Future of electrotechnics: ferrofluids

Fu u e o elec o echnics: e o luids 9 FUTURE OF ELECTROTECHNICS: FERROFLUIDS D. Maye Uni e si y o Wes Bohemia, Facul y o Elec ical Enginee ing, Uni e zi ni 26, 306 14 Plzen. maye @.zcu.cz Summa y: Magne ic liquids enabled de elopmen o new de ices and echnologies ha a e a use ul al e na i e o he exis ing ones. Many o hese applica ions a e s ill in p og ess and do no ep esen any b eak- h ough disco e ies ye . Ne e heless one may expec ha owing o hei ema kable quali ies magne ic luids will become in he u u e a pa o o iginal p ojec s. The esea ch o magne ic liquids has a s ongly mul idisciplina y cha ac e . I is hus desi able o echnicians o di e en specializa ions o o he specialis s (such as physicians, biologis s, pha macis s e c.) o be acquain ed wi h he quali ies and exis ing applica ions o hese pe spec i e ma e ials. 1. INTRODUCTION In he 19 h cen u y some physicis s (M. Fa aday, T. J. Seebeck and o he s) e i ied hei p esump ions o he quali ies o he magne ic ield using liquids whe e ine me al dus was dissol ed. A disad an age o his en i onmen was i s ins abili y - unde he in luence o g a i y he dus ended o se le down. A e mo e han 100 yea s physicis s eopened he issue and ound ou ha hese liquids can be s able i he dus pa icles a e e y ine. Wi h ex emely ine e omagne ic pa icles hei sedimen a ion occu s a e a longe ime. In hese liquids he magne ic iscosi y phenomenon was disco- e ed i means ha when hey a e exposed o he magne ic ield hei iscosi y inc eases. These liquids we e labeled as magne ic heological. Soon p oduc ion echnology was de eloped, hei physical - chemical quali ies we e examined and hei applica ions we e sea ched o in echnical, medical and biochemical p ac ice. The 1 s pa en s o using e o luids we e gained by Jacob Rabinow [13] in 1940. In he 50s and 60s o he 20 h cen u y esea ch in e o luids was emba goed. Since 1970s he knowledge was disclosed and many pape s in jou nals and books ha e been published. Many in e es ing and use ul applica ions ha e been ealized and pa en ed. Complex ma hema ic-physical heo ies ha e been desc ibed ha p o ide in o ma ion abou s uc u e and beha io o e o luids in s a iona y and dynamic s a e. The solu ion o magne ic ields (elec o- magne ic, he mal powe e c.) is di icul in sys ems con aining e o luids as hey a e in a e y non-linea and aniso opic en i onmen . Al hough many ma e ials we e published on e o luids, he de elopmen and mainly usage o hese p ospec i e ma e ials is no ully exploi ed ye . Con empo a y de ailed knowledge abou he e o luids is deal wi h in wo ks [1], [2] [11] [12] [15]. 2. PHYSICAL-CHEMICAL PRINCIPLE OF FERROFLUIDS Fe o luids a e pe manen ly s able colloid suspensi- ons o e omagne ic pa icles in ca ie liquid. Sus- pension s abili y he e means he quali y ha he sus- pension emains pe manen ly homogenous, i is he e omagne ic pa icles do no sepa a e om he ca ie liquid and do no se le a he bo om o he con aine (do no sedimen ), nei he c ea es mu ual agg ega ions. To each s abili y e o luids mus con ain e omagne ic pa icles o he size 5 o 15 nm (1 nm = 10 -9 m), so called nanopa icles. Nanopa icles a e usually o med by one Weiss domain. As a esul o spon aneous magne iza ion i has a magne ic momen and ep esen s an elemen a y magne ic dipole. Elemen a y dipoles in luence each o he . To p e en hei agg ega ion hey a e co e ed wi h s abilize , i.e. a polyme ous (mac omolecula ) coa ing, so called de e gen o med by he chains o pola molecules (e.g. a y acid), long 1 o 2 nm. E e y chain is a one end bound wi h a nanopa icle and a he o he end loosely a ac ed by he molecules o he ca ie medium, Fig. 1. De e gen is hus a su ace ac i e ma e ial ha p e en s di ec con ac be ween nanopa icles, causes epulsi e o ces be ween hem and so p e en s hei agg ega ion. chains o molecules o de e gen ca ie medium magne ic nanopa icle Fig. 1. Fe omagne ic nanopa icle wi h de e gen coa ing in ca ie liquid. The mos common ma e ials o e omagne ic nanopa icles a e magne i e (Fe 3 O 4 ), maghemi e (Fe 2 O 2 ), cobal (Co), i on (Fe) o i on ni ide Fe x N). The ca ie liquid can be wa e , a ious oils, usually syn ac ic on hyd oca bon base, glycol and hei compounds. Typical magne ic liquid con ains (in olume): 5% e omagne s, 10 % de e gen and 85 % ca ie liquid. I s ela i e pe meabili y µ ≈ 5 10 Ad ances in Elec ical and Elec onic Enginee ing d ops wi h empe a u e and a Cu ie empe a u e ge s he alue µ = 1, sa u a ion magne iza ion is abou 1,3 T and wo king empe a u e is om –125 o 200 o C. Wi h highe empe a u e and empe a u e changes chemical de e io a ion o de e gen chains occu on he su ace o nanopa icles, which leads o des abiliza ion o e o luid. Fe o luids du abili y is e.g. om 8 o 10 yea s. High quali y e o luids wi h long du abili y a e mo e expensi e. Nanopa icles mo e in ca ie liquid by he mal (B own) mo ion. I he liquid is no in he magne ic ield, magne ic momen s o nanopa icles a e andom- ly o ien ed and he liquid is non-magne ic, Fig. 2. I he liquid is in he magne ic ield, he nanopa icles a e pola ized, i is hey u n in he di ec ion o he magne ic ield and make chains lying in he di ec ions o he lines o o ce. This p ocess leads o conside a- ble changes o physical chemical quali ies o e o- luids. As o hei mechanic-elas ic quali ies i is mainly iscosi y – he magne ic iscosi y phenome- non. S able e o luid emains liquid e en in a s ong magne ic ield, i means i s pa icles do no sedimen and do no agg ega e in a s ong magne ic ield. Un- less exposed o he magne ic ield, i is iso opic, bu in he magne ic ield i becomes s ongly aniso opic. The dependence o he magne ic induc ance on he in ensi y o he magne ic ield has in e o luids a cou se simila o ha o solid e omagne ics: wi h g owing H inc eases B and asymp oma ically app oa- ches he s a e o sa u a ion. In he linea magne ic ield as a esul o losses (hys e esis and eddy cu - en s), du ing emagne iza ion o nanopa icles hese a e hea ed and he ca ie liquid is hea ed as well which leads o dec easing o i s iscosi y. Fig. 2. Fe o luid wi hou he in luence o ou e magne ic ield: magne ic momen o nanopa icles has andom dis ibu ion. In some applica ions e o luids a e used wi h mic opa icles i is pa icles o he size om 5 o 15 µm. Fo hese liquids he name is used – magne o- heoloigical liquids. Mic opa icles a e mul i-domain ones, (non single-domain ones like nanopa icles) and a e no magne ically pola ized, do no ha e a magne- ic momen . Magne o- heological liquids con ain a conside ably la ge olume o e omagne ics, up o 70 % (o weigh ). They a e no usually s able, i means hei mic opa icles sedimen and agg ega e, he de elopmen o s able magne o- heological liquids is one o he aims o he p esen esea ch. They a e used in si ua ions when ex emely s ong magne ic iscosi y phenomenon is equi ed, in magne ic ield hey lose hei liquidi y and become solid. Fe o luids do no exis in na u e; hey a e de e- loped syn he ically. Olde p oduc ion echnologies we e based on long e m magne ic c ushing o mag- ne i e o e i e pa icles in ball mills in de e - gen solu ion. The p ocess o g inding las ed om 500 o 1000 hou s and a e inishing he cen i ugal sepa- a ion o bigge pa icles ollowed. A p esen as e and mo e e ec i e ways a e used, based on a ious chemical p ocesses leading o p ecipi a ion o nano- pa icles om solu ions o e ous sal s. The ob ained p oduc mus be pu i ied, i is bigge pa i- cles a e emo ed o by cen i uga ion o by sedi- men a ion caused by g a i y o non-homogenous magne ic ield. The p oduce s o e a wide ange o e o luids o magne o- heological liquids ha di e in hei composi ion, physical chemical quali ies and p ice and hey ecommend wha applica ions hey a e sui able. Among impo an p oduce s o hese liquids and equipmen using hem a e e.g. Ame ican company Lo d Co po a ion Inc. and B i ish company Liquids Resea ch L d. [17]. 3. FERROHYDRODYNAMICS THEORY In his pa we in oduced a sho synopsis o ma hema ical desc ip ion o sys em wi h he e o- luid, in he o m o bounda y alue coupled p o- blem, based on magne ically/mechanical ields. Magne ic ield in he domain Ω is desc ibed by he equa ion 1 o o ( ) = µ − ∈Ω A J, A , (1) oge he wi h bounda y condi ion o A, whe e by ec o is de ined he poin in he ield, µ is he pe meabili y, γ is he conduc i i y and J is he cu en densi y. The magne ic ield s eng h is 1 o (di = 0) µ H = A A (2) Fo 2D o a ional symme ic sys em ( , z) wi h e o luid (see e.g. Fig. 4) hold ue ( ( ) 1 1 A A z z ϕ ϕ µ µ ∂    ∂ ∂ ∂ + = −     ∂ ∂ ∂ ∂     J , (3) A ϕ ∈Ω , oge he wi h bounda y condi ion and he magne ic ield s eng h is 1 1 , z A A A H H z ϕ ϕ ϕ µ µ ∂ ∂   = − = − +   ∂ ∂   (4) Fu u e o elec o echnics: e o luids 11 F om gene alized Na ie -S okes equa ions o con en ional luid mechanics may be deduced o incomp essible iso opic e o luids and o he mo ionless sys em he e odynamic Be noulli equa ion. In he s eady s a e ha he o m [14], [15]:: 0 cons . p g h M H ρ µ ∗ + − = , p ∈Ω (5) wi h ollowing bounda y condi ion: * n c p p p p + = + (6) we e s m p p p p ∗ = + + is composi e p essu e, p is he modynamic p essu e, p s is magne os ic i e p es- su e, m 0 p MH µ = is luid-magne ic p essu e, whe e 0 d H M M H =  , 1 2 n 0 n 2 p M µ = is magne ic no mal ac ion, p c is capilla y p essu e, p 0 is p essu e in nonmagne ic luid, ρ is pa icle mass densi y and g = 9,8 m/s. Fo some applica ions he dynamics o magne ic iscosi y phenomenon is impo an (e.g. o e ohyd odynamic dampe ). In his cases is impo - an he de e mina ion o he esponse o iscosi y o he change o he ou e magne ic ield. I s alue is calcu- la ed in nanoseconds. 4. THE EFFECT OF THE MAGNETIC FIELD. EXPERIMENTS To unde s and he beha io o e o luids in he magne ic ield, some simple expe imen s may be p esen ed ha show his physically complex en i on- men some imes beha es con a y o expec a ions. Fe o luid wi h ee bounda y in nonhomoge- nous magne ic ield. I B eaches ce ain c i ical alue, su ace ins abili y o e o luids occu s and i s su ace changes in a sys em o spikes di ec ed in he cou se o magne ic lines o o ce. These spikes a e he esul o complex s uc u al o ce a io in non- linea aniso opic en i onmen o he liquid whe e magne ic o ces apply as well as g a i a ional o ce and su ace ension. In Fig. 3 he e is a Pe i dish wi h e o luids and pe manen magne unde nea h. Fig. 3. Fe o luid in a Pe i dish in he magne ic ield o a pe manen magne : i s su ace changed in o a se o spikes. Fe o luid a ound a cu en -ca ying wi e. In Fig. 4 he e is a Pe i dish wi h e o luid. Conduc o wi h cu en I, goes h ough he dish, which, as i is known, induces in i s en i onmen magne ic ield o in ensi y H = I/2 , whe e is pe pendicula dis ance om he conduc o . Fe o luid is abso bed by he non-homogenous magne ic ield, so i s o iginally la su ace changes i s shape. In axial sec ion e o luid su ace is bounded by he cu e ype y = cons ./ . Fig. 4. Fe o luid nea a cu en -ca ying wi e. Theo e ical solu ion. On Fig. 5 is 1D domain Ω, 2 , R ∈ , o e o luid. Acco ding o Be noulli eq. (5) is * * 0 2 2 0 2 ( ) ( ) p gh MH p gh MH ρ µ ρ µ + − = − − I 2 = → ∞ , hen 2 ( ) 0, H → 2 ( ) 0, M → 2 0. h h = = I we neglec he capilla i y, p c = 0, hen acco ding eq. (6): 1 ( ) cons . ( ) Cons . h H = = (7) The bounda y be ween subdomains e o luid/ai is no solid. He ewi h is cha ac e ized he coupled p oblem. h h R 1 2 Fig. 5. To he in luence o magne ic ield o he cu en - ca ying wi e on he le el o e o luids . 12 Ad ances in Elec ical and Elec onic Enginee ing Le i a ion in e o luid. I is known ha pe ma- nen magne canno ha e s able le i a ion in s a iona y magne o s a ic ield. (so-called Ea nshaw Theo em). The si ua ion changes when he medium, in which le i a ion occu s is e o luid. I we place a sealed dish wi h e o luid in a non-homogenous magne os a ic ield, he e o luid will be d awn o he places wi h inc easing ield in ensi y, Fig. 6. Fe o luid ension appea s and i a non-magne ic body is imme sed, he inne ension is b oken and he body is exposed o o ces ha ( oge he wi h he g a- i a ional o ce) caused s able le i a ion o he body - so called passi e le i a ion o he non-magne ic body. Fig. 6. To he le i a ion in e o luid. 5. USAGE OF FERROFLUIDS He e some ypical examples o use ul usage o e o luids in echnical a eas. Fe ohyd odynamic dampe . Dampe s used in machine y enginee ing dissipa e kine ic ene gy o ib a ing mechanism and so damp mechanic shocks and consequen ib a ions. Con en ional hyd aulic dampe s ha e cons an damping, only in special cases hei damping can be changes by egula ion o luid low wi h a h o le al e. Fe ohyd aulic dampe s p o ide a mo e elegan solu ion. They a e illed wi h e o luid ha is exposed o he magne ic ield o he coil induced by a con olled cu en signal. I changes he iscosi y o he e o luids and he dumping inc eases. The cu en signal is, acco ding a pa icula s a egy con olled by an on-line senso ha eads he causes o ib a ions. In Fig. 7 he e is one o cons uc ion a ian s o he e ohyd odynamic dampe . Fe ohyd odynamic dampe s we e used in di e - en equipmen s om ine measu ing appa a uses o washing machines, lo y sea s o he chassis dumpe o means o anspo and a e e y p ospec i e. I a ca mo es a he speed o 72 km/h, i makes 2 cm dis ance in 1 ms. Con en ional dampe s eac a e 15 ms, he ca hen makes 30 cm be o e he dampe eac s. The e omagne ic dampe eac s much as e , a e 5 ms. The dampe s eac s in 10 cm dis ance and Fig. 7. Fe ohyd odynamic dampe : 1 - e o luid, 2 – exci ing coil, 3 – plunge o dampe , 4 – dampe shell, 5 – gap, 6 – h o le al e. he ca ge s o e a bump wi hou „bouncing“. Mo e ansmission o he ib a ions o he d i e cabin, limi s ansmission damping p e- en s he bouncing o wheels and hus a loss o adhesion be ween he i e and he su ace which inc eases he s abili y o he ca , mainly in cu es. Magne ic dampe s hus inc ease he sa e y and com o abili y o he ide, sho en b aking eac ion, imp o e he beha io o he ca and ex end i s du abili y, mainly i s i es. Ano he example is he dampe o lo y sea s. In Fig. 8 he en elopes o sea ib a ions when using con en ional (non-con olled) hyd aulic dampe wi h bo h s ong a weak damping and magne ic (con olled) dampe . Magne ic dampe hus inc e- ases he com o and sa e y o d i ing. sea sp ing dampe displacemen sou ce o ib a ions equency sligh ly damping, non-con olled s ongly damping, non-con olled con olled damping wi h e o luid Fig. 8. Vib a ion o lo y sea wi h con en ional (non- con olled) hyd aulic dampe and wi h a con olled magne ic dampe . Fu u e o elec o echnics: e o luids 13 Magne ic dampe s a e used also in house appliances, such as washing machines. In con- s uc ion o buildings in seismically ac i e a eas buildings a e planned ha will ha e buil in magne ic dampe s and so will be esis an agains ea hquake. Fe ohyd odynamic sealing. Using e o luids enables pe ec sealing o a o a ing sha . The way o sealing o a ing sha wi h e o luid o magne ic ma e ial sha is in Fig. 9. In mu ually sepa a ed a eas he e a e di e en p essu es p 1 , p2. On a pe manen magne in he shape o a sho cylinde s and pole ods om magne ically so ma e ial. Be ween he sha s and he pole ods he e is an ai gap o he size o se e al en hs o cen ime e . In he ai gap a s ong magne ic ield concen a es. To his a ea e o luid is pushed ha is ixed by he in luence o he magne ic ield and has he unc ion o sealing. Fo magne ic induc ance in he ai gap B ∼ 1 T his sealing can keep he p essu e di e ence | p 1 - p 2 | ∼ 0, 2 o 1 a m. Based on he gi en p inciple mul i-laye sealing a e buil , ha enable o inc ease he o e p essu e| p 1 - p 2 | o he alue o e 10 a m. pe manen ní magne pólo ý nás ec e okapalina hídel (magne icky odi ý) p 1 p 2 pe manen magne pole piece e o luid sha Fig. 9. Sealing magne ic sha using e o luids. In compa ison o con en ional (mechanical) sea- ling he gi en p inciple has conside able ad an- ages. I is simple and (and hus cheape ) mo e eliable and has a lowe ic ion momen um ( e o- luid unc ions as a lub ican ), high igh ness, long du abili y (p oduce s claim up o10 yea s) and can unc ion in a wide empe a u e ange om –100 o C o 200 o C. This way o sha sealing is used also o iden ical p essu es (p 1 = p 2 ), as e ec i e dus p oo sealing e.g. o p o ec bea ing ope a ing in dus y, chemically agg essi e, oxic o biologically ac i e en i onmen . Speake s wi h e o luid a e cons uc ed as common elec odynamic speake s: in he magne ic ci cui wi h pe manen magne he e is an ai gap, in which a coil ib a es ed by acous ic signal and connec ed wi h he memb ane, Fig. 10. Unlike he con en- ional solu ion, a ound he coil he e is no ai , bu e o luid. By he in luence o he s ong magne ic ield o pe manen magne he e o luid is pe ma- nen ly kep in he ai gap. The acous ic pe o mance is limi ed by he accep able cu en load o he coil. As he he mal conduc i i y o e o luid is en imes bigge han he mal conduc i i y o ai , i enables o inc ease cu en densi y in he coil and hus he acous ic pe o mance o he speake . memb ane coil e o luid pe manen magne Fig. 10. Speake wi h e o liud in he ai gap. Elec ical machines wi h e o luids. Fo powe ans o me s e o luid is used as a cooling medium. Unlike ans o me oil i has a highe he mal conduc i i y, while i s elec ic s eng h is basically he same. Fo o a ing elec ical machines e o luid is applied in he ai gap be ween o o and s a o . I makes cooling be e , bu i s o all i lowe s he eluc ance o he magne ic ci cui o he machine and hus he magne izing cu en o he machine. On he o he hand, i inc eases he hyd- aulic eluc ance o he o a ion. This equi es usage o e o luid wi h low iscosi y and high pe me- abili y. The men ioned way is limi ed o slowly unning machines. Expe imen s show ha o o a ing machines o 1000 u ns /min he ad an ages p e ail ela ed wi h usage o e o luids. 6. FUTURE OF FERROFLUIDS E en i e ohyd odynamics ep esen s a young science i enabled o ealize new appa a uses and echnologies, pa o which has been p esen ed he e. Many a e s ill in p og ess and do no ep esen b eak h ough disco e ies. Ne e heless i is expec ed ha o he new machine y and elec o- echnical componen s and new p oduc ion echno- logies will a ise and ha is why a ious esea ch eams and p oduc ion companies s udy e o luids. Mo e de ailed in o ma ion is p o ided in he bibliog aphy. Acknowledgemen This wo k has been suppo ed om he G an Agency o he Czech Republic as a p ojec No.102/07/0147 and he Resea ch P ojec MSM 4977751310. REFERENCES [1] BERKOVSKI,B.M.– BASHTOVOY,V.: Mag- ne ic luids and applica ions handbook. Begell House, Ing. New Yo k, Walling o d, 1996. 14 Ad ances in Elec ical and Elec onic Enginee ing [2] BLUMS, E – CEBERS. A – MAIOROV,M. M.: Magne ic luids. W de G uy e , Be lin, 1997. [3] ENGELMANN, S. e al.: Koncep o a new ype o elec ic machines using e o luids. Jou n. o Mag. and Mag. Ma e ., 293, 2005, pp. 685-689. [4] HÄHNDEL, TH. e al: Magne ische Flüssig- kei en–Eigenscha en und Anwendungen. Fo um de Fo schung, 3, 1997, 5.1, pp. 53-61. [5] KOP  ANSKÝ, P. – MARTON, K. e al.: The DC and AC dielec ic b eakdown s eng h o mag- ne ic luids based on ans o me oil. Magne o- hyd odynamisc, 2005, 41, pp. 391-395. 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