nhm l mt vt liu tuyt vi ch to my bay, th vic s dng vn p tht l mt tng

l lng, ko li li k thut thit k thn my bay tr li thu ban s ca cng ngh hng khng, nhng v

s khan him nhm trong thi chin h phi dng g thay th. Gn 8.000 chic Mosquito c

ch to v s dng trong Th chin th 2.

S khc nhau gia g v nhm chng minh rng s tng tc gia sng in t v vt liu qua cc

hin tng nh phn x, truyn x, hp th, nhiu x v khc x u ty thuc vo in tnh v t tnh

ca vt liu . iu ny rt tht v bn cht sng in t, nh ci tn din t v qua s pht hin

ca nh vt l hc James Maxwell, l do s hn phi rt l hnh phc gia in trng v t trng. Cho

nn, tc dng ca sng in t ln mt vt liu gy ra s dao ng lng cc in (electric dipole) hay

lng cc t (magnetic dipole) ca phn t vt cht.S dao ng ca cc lng cc tr nn cc i khi c

s cng hng xy ra mt tn s no ty vo c tnh ca vt cht. S cng hng a n s hp

th nng lng sng ti a tn s v tan bin thnh nhit nh ta thy nc trong thc n trong l vi

ba tn s 2,45 GHz. Mc d s hp th sng, nh ta thy, l mt hin tng xy ra thang phn t,

nhng ngi ta c th thit lp mt cng thc n gin o phn x v hp th ca sng in t

trn b mt cc loi vt liu cc bc sng (tn s) khc nhau da vo phng trnh sng in t ni

ting ca Maxwell.

y ngi vit xin c m ngoc ni s lc v phng trnh sng in t ca thin ti James Maxwell

ngi Scotland vo th k 19.Khi ni n sng in t ngi ta bt buc phi ni n phng trnh

Maxwell. Bng bn cng thc n gin nhng sc tch, phng trnh sng Maxwell kt hp in

trng v t trng, hai thc th khc bit, m t bn cht ca sng in t. Ci v i ca phng

trnh ny khin c gio s Richard Feynman (Nobel Vt l 1965) phi tht ln rng "Mt vn nm sau

hu th vn xem phng trnh Maxwell l mt pht hin v i nht ca th k 19. So vi n, nhng s

kin lch s khc cng thi s i vo qun lng nh mt vic tm thng tnh l". Phng trnh

Maxwell cho thy nhng loi sng khc nhau t sng radio c bc sng cc di hng chc kilomt n

tia X, tia gamma nng lng cao vi bc sng cc ngn picomt (10-12 m) u l b con h hng c

gi chung l sng in t. Phng trnh Maxwell cn tnh c vn tc truyn sng cng l vn tc


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truyn nh sng (khong 300.000 km/s). T s tng ng c bn ny Maxwell kt lun nh sng

thy c bao gm nh sng , vng, xanh, tm, chng qua cng l mt vng sng trong ph in t.

S v i ca phng trnh Maxwell l n khai minh mt lot s kin khoa hc mang tnh t ph c

tm vc thi i nh: (1) thng nht in trng v t trng, (2) thng nht cc loi sng, (3) nh sng

cng l sng in t v (4) t mt nn tng nh lng l gii s tng tc gia sng in t v vt

cht. Trong cuc sng hin i, con ngi c "tm" trong sng in t. Khng gian sinh hot ca

chng ta trn ngp sng radio cho vic truyn thanh, truyn hnh, sng radar, sng vi ba, sng in thoi

v ng nhin nh sng, tia hng ngoi v t ngoi t mt tri. Nu khng c phng trnh sng

Maxwell c l s khng c nn thin vn hc hin i v cng s khng c nhng cng c vin thng t

nhng i thu pht sng khng l, nhng thp ng-ten cao ngt ngng n chic in thoi di ng

nh b. N to mt cuc cch mng trong cc phng thc lin thng gia con ngi v ng loi m

cn ni kt con ngi vi v tr bao la.

Tr li ci sn g ca Mosquito. G c dng thnh cng cho chin u c Mosquito, v g tn x

v hp th sng radar lm gim phn x do dn in ca g thp, nhng khng ai mun quay v

thi mng mui ca ngnh hng khng. S ngu nhin y may mn ny ch ra mt nguyn l th s l

ta phi dng vt liu c dn in trung bnh gia tng s hp th nng lng sng lm gim sng

phn x. Vic nghin cu vt liu hp th sng radar (radar absorbing materials, RAM) ra i. Xut pht

t phng trnh Maxwell, vic nh lng phn x ca sng in t c din t bng mt cng thc

n gin cho chng ta nhng con s ty theo in tnh, t tnh ca vt liu v tn s sng. in tnh

c biu hin qua dn in hay in thm (permitivity) v t tnh c biu hin bng t

thm (permeability) [2] (Ph lc a). Uy lc ca phng trnh Maxwell ta sng khi cng thc ca phn

x vng sng radar (tn s 1 - 18 GHz) cho thy sng radar phn x gn nh 100 % t cc b mt kim

loi.

Sau Th Chin th 2 cc chuyn gia tng hnh hc vn khng ngng nghin cu vt liu hp th sng

radar. Ha bnh tr li nhng th gii li chm m trong chin tranh lnh v nhu cu "nhn trm" ln

nhau thc y vic ch to nhng chin my bay trinh st c ph lp sn hp th radar v c th bay

tht cao trnh radar ca i phng. T cng thc nh lng phn x, cc nh khoa hc tnh

c dn in (qua in thm) v t tnh ( t thm) ca vt liu thch hp cho vic hp th

trong vng vi ba (tn s 1 n 18 GHz). H xc nhn cc loi bt carbon c dn in thp hay bt st

(carbonyl iron), oxit st hay oxit hp cht st (th d: ferrite) c t tnh thch hp. H ch ra mt loi sn

cha ht carbon hay cha bt st hay mt phc hp cha c hai. Lp sn s c ph ln thn my bay

vi dy thng l 5 - 10 mm. Nhng vt liu ny hp th 50 - 90 % nng lng sng radar trong vng

vi ba c tn s 1 - 18 GHz.

Lp sn dy cha bt st, oxit st d b r st v lm gia tng trng lng ca vt th c ph kh ng

k. Yu t ny gy ra nhiu im bt li trong khi tnh bn ca sn v vic gim trng lng thn my

bay, tu thy lc no cng l nguyn l hng u ca cc thit k cng ngh. Ngoi ra, lp sn ch hp

th radar mt tn s, nu khng may tn s radar ca i phng nm ngoi tn s hp th th lp sn

v hiu. Kt qu l ta mc li radar, trong tm nhn ca phe ch v vn mnh ca ta s nm trong

bn tay lng l ca i phng ng s!

S ra i ca si carbon (carbon fibres) vo thp nin 60 ca th k trc to ra cc composite c cht

nn polymer v si carbon va nh va bn cho cc ng dng cu trc. Ging nh ht carbon, si carbon

cng c dn in thch hp cho vic hp th vi ba nn composite si carbon va l vt liu cu trc


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va l vt liu tng hnh. Khng c g ngc nhin khi composite si carbon l mt ng vin sng gi trong

tt c vt liu cho ch to my bay, tu chin tng hnh hin i.

Trong cc ng dng thc tin, vt liu l tng cho vic hp th radar cn mt lc ba iu kin l bn,

nh v hp th tht nhiu trn mt bng tn tht rng (tng phn vi mt tn s).Nh vy, cc nh

khoa hc gii quyt vn ny nh th no?Chng ta hy i vo cc phn k tip.

4. Polymer dn in

K t lc tnh c c khm ph vo nm 1975, hn 30 nm qua polymer dn in l mt tinghin cu si ng trn mt c s ln ng dng. Nhng cng trnh nghin cu ca vt liu ny c thu tm trong mt quyn sch ting Vit c xut bn nm 2008 [3]. Polymer dn cng tng c nghin cu nh mt vt liu tng hnh a nng v "thng minh" [4].Tng t nh

si carbon v bt carbon, polymer dn in l mt vt liu hu c c kh nng dn in. Nhngkhc vi kim loi v carbon, dn in ca polymer dn c th c iu chnh bng phngphp in ha (electrochemical). Khi p dng vo vic hp th nng lng sng radar, kh nngiu chnh dn in a n vic hp th sng nhng tn s khc nhau, hay c th bin vtliu gi "ngu si" phn x sng radar, nhng khi cn thit tr li chc nng "thng minh" chu, hp th sng radar. Cc chin lc chin thut trong chin tranh l tp hp ca s "gi di"v "nh la", v polymer dn rt linh hot v phng din ny.c im ca polymer dn l kh nng hp th nng lng vi ba rt hu hiu. Mt th nghimnh dng l vi ba gia dng cho thy khi ta cho vi ba tc dng ln polymer dn, n s hp thnng lng sng v pht nhit [5]. Ngoi ra, ta ch cn 2% polypyrrole (mt loi polymer dnin thng dng) trn vo lp cao su hay lp sn dy 2,5 mm l hn hp ny c th hp th t

nht 90% nng lng vi ba ca bng tn 12 - 18 GHz [6-7]. Mt lot polymer dn in vi cccu trc ha hc khc nhau nh polyaniline, polythiophene v poly(phenylene vinylene) chohiu ng tng t nhng tn s radar thch hp. Mt c im khc l polymer dn in c th hai dng: (1) dng trung tnh cch in v (2) dng dn in kt np vi dopant A. Polymerdn c t trong bnh in gii v bng s thay i in p ca bnh, hai dng s c chuynhon nh sau,

....MMMMMMMMMMMMMM..... (dng 1) + A

....MMMM+A-MMMMM+A-MMM+A-MMMM..... (dng 2)


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Hnh 7:Khi c in p (trong hnh: "on") polymer dn in phn x sng radar; khi khng c

in p (trong hnh: "off") n hp th radar. tn s khong 1,03 GHz, vt liu ny hp th gn

99,9% (-50 dB) nng lng sng ti. Trong khong 1 n 1,1 GHz, nng lng b hp th l 90% (-20 dB) [8].Dng 1 l dng trung tnh cch in v dng 2 l dng dn in.Dng 1 khng hp th v dng 2hp th sng radar [Hnh 7]. Nh vy, polymer dn in tr nn mt vt liu thng minh "h hthc thc" nh la i phng, khi cn thit s l mt vt liu tng hnh, khi khng cn thit lil mt vt liu "ngu si" phn hi sng radar [8-10]. Tuy nhin, bnh in gii thc hin iuny d cho kch c no hay hnh dng no cng khng phi l mt linh kin thch hp cho ccthit k my bay hay tu tng hnh.Vo nm 2001 mt nhm nghin cu ti Vin Hn lm Khoa hc Trung Quc (ChineseAcademy of Sciences) tng hp polyaniline - mt polymer dn in thng dng - c dng

hnh ng kch c micromt [11]. Bnh thng, polymer dn in nh tn gi nh ngha chc tc dng in. Nhng khi polyaniline dng hnh ng th c tc dng t tnh trong bng tnquan trng ca sng radar (1 - 18 GHz). Phi ni y l mt pht hin th v bi v rt him cmt vt liu nh nh polymer mang kh nng hp th sng radar bng c hai c ch t v in.Nhng vt liu t tnh hp th radar nh bt st hay oxit st c t trng rt nng v d b n mn(r st). y khng phi l vt liu l tng cho my bay, ni m c tnh nh cn l mt iukin thit k quan trng hng u, hay tu chin phi thng xuyn tip cn vi nc bin. Hnna, vic trn thnh phn in nh carbon vo thnh phn t nh oxit st khng cho hiu ngng vn (synergetic effect) [7]. Polyaniline ca nhm nghin cu Trung Quc tha mn iukin nh cn v mang c hai c tnh in v t. Tuy nhin, sau bi bo co nm 2001 ngi vit

khng tm thy nhng t liu tip theo ni n vic trin khai polyaniline dng ng trong cc pdng tng hnh.

5. Vt liu nano


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ng than nano v cc ht nano kim loi v oxit kim loi cng ngy cng a dng v phng cchsn xut cng c nng cao to ra nhng sn phm i tr. Vt liu nano mang n nhiu ngdng v hiu ng tng hnh cu vt liu ny cng l mt ti nghin cu cu cc nh tng hnhhc. Ngi ta khi phi hp polymer dn in v ng than nano to thnh composite thy

composite ny hp th vi ba nhiu hn khi ch c n c mt thnh phn [12]. iu ny chngt c hiu ng ng vn xy ra gia polymer dn v ng than nano.Hin nay cha c nhiu cng b v s hp th radar ca ht nano. Khi kch thc mt vt liuc thu nh th din tch b mt ca ton th ht nano s gia tng vi mt s ln tng ng.Ht st, oxit st v ht carbon dng trong vic hp th sng radar c ln micromt.Ht nanoc kch thc khong 1.000 ln nh hn ht micromt. Bng mt phng php tnh ton ngin, ta bit c din tch b mt cu ton th ht nano s 1.000 ln ln hn so vi htmicromt cng mt th tch. Khi c s gia tng b mt, nhng c tnh ca vt liu s gia tngrt nhiu ln. c tnh hp th nng lng sng vi ba radar gy ra bi s gia tng t tnh v in

tnh ca vt liu nano cng khng nm ngoi nguyn tc ny. Mc d cc loi ht nano kim loi,oxit kim loi, cht bn dn, ng than nano cng lc cng a dng v tinh vi, ngi vit vn chatm thy cc bo co cng khai ni v tim nng ca vt liu nano trong cc ng dng tng hnh.Mt iu th v khc m vt liu nano c th cng hin cho hiu ng tng hnh l kh nng hpth nng lng trong vng hng ngoi (infrared). Trong ph in t, vng hng ngoi nm cnhvng vi ba (Hnh 4). Nhit pht ra t ng c, hay do s ma st ca khng kh phn u, phnui v ra cnh my bay trong cc phi v. Nhit sinh ra bc x hng ngoi m cc b cm ngtrong tn la "lng v dit" ca i phng c th cm nhn. Khi pht hin, tn la ch bmtheo ngun nhit v ph tung mc tiu. V vy bc x hng ngoi cn phi c pht tn khngb b cm ng ca i phng pht hin. Vic trin khai vn tay n vng sng "lng ging" k

cn vi ba l mt vic kh thi ca vt liu nano.

6. Vt liu hp th radar bit ng bin

Cc i radar qun s pht sng truy lng my bay hay tu chin i phng nhng tn s

b mt. l mt trong nhng thng tin cc mt quc gia. Tn s ny u trong vng 1 18 GHz v y l bng tn rt rng trong khi vt liu hp th radar ch c th mang n hiu qu mt tn s nh hp. Nu ta c mt vt liu hp th radar tuyt vi nhng nm ngoi tn s cai phng th hin tng "trt qut" xy ra v hnh bng ca ta s l l hin ln mn hnh radarca i phng. Nu l radar ca tn la th ch trong mt khonh khc tn la ho hc tm tam m vo! Vic truy lng v phn kch cng ging nh hot cnh trong phim hot hnh "Tomand Jerry".Ch mo Tom dng mi cch tm cu t nht Jerry nhng Jerry khng chu thua,va tm cch trn lnh va quay li phn kch nh ph u mo Tom to nn mt cnh bt nho


16/29

v cng th v.Cu t Jerry lc no cng thng v cu khn kho bit tin liu c ca moTom nn tin thoi nhp nhng nh mo Tom nhng trn n nn thn.Vic tin liu ca i phng nm trong sch lc "Bit ngi bit ta". Nhng vic tinliu tn s radar ca phe bn kia "bit ngi" dnh phn thng cho ta th nh chuyn m kimy bin, "non cao t rng bit u m tm"! C l ta phi cn n s thng minh v can m

ca ip vin tm c James Bond 007 cng vi nng kiu n khu gi i vo t ch mang vnhng thng tin cc mt cho phe ta. Nhng cc cng trnh nghin cu ca nhm Chambers(University of Sheffield, Anh Quc) s cho James Bond v vn ngh hu sm! NhmChambers cho thy khng cn phi s dng ti sc ca mt James Bond ho hoa nhng thchthp th lm ci vic "nh cp" ti liu mt khi ta c kh nng thit k c mt b mt cph bi vt liu hp th radar c c tnh ng bin ng (dynamic adaptive radar absorbingmaterials, DARAM) cn gi l mt ph "thng minh". Mt ph ny gip ta ch ng c sngti radar d bt c tn s no [8-9, 13].Theo nhm Chambers, ta cn hai iu kin. iu kin th nht l nu in tr (hay dn in, dn in t l nghch vi in tr) ca lp ph c thay i th tn s hp th radar c thc di chuyn qua li (nn l "ng", dynamic) trn mt bng tn rng ln. iu kin th hai l

pha sau lp ph ta t mt b cm ng pht hin tn s radar i phng. Khi lung radarchm vo lp ph, b cm ng s lp tc xc nhn tn s radar v s bo cho lp ph bit kpthi "ng bin" (adaptive) iu chnh n tn s hp th bng mt vi mch lin thng (Hnh 8)[9]. Khng nhng nh th, khi sng radar ca i phng chuyn sang tn s khc h thng nycn c kh nng di chuyn s hp th sng n tn s tng ng trit tiu ngun e da mi[8]. Tt c trnh t ca mi thao tc phi din bin cc nhanh, vi giy ng h, v phi i trcnhng bc k tip ca i phng nhanh chng lm nha nhng lung sng radar e da.Thtl mt h thng thng minh. Nhng y ch l mt cu tng in hc cho thy nguyn l ngx ca mt h thng va hp th nng lng sng in t va t ng di chuyn n tn s hpth (absorb while scanning).


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Hnh 8:H thng vt liu hp th radar bit ng bin (DARAM)

v b cm ng xc nhn tn s ca sng ti.

Vy vt liu no c th s dng to ra lp ph thng minh thc hin cu tng ny?Ngi tangh ngay n polymer dn in.Nh cp trn, dn in (hay in tr) ca polymer cth thay i bng mt qu trnh in ha.Ngoi ra, polymer dn cn mang tnh cht ca mt tin [8].Khi c mt kch hot in ha, vt liu ny c th thay i dn in v in dung

(capacitance) ca t in to thnh mt lp ph hp th radar "thng minh". y l mt ctnh vt tri hn bt carbon hay bt st, oxit st c in ch mang in tnh hay t tnh bt bin,th ng v "v cm" trc mi kch thch t ngoi. Tuy nhin, vic s dng polymer dn tronglp ph hp th radar gp hai tr ngi ln.Tr ngi th nht lin quan n c tnh khng nnh nhit (thermally unstable) c hu ca cc loi polymer m ta thng bit qua ci tn thngdng l plastic. Trong kinh nghim thng ngy, chng ta u bit polymer bin tnh v phnhy c nhit cao (80 200 C) v bi tia t ngoi trong nh sng mt tri. dn inca polymer s b bin i hay b trit tiu bi nhng tc nhn ny. Tr ngi th hai l s thayi tn s hp th sng radar ca polymer dn trong lp ph thng minh c th ko di nhiupht. S ng bin ny qu chm so vi tc bin ho trong vng "vi tch tc" ca radar i

phng loi tr vic s dng polymer ch to lp ph "thng minh".Nhng cc b tc ny khng lm nhm Chambers chn bc. Vic ch to vt liu l tng vanh, va mng v c th hp th sng radar bao trm mt bng tn rng (broadband) c th ngph vi tt c tn s ca i phng l mc tiu ti hu m cc nh tng hnh hc mun t ti.Nhm Chambers chuyn hng nghin cu v trin khai m hnh in hc cho vic ch to mtcu trc hp th nng lng radar trn mt bng tn rng. H khng ch trng n cc vt liu bit nh ht carbon, bt st, hp cht oxit hay polymer dn m tp trung vo mt cu trc m


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n v to thnh c c ba c tnh l in tr, in dung v in cm (inductance) (Ph lc b).Trn phng din thc nghim, h chn diode c ba c tnh cn thit trn, mt linh kin thngdng trong cc ng dng in t, lm n v cu trc. Trn mt b mt ca bng mch in(printed circuit board) c din tch 18,5 x 12,5 cm dy 0,8 mm h ci vo 180 diode. Khi c dngin chy qua, cu trc ny c th hp th 90 - 95% nng lng ca sng ti radar trn mt bng

tn t 8 GHz n 14 GHz. y l nhng con s ng kinh ngc cho mt cu trc tng i ngin v rt mng. Khi mt vt th c ln ca chic my bay to trung bnh hp th 90 - 95%nng lng sng, tit din radar ca n tng ng vi mt con chim b cu! Khng c dngin, cu trc gi v "ngu si" phn hi sng radar.So vi cc vt liu hp th truyn thng, c th ni y l mt cu trc t n nh cao ca"thng minh" v mang n nhng iu kin l tng cho vic hp th sng radar. Cu trc dioden gin ny c th c trin khai bng cch kt hp vi cc linh kin in t phc tp khc m rng hn na bng tn hp th. Tic thay, ngi vit khng tm thy nhng bi bo co cng ti sau bi vit u tin nm 2004

7. Siu vt liu

Tnh cht ca mt vt liu nh c tnh, ha tnh, l tnh, in tnh, t tnh, quang tnh l nhngc tnh ty thuc vo bn cht ha hc ca cc phn t lm nn vt liu . l nhng iuhiu bit kinh in.Khi mun bin i tnh cht ca mt vt liu, ngi ta thng c khuynh

hng dng cc phng php ci bin ha hc thc hin.Th d, cho vo st mt vi phntrm carbon ch to thp c cng v bn tt hn st.in tnh ca cht bn dn siliconcng uc bin i bi cc cht tp gi l dopant.Trong vt liu polymer, ta c th tin onc cc tnh cht ca polymer t cc c tnh ca phn t (monomer) to thnh.T , ta c thdng cc loi monomer khc nhau, bin i mt cch "ha hc" to mt composite vi ccc tnh ti u. Nh th, bin i tnh cht vt liu qua cc phng php ha hc nh tng hp,gia cng vi cht ph gia hay to cc loi composite tr thnh nhng phng thc ch binlu i trong khoa hc vt liu.Tng t, trong quang hc v in t hc c tnh in v t ca vt cht l yu t quan trngtrong vic tn dng v ch ng nh sng theo cc nhu cu. Chng hn nh cc loi thu knh mynh, knh mt hay si quang (optical fibre) c thit k da vo vic iu chnh ha tnh ca vt

cht cung cp cc c tnh thch hp cho tng ng dng. Th d, ngi ta iu chnh thnhphn ha hc ca si quang c th truyn quang trn on ng di; cho ch vo thy tinh gia tng chit sut lm lng knh hay ly tch gia dng sang trng; cho ht nano vng vo chto thy tinh nhiu mu dng cho vic trang tr nh trong cc khung ca gio ng. Mt th dcho mt ng dng quang hc khc l ngi ta dng cc loi plastic (polymer) trong sut thay chothy tinh gim trng lng cho cc loi knh cn.Nhng chit sut ca cc loi polymer linh hn thy tinh, cho nn knh plastic dy hn knh thy tinh. gia tng chit sut, knhplastic thng c ci bin bng cch dng cc nhm chc cha bromine (Br) lm gim dy


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ca knh nhng vn duy tr c tiu c ( ca knh), nh hn v hn ht l gn gi c vp ca gng mt ngi mang.Nhng th d trn cho thy khi phn t c ci bin th cc tnh cht quang hc v in t cavt cht cng c ci bin. Nhng y khng phi l con ng duy nht nh ot nhngc tnh in t ca vt cht. T nm 2000, mt loi vt liu nhn to gi l "siu vt liu"

(metamaterial) c khm ph v ang tr thnh mt ti nghin cu "nng" trong cc i hc,vin nghin cu doanh nghip v quc phng. Khc vi vt liu ch bin t thin nhin nh chtv c (cht bn dn), hu c (carbon, polymer), kim loi v oxit kim loi, siu vt liu l mtcu trc c thit k hon ton nhn to bng cch b tr (engineer) nhng n v cu trc saocho cc c tnh in t quan trng nh t thm v in thm c tr s theo mun k ctr s m. c tnh v hnh dng ca nhng n v ny s c cp phn sau.C quan "European Virtual Institute for Artificial Electromagnetic Materials andMetamaterials"nh ngha siu vt liu l "mt sp xp ca cc thnh phn cu trc nhn toc thit k t c nhng c tnh in t thun li v khc thng"(an arrangement ofartificial structural elements, designed to achieve advantageous and unusual electromagneticproperties) [15]. Tp ch Nature xem vic pht hin siu vt liu nh l mt ct mc quan

trng trong lch s vt l mang tm vc ngang hng vi vic khm ph laser, pin mt tri haythng tin lng t (quantum information). Khng g ngc nhin khi c quan DARPA (DefenseAdvanced Research Projects Agency) ca chnh ph M t ra rt ho phng trc nhng nnghin cu siu vt liu v trong mi nm qua tch cc ti tr cc cuc hi tho lin quann l thuyt v ng dng ca vt liu ny. Cho n nay (nm 2010), cc bi bo co v nhngthnh qu nghin cu ca siu vt liu c ng ti ln cc tp ch chuyn ngnh ni tingnh Science, Nature, Nature Materials, Physical Review Letters, gia tng theo cp ly tha cn s ngn, cha k n nhng bi tng quan c sc trong Scientific American, Physics Today,Physics World, Materials Today, MRS Bulletin v.v...Nh chng ta s thy phn k tip, so vi vt liu c in t thin nhin nhng c tnh in tca siu vt liu rt khc thng v phn trc cm, i ngc li nhng thng thc m ngi ta bit t nhng kinh in chnh quy. Chnh v s khc thng v phn trc cm, vt liu nhnto ny c biu hin bng tin t "siu" dch t ch "meta" c ngun t ting Hy Lp, ngha l"vt" (beyond). Siu vt liu "vt" qua nhng vt liu c in nm ngha l khi n v cbn ca vt cht nh chng ta thng bit l phn t, th trong siu vt liu l nhng n v cutrc nhn to c kch c t milimt n nanomt. Siu vt liu cho ta mt khi nim mi vphng thc mi tp trung vo vic ci bin cc n v cu trc hn l thay i c tnh ha hcca phn t nh trong cc vt liu c in. Nhng n v ny c th xem nh "phn t" theongha rng, quyt nh cc chc nng v c tnh in t ca siu vt liu. Chng c th l quemicro/nano vng, si micro/nano bc, mng li vi m hay l mt vng kim loi c khe h [16][Hnh 9]. Hnh dng, kch thc v cch sp xp ca nhng n v ny c tnh ton trc thch ng cho mt ng dng gy ra do kt qu ca s tng tc gia siu vt liu v sng int.


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Hnh 9:Mt th d v n v cu trc ca mt siu vt liu dng trong vi ba: (a) Vng kim loi

ng c khe h kch c milimt, (b) C su vng th lm nn mt n v c in ln hai bng nncch in cao 1 cm gn thng gc vo nhau [16].

Trong vic thit k siu vt liu nhng n v to thnh phi nh hn bc sng ca sng in tuc s dng. Nh th, sng in t khng th "nhn" c tng chi tit ca n v m ch"thy" mt vt liu ng nht, ging nh ta nhn mt ly thy tinh cha nc ch thy nc vthy tinh, m khng thy phn t thy tinh hay phn t nc. Th d, sng vi ba c bc sngvi centimt, n v cu trc tng tc vi vi ba c th cp milimt. Ta thoi mi thit k vci bin nhng n v cu trc kch c ny thao tng ng i ca sng m sng "khng hhay bit". i vi nh sng thy c (c bc sng vi trm nanomt), vic thit k i hi knng ca cng ngh nano vi s chnh xc cp nanomt.Siu vt liu c l khng xut hin nu khng c l thuyt ca mt nh khoa hc ngi Nga tnl Veselago v s bn nhy ca gio s vt l John Pendry ti Imperial College London (AnhQuc). t thm v in thm ca vt cht thin nhin phn ln c tr s dng v khng cvt cht no ng thi cng c tr s m.Nhng vo nm 1968, Veselago ny sinh mt tng li rng nu ta c mt vt liu m t thm v in thm ng thi c tr s m th s is i thay nh th no?ng pht biu nhng tnh ton v suy lun trn tp ch Soviet PhysicsUspekhi [17].Tuy nhin, bi bo vt l mang ngha mng lung ny d dng chm vo qunlng. Bng nhin, mt ngy p tri no vo nm 2000 khi cng ng nghin cu khoa hca ra khi nim "siu vt liu" vi tim nng ch to nhng vt liu c t thm v inthm bt k tr s no c dng ln m v zero, th nhng iu tin on trong bi bo co caVeselago nm im lm hn 30 nm qua trong mt x xnh y bi c li ra nh sng. L lunca ng tr thnh mt chun mc v vt liu c t thm v in thm m l mt siu vtliu ti thng m cc nh khoa hc mun t ti.Cng rt d hiu khi bi bo co ca Veselagoch vn vn 6 trang giy nhng c trch dn hn 3.000 ln trong 10 nm qua, mt con s rtto chng t a v tin phong v tm quan trng ca l thuyt Veselago.Quang hc c bn cho ta bit rng t thm v in thm c lin quan trc tip n chitsut (Ph lc c). Chit sut ca khng kh (xem khng kh nh l chn khng) l 1 trong khichit sut ca nc l 1,33, thy tinh l 1,5. Tr s ca chit sut l dng. Khi tia sng (haysng in t) i t mi trng ny n mi trng kia c chit sut khc nhau th ng i canh sng s b khc x mt tip gip. l nhng hin tng thng thy khi tia sng i tkhng kh sang mi trng nc (hay thy tinh). Vt liu "gi tng" trong bi vit caVeselago c t thm m v in thm m nn chit sut cng l mt s m. Ngi ta gi


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y l vt liu c chit sut m (negative refractive index materials, NIM) v nh cp trny l siu vt liu cc nh khoa hc ang nhm ti v cc tim nng ng dng "i i" trongquang hc, in t hc, in t hc v quang in t (photonics). Hnh 10a v 10b [18] minh hahai trng hp khc x trong mi trng c chit sut dng v chit sut m.

Hnh 10a:Nguyn l khc x ca nh sng trong (a) mi trng chit sut dng v (b) mitrng chit sut m.

Hnh 10b:Khc x trong cht lng c chit sut dng (hnh tri) v trong cht lng gi tngc chit sut m [18].

Nhng kt qu khoa hc thng n trong tay nhng ngi nghin cu qua mt s ngu nhin.

tng v siu vt liu tng hnh cng ngu nhin pht xut t mt mu vt liu hp th radar.Vo gia thp nin 90 ca th k trc, cng ty Marconi Materials Technology (Anh Quc)chuyn sn xut nhng vt liu tng hnh thng mi nh Pendry t vn v c ch hp thradar ca mt mu si carbon. Pendry pht hin mt iu th v l in tnh ca carbon to ra shp th sng radar khng nhng t bn cht c hu ca carbon m cn do hnh dng di vmng ca si carbon. Vic khm ph v s lin h gia in tnh v hnh dng vt liu ny khinng lin tng n mt lin h tng t cho t tnh. Cu hi ng t ra l: c kh nng no chto mt vt liu phi t (non-magnetic) tr nn vt liu mang t tnh bng cch iu chnh hnhdng ca vt y? Suy ngh ny khng phi l hoang tng m c c s l lun phng pht trongcc cng thc ca phng trnh Maxwell. Rng l khi c mt dng in chy ngang mt vt(chng hn nh si dy hay vng kim loi) th t trng cm ng s pht sinh xung quanh vt

. T nguyn l c bn ny, s tng tng a Pendry tin thm mt bc na l nu ta to rav s vng cc nh bng ng (ng l kim loi phi t) ri ci chng vo mt cht nn, sau cho dng in chy qua cc vng ny th ta s c mt cm ng t. Nu ta kt hp si carbonmang in tnh ca cng ty Marconi to ra mt hn hp vi cc vng t hon vi cm ng t th tac c may ch ng c bc x in t theo mun. Pendry v hnh trung c nhng nimtin khi to ra mt siu vt liu c kh nng to hiu ng tng hnh trong mt bng tn v cngrng ln ko di t vng vi ba n vng nh sng thy c.


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Hnh 11: Siu vt liu hay vt liu c chit sut m u tin. n v cu trc (Hnh 9) c lp

rp thnh mt h thng lp th c trnh t nht nh vi cc vng ng c khe h cho t thmm v si dy ng cho in thm m trong vng vi ba.

in thm m hin din trong mt s vt liu nhng t thm m l mt iu him c. Vtliu cng c ng thi hai tr s m v do c chit sut m theo tng ca Veselago, licng him hoi nu khng ni l v vng. Nhng, "ng tuyt vng, em i ng tuyt vng"!

Ngi hng Pendry sng sng xut hin nh mt v cu tinh Vic to ra in thm mtng i d dng hn t thm m nn Pendry c bit quan tm n t tnh ca vt liu.Xut pht t vt phi t bin thnh vt mang c tnh t, Pendry suy din tip kh nng bin ihnh dng v kch thc ca vng kim loi ng to mt t thm m. ng v cc cng s ngh vng kim loi c mt khe h c th cng hng v to ra t thm m khi tc dng visng in t [19]. ng gi tn n v cu trc ny l vng cng hng h (split ring resonator).Nhm nghin cu San Diego ti i hc California (San Diego, M) tip tc pht huy cu trcny v ln u tin trong lch s khoa hc to ra mt vt liu c chit sut m c cng btrong ba bi bo co quan trng [20-22]. y l mt cu trc lp th c to thnh bng nhng

vng kim loi ng c khe h in mt trc v si ng theo chiu dc mt sau [22] (Hnh11). Khi sng vi ba tc dng ln cu trc lp th ny th vng ng sn sinh ra t thm m vsi ng cho in thm m. T gc khc x ca sng, nhm nghin cu San Diego tm thytr s chit sut ca siu vt liu ny l -2,7 [22].Cng trnh thc nghim ca nhm San Diego khng nh l thuyt ca Veselago v bin gicm Veselago thnh hin thc. Cu trc lp th vi cc kt qu th nghim ca nhm San Diegol mt bc nhy vt trong quang hc v in t hc, mt nim hn hoan ca phe "khng nh"nhng li l mt mi hoi nghi ca phe "ph nh". Cc nh khoa hc khng d dng chp nhnnhng g qu mi i ngc vi ci trc cm vn c, hay nhng t ph mun chc thng quntnh ca t duy. Hoi nghi l mt c tnh chung ca cc nh khoa hc v n m ch rng di

kt qu c pht hin ca mt cuc th nghim vn cn nhiu vic phi lm v nhiu iu phic l gii nghim tc theo ng cc quy lut vt l. Nhng trc khi m bi m ca nhngcuc tranh lun lng ng th c rt nhiu xut ng dng cho siu vt liu.Vt liu chitsut m m ra hng lot nhng tim nng mi cho cc dng c quang hc, in t, quang int, sinh hc v sinh y hc. c bit, nhng thnh cng vng sng vi ba tr thnh nn tng chovic ch to siu vt liu vi b tr cp nano (nano-engineering) trin khai ti cc vng sngvi bc sng ngn hn nh vng hng ngoi v nh sng thy c - nhng vng sng cho con


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ngi nhiu ng dng hu ch.Da theo tng ca Veselago, chng ta hy xem siu vt liu c th ng gp no ci thinmt dng c quang hc n gin nht: ci thu knh.

8. Siu vt liu v siu thu knh

Veselago tho lun ng i ca nh sng qua mt thu knh gi tng c chit sut m m bygi ngi ta gi l "siu thu knh" (superlens) (Hnh 12). Vo nm 2000, Pendry hi sinh vtrin khai l thuyt Veselago qua bi vit c ta l "Negative refraction gives a perfect lens"(Chit sut m lm ra chic thu knh l tng) [23]."Chic thu knh l tng" trong bi vitca Pendry c ngha l siu thu knh.Bo co ny lm si ni cng ng nghin cu quanghc v in t hc. Trong khi Veselago cn ngp ngng giai on l thuyt v gi tng thPendry hng hn khng nh v chng minh rng vic ch to vt liu chit sut m l kh thi vsiu thu knh hin hu vi nhng tim nng tuyt vi.

Hnh 12:ng i ca nh sng i qua (a) mt thu knh bnh thng c chit sut dng v(b) siu thu knh c chit sut m.

Ti sao gi l "siu thu knh"? Thu knh l mt b phn trung tm ca cc dng c quang hct ci my nh bnh thng, knh hin vi quang hc n knh vin vng thin vn. D c chto cc k hon ho, khng cha nhng khuyt tt gy ra trong qu trnh sn xut, thu knhquang hc vn khng cho hnh nh r rt ca vt quan st khi vt ny c kch thc tng ngvi bc sng nh sng. Nu bc sng ca nh sng trng l 550 nm (nanomt) (trung bnhcng ca bc sng nh sng tm 400 nm v nh sng 700 nm) th hnh nh ca vt nh hn550 nm ( ln ca vi-rt) trong knh hin vi quang hc s b nho v nhiu x (diffraction). Tuynhin, siu thu knh lm t vt liu c chit sut m s khng b nh hng ca s nho nh donhiu x. iu ny cho thy siu thu knh s cho con ngi mt dng c quang hc quan stmt vt c ln nh hn bc sng ca nh sng [24].Kh nng "k qui" ca siu thu knh lm tan bin s nhe nhiu x lp tc c nhng nghng dng trong li-t quang hc (optical lithography) hay to ra nhng vi mch n cp nanomt,sn xut cc loi a quang hc (DVD, CD) vi lng tr d liu vi trm ln nhiu hn v tim


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nng x l d liu bng nh sng trong my vi tnh hay dng c in t. Ngoi ra, knh hin vic mt vai tr c bit trong sinh hc. Vic quan st vi-rt, phn t sinh hc nh DNA, proteinbng knh hin vi in t l mt thng thc, nhng tic thay tia in t mang nng lng cao s"git" nhng phn t sinh hc ny trong lc quan st. Knh hin vi quang hc dng nh sng quasiu thu knh s cho ta thy vt cht sinh hc trong trng thi "sng" v y s l chic cha

kha gii m nhng b mt sinh hc duy tr s sng ca mi sinh linh tn cng phn t.Ngy xut hin cc sn phm ny trn thng trng c l cn xa, nhng theo Pendry gic mca Veselago dt khot ang tr thnh hin thc v mt thp nin tri qua cho thy nhngthnh qu y kinh ngc trong lnh vc ny.

9. Siu vt liu v hiu ng tng hnh

Vo ma thu nm 1943 trong lc th chin th 2 ang i vo thi k quyt lit, th bng nhin

mt ngun tin c bn ra t phe ng Minh cho rng hi qun M thnh cng trong vicch to mt loi tu chin tng hnh khi nhn bng mt thng ln h thng radar. Ngun tin chorng chin hm USS Eldridge c dng lm th nghim trong Project Rainbow (K hoch Cuvng) nhm trin khai k thut tng hnh dng trng in t b cong khng gian v thigian. Thm ch c mt s "nhn chng" cho rng h thy chin hm ny tng hnh mypht ng h lin trn ng di chuyn t Philadelphia n cn c Norfolk! Ngy hm nay,ngi ta tha hiu ch l mt cuc vn ng chin tranh tm l lm lung lc ch chinu ca phe c quc x. l li n i, nhng vic ch to mt vt liu c th tng hnhtrc con mt tht trn tc v con "mt thn" radar, tng chng nh ch hin hu trong tiuthuyt vin tng hay chin tranh tm l, khng ng gi y ci vt liu l tng cng nmtrong tm tay ca cc nh khoa hc.Hiu ng tng hnh ca siu vt liu c l ging nh mt cu chuyn trong b phim kinh d NhtBn mang ta l "Kaidan"(Qui m) hay l "Nhng chuyn ma qui ". Chuyn k rng cmt ngi m n t b cc hay ni ting khp vng tn l Hoichi sng trong mt ngi cha.Trong mt m khuya, mt samurai (v s) n cha mi Hoichi n doanh tri ca mt vTng Qun (Shogun) nh n. V ha thng trong cha bit y l hn ma t trn trong mttrn nh vi trm nm trc n quy ph Hoichi. Doanh tri ca Tng Qun tht ra l mt bitha ma gn cha chn cc t s samurai.ng bn dng bt lng v ba ln ngi Hoichi lm choHoichi "tng hnh" trc cc hn ma.m sau, hn ma samurai tr li tm Hoichi nh tipbn trng ca cho v Tng Qun nghe. Nhng hn ma khng thy Hoichi m ch thy mt citai ngi ang l lng i qua li trong khng trung. Tm khng c Hoichi, hn ma ni gintut gm ct t ci tai. Ci tai rt xung t, mu tun xi x. Ha ra, ci tai chnh l taica Hoichi m v ha thng b st qun v ba ngy hm trc...Nhng dng ch ngon ngoo ba ch trong cu chuyn ma qui Nht Bn, nhng chuyn hinhnh, tng hnh trong c tch thn thoi, ci o chong ca Harry Potter hay l "ngi v hnh"trong tiu thuyt ca H. G. Wells phn nh ci kht vng s hu kh nng bin ha thn thngca con ngi. Nhng sn phm tng hnh vin tng c tc dng ngc, tr thnh ni m nhthng trc, mt mc tiu nhm n ca cc nh khoa hc.Liu gi y trong gii hn cho phpca cc quy lut vt l, c vt liu no khi ph ln s lm cho ta hon ton n khng thy bnghnh, i khng li du chn, v s cho con ngi mt phng tin lm tha mn gic m ngn


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i?Theo cc thnh qu nghin cu trong 10 nm qua, siu vt liu ang c cu tr li khngnh.Cng ging nh vt liu hp th radar, s tc ng ca sng in t trong hiu ng tnghnh cho thy vic thit k siu vt liu cng phi t trng tm vo in tnh (c din t bng in thm) v t tnh (c din t bng t thm).L thuyt v hiu ng tng hnh ca siu vt liu c nhen nhm qua lin h t vn gia

Pendry v cng ty Marconi Materials Technology. T ci siu thu knh c chit sut m v mt"vng tri t do" cho php thit k nhng siu vt liu c t thm v in thm mi tr s,Pendry tung ra mt tuyt chiu khc. Nm 2006, ng v cc cng s tho lun mt kh nng linquan n hiu ng tng hnh l c phng php no, t nht trn mt l thuyt, ngi ta c thun nn theo mun ca mnh ng i ca sng in t trong mt khng gian c bit no .Khng gian c bit ny thc ra l siu vt liu. Pendry v cc cng s xut pht t mt tngcc k n gin v chng minh rng s un cong ca trng in t c th c thc hinbi phng php chuyn i ta (coordinate transformation) [25-26]. Ngha l, theo quanim ton hc nhng ni ng i ca sng b un cong th ta cng b chuyn hng, vtrong h ta mi h suy din ra tr s ca t thm v in thm sao cho phng trnhsng Maxwell vn c tha mn. Nh th, Pendry sng to ra mt phng php mi gi l

"quang hc chuyn i" (transformation optics). Phng php ny gip ta tm ra nhng tr s ca t thm v in thm thch ng, hay ni khc hn nhng tr s chit sut cn thit dnd un cong ng i ca sng in t trong mi trng siu vt liu.Kt qu ca Pendry v cc cng s cho thy khi siu vt liu c ph (cloak) ln mt vt, sngin t (hay nh sng) thay v i theo mt ng thng m sm vo mc tiu gy ra phn x,nhiu x nh ta thng bit th by gi s b un cong trong lp ph, i vng theo ngoi vi cavt c ph, ging nh ging nc nh nhng chy vng quanh khi nh ln gia dng sui(Hnh 13) [25]. Mt siu vt liu nh th ch ng c ng i ca sng in t trn thc cht "nh la" lung sng n phi i vng mc tiu v khng c mt lung sng no b phnx tr li ngi quan st. Mc tiu m ngi ta mun soi mi nhn vo, by gi b tng hnh vhon ton "trong sut".

Hnh 13:ng i ca sng in t trong siu vt liu: (A) Biu hin hai chiu, vt b ph lqu cu trn c bn knh R1, v lp ph c b dy (R2-R1) v (B) Biu hin ba chiu.

L thuyt "quang hc chuyn i" ca Pendry v cc cng s cng ch l l thuyt sung nukhng c s cng tc ca tin s David Smith; mt nh thc nghim ti ba ca Duke University

(M) v tng l thnh vin ca nhm nghin cu San Diego. Thc hin theo l thuyt caPendry v da theo kinh nghim ch to vt liu chit sut m trc kia [20], cp bi trngPendry-Smith cng vi cc nghin cu sinh ch to ra mt siu vt liu m n v cu trc lnhng chic vng h cng hng kim loi ng vi kch thc c dn dn bin i nhngv tr khc nhau chit sut cng bin i theo yu cu ca hiu ng tng hnh [27]. Vi siuvt liu ny, h chng t bng thc nghim nhng lung vi ba "ngoan c" b un v bt bucphi i vng quanh mt b mt (mt vt hai th nguyn) v v th khng b phn x; trn thc


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cht vt ny tng hnh trong sng vi ba.Nhn li, siu vt liu qu tht l "siu". Khi nhng vt cht ng nht nh nc, khng kh, thytinh ch c mt tr s chit sut nht nh, th siu vt liu khng nhng c th c chit sut mm cn l mt tp hp ca nhng mnh khm (mosaic) quang hc p tuyt vi mang tng tr schit sut khc nhau theo quyt nh ca con ngi.

Kh nng dng siu vt liu ph ln mt vt khin vt tng hnh trong sng vi ba ln u tin c chng minh bng th nghim. Bi vit vn vn ba trang giy ca nhm Pendry-Smith trntp ch Science [27] thc s lm chn ng cng ng nghin cu quang hc v in t hc nhtl gii nghin cu khoa hc qun s.Nh cp phn trn, s tng tc vi sng in ti hi kch c ca cc n v cu trc ca siu vt liu phi nh hn bc sng c s dng.Bc sng ca sng vi ba radar (tn s GHz) c kch thc milimt n centimt nn vic chto cc n v siu vt liu c phn d dng. Bc sng ca nh sng thy c trong vngnanomt (nm), tri di t 380 nm (nh sng tm) n 700 nm (nh sng ). Nu mun hiu ngtng hnh xy ra trong vng nh sng thy c nh ci chong ca Harry Potter th ta cn thit

k nhng n v cp nanomt. iu ny i hi k nng ch to vt cht, b phn hay cu trcchnh xc cp nanomt v hin nay vn cn l mt th thch to ln i vi cc nh nghin cunano.

10. V nhng kin i nghch

Trong lch s khoa hc s xut hin mt l thuyt t ph thng gy ra nhng tranh ci ko dihng chc nm i khi c mt th k cho n khi chn l c chp nhn. Thuyt lng t vthuyt tng i l nhng th d in hnh.Siu vt liu cng khng l ngoi l. Trong mc chi tm chn l, nhng hoi nghi, tranh lun, bin lun, ph bnh, t ph bnh l t duy cn thitca nh khoa hc v cng l b mt lnh mnh bt kh phn ca nghin cu khoa hc. Vic chng v un nn nhng con ng i ca sng in t trong siu vt liu cho thy nhng hintng k l v gy ra nhiu cuc tranh lun th v thm ch gay gt ti cc hi tho khoa hcgia phe "khng nh" v phe "ph nh". Liu cc d liu thc nghim ca siu vt liu c phil mt do khc cho vic sa i li nhng l thuyt kinh in v ci bin tt c th tch linquan n quang hc v in t hc, hay chng ch l hi chng "cng b hay l cht tit"(publish or perish) vi mc ch kim tin kinh ph nghin cu ca cc khoa hc gia?Gio s Ben Munk (Ohio State University, M), mt lo lng trong nghin cu in t vchuyn gia tng hnh hc, l mt ngi trit phn i vt liu c chit sut m.ng khng tins hin hu ca mt vt liu c t thm v in thm ng thi cng m. Nhng phn binca ng c vit thnh mt quyn sch c nhan "Metamaterials: Critique and Alternatives"(Siu vt liu: Ph phn v Cch nhn khc) [33]. Munk truy nguyn bi bo co ca Veselago[17] v cho rng l thuyt Veselago ng trn phng din ton hc nhng khim khuyt


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(deficiency) trn phng din vt l v l lun Veselago hm cha bin s thi gian c tr sm.Nu c chit sut m th thi gian cng s m. ng hm hnh bo rng ng v hu chonn nu thi gian c m th ng mun xin mt khonh thi gian c ci dim phc "cilo hon ng" ngc dng thi gian tm li mt cht xun xanh! Theo ng, gic m caVeselago cng ch l gic m, th gian ny s khng c siu thu knh chit sut m.

ng nhin, ng cng cng kch l lun v ngh ca Pendry, mt h lun ca Veselago, vcch thit k vt liu chit sut m m nhm San Diego thc hin [22]. Ci m nhm SanDiego gi l sng khc x v t tnh ra tr s chit sut -2,7 cho cu trc mang cc vng hcng hng v ng thng (Hnh11) chng qua l mt s nhm ln. Theo Munk, sng khngphi l sng khc x m ch l bc x ca sng b mt (surface wave), mt c trng ca micu trc c mt trnh t sp xp nht nh bao gm c cu trc ca nhm San Diego.Cho n nay (2010) ngi vit khng tm thy phn bin no t phe "khng nh", nht l tnhng chuyn gia u ngnh nh Pendry hay Smith, v ph bnh ca Munk v trong khi nhng d liu thc nghim v siu thu knh v hiu ng tng hnh ca siu vt liu vn o tnh dng thc lin tc xut hin trn cc tp ch khoa hc. Ting ni ca Munk c phn n cnhng ng ch mun duy tr cht lng cho nn khoa hc chnh thng (good science), ph tan

tm l by n cng ha nhau chy v mt hng v ging ln ting chung cnh bo n cc cquan cung cp kinh ph nghin cu v ci "chn" v "gi" ca mt hin tng khoa hc. D nghay sai, Munk qu l mt "mnh h", hay ng hn l mt "lo h" mang mt tm hn chnhtrc, khng ngt ting gm gia dng thc l.

11. Tng hnh: nhn t "bch chin bch thng"?

Nm 1999 trn chin trng Kosovo (Yugoslavia c), "Chim ng m" F-117 b tn la SAMca tuyn phng khng qun i Yugoslavia bn h. B t lnh khng lc M chng mng nvic gii thch chi tit v vic, d sao y ch l chic my bay ca nn cng ngh "c k" trongthp nin 1970, cn chi phi ni nhiu! Nhng s kin ny ch ra ci gt chn A-sin (Achilles'heel) ca my bay tng hnh m chnh ph M u t hng trm t la. F-117 hay chic B-2hin i hn v ton th cc chin hm tng hnh ca cc cng quc nh M, Nga, Anh, Php,Thy in khng phi hon ton v hnh trc radar. y, chng ta cn tr li nn tng cacc quy lut vt l xoay quanh phng trnh sng Maxwell. Trong ton th bi vit ny, chng tach tho lun sng radar vng vi ba c tn s cp gigahertz (GHz) tng ng vi bc sngtrong phm vi milimt v centimt. Hnh dng b mt phn tn sng radar v vt liu hp thradar u c thit k v ch to i ng vi sng vi ba. Nh F-117, B-2 c th bay llt, cc chin hm tng hnh c th nhn nh lt sng trong vng vi ba nh ra vo ch khngngi m khng h s b pht hin.Nhng, khi h thng radar pht sng bng tn megahertz (MHz) vi bc sng tng ng n v mt, tnh hnh hon ton i khc. Trong cc cng thc ton hc din t phn x cusng, tn s (hay bc sng) lc no cng l mt bin s quyt nh (Ph lc a). Quy lut vt llc no cng mun vy cht nhng c mun ca con ngi; nhng g tng hnh vi ba ticthay s hin hnh trong sng megahertz. Ni mt cch nh lng, tit din radar trong vng viba s gia tng 10 100 ln trong sng megahertz [34]. Loi sng ny khng phi l nhng lnsng xa l i vi nhng sinh hot hng ngy. l nhng ln sng radio, tivi mang n s


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thoi mi n cho chng ta qua chic mn hnh nh trong gia nh hay bng iu nhc m du tbn kia b i dng. Khi sng radio c s dng trong radar, ta c th gim st (surveillance)s di ng ca mt vt th cch xa hng ngn dm. Tuy nhin khc vi vi ba, sng radio khngc kh nng nh v (locating) chnh xc khong cch gia ngi quan st v mc tiu. V y lnguyn nhn chnh ti sao vi ba l loi sng thng dng trong cc ng dng dn s ln quc

phng cho vic gim st v nh v cng lc.Mt khc, nu ta nhn li cng thc phn x ca vt liu hp th radar, ngoi in thm v t thm, b dy ca lp ph cng l bin s quyt nh phn x (Ph lc a). phn x sthp nht khi b dy ca lp ph bng 1/4 bc sng [7]. Trong vng vi ba, ta ch cn lp phvi milimt. Trong vng megahertz, bc sng di vi mi n vi trm mt v vic to mtlp ph c b dy 1/4 bc sng vi mi mt l mt vic khng tng in r!Nh vy, trong ngha tuyt i tng hnh cha hn l mt nhn t "bch chin bch thng" vn ty thuc vo nhiu iu kin hon ton b g p bi cc quy lut vt l. Thit k my bay,tu chin tng hnh khng phi ton bch. u nhng k h s b l din nh chic tai ca chtiu Hoichi trong cu chuyn "Qui m"m radar hay b cm ng tm xa ca i phng sd dng pht hin. Nhng phng php hay vt liu lm m mt radar to ra nhng bt ph k

thut ch ko di nhiu lm vi nm. Cng ngh cm ng (sensing technology) cng lc cng tora nhng b cm ng (sensors) tinh vi, nhy cm tn dng nhng vng sng in t cha bao gis dng nh vng terahertz (THz) nm gia vng vi ba v hng ngoi hay vng t ngoi(ultraviolet). Trong khi , nghin cu siu vt liu vn cn trong trng nc v cc ng dngthc tin c l cn xa vi.

12. Li kt

Nhng iu trnh by trong bi vit ny da trn cc bi bo co v t liu cng khai. Nhng ych l phn ni ca tng bng ngm; mt cnh ca s nh h m cho ta c hi a mt khe khnhn vo mt kho bu c mt. Con ngi thch u tr v chinh phc nhau. Radar to iu kincho s pht trin ca k thut tng hnh, ngay sau li c k thut phn tng hnh. Mt chiuthc c tung ra khng sm th mun cng s b ha gii bng mt chiu thc cao hn, ri lib khng ch bng chiu thc khc cao hn na. Nghe nh truyn kim hip Kim Dung, nhngrt tht. Con ngi pht sng truy lng mc tiu, ri con ngi ch ng nhng ng i casng, thm ch dp tt n. Ci tuyt vi l tt c mi thao tc u tun th theo phng trnhMaxwell, mt mn qu v gi m Maxwell cng hin cho nhn loi. D cho sng c nhynht ra sao, d con ngi c un nn nhng ng i ca sng theo giai iu no, phng trnhMaxwell l mt chun mc bao trm mi hnh trng v thao tc ca sng in t. May thay, nkhng phi l nhng cng thc ton hc y k hiu kh hiu, dy c con s thch thc t duyloi ngi. N n gin khng ng. Nhng cng thc c dn xut sau ny, cc a con caphng trnh Maxwell, nh lng phn x ca sng hay tin on nhng c tnh in t,chit sut ca siu vt liu cng l nhng biu hin ton hc n gin. R rng, trong cuc utr "tng hnh" v "phn tng hnh", khng phi l thuyt phc tp m l cc phng thc ch to(fabrication) vt liu, d l vt liu thin nhin hay siu vt liu, cng vi s kt hp a ngnh sl yu t quyt nh s pht trin ca k thut tng hnh li ko theo nhng ngnh cng ngh linquan, lc no cng trong trng thi ng v vn ti bng nhng bc i vn dm.


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Cong Nghe Tang Hinh