MICRO-NANO POROUS MATERIALS FOR HIGH PERFORMANCE THERMAL INSULATION

Transcription

1 MICRO-NANO POROUS MATERIALS FOR HIGH PERFORMANCE THERMAL INSULATION Daniel Quénad and Hébet Sallée CSTB Cente Scientifique et Technique du Bâtiment, Envelope & Coatins Depatment - Division "Physical Chaacteisation of Mateials", Fance Abstact As claimed in the Geen Pape fo Eney Efficiency, ecently published by the Euopean Commission, the eney demand of buildins, especially in the existin stock, is temendoulsy impotant fo a sustainable development of ou society. In fact, the eney consumption of buildin (esidential & tetiay secto) accounts fo ove 40 % of final eney consumption in the Euopean Union, moe than industy o tanspot. In paticula, the eney consumption of the 160 million of existin buildins has to be educed by 80% to 90%. To achieve this objective, exploitation of the eney-savin potential is vital, especially the eduction of heat loss thouh buildin envelopes. Howeve, limited space fo themal insulation is a majo poblem fo buildin enovation. To tackle this challene, new poducts, named Vacuum Insulation Panel, ae unde developpement usin poous coe mateials wapped unde vacuum in aitiht films to educe the themal conductivity and theefoe incease the themal esistance of panels with educed thickness. An investiation of the physical popeties (poosity, density, specific aea, poe-size distibution) of fumed silica used as coe mateials has been pefomed. In addition, a complete analysis of the themal conductivity vesus pessue, humidity and tempeatue has been pefomed. Finally, a simplified semi-empiical model is suested to descibe all these effects. Keywods: Fumed Silica, Vacuum Insulation Panels, Moistue Adsoption, Themal Conductivity.

2 1. INTRODUCTION The esidential and tetiay secto accounts fo moe than 40 % of final eney consumption in the Euopean Community, moe than industy o tanspot. By end use, space heatin emains the main impotant, compaed to wate heatin and lihtin, with moe than 50 % of total eney consumption in buildins. Theefoe, eney efficiency is the fist and most costeffective way to educe both eney use and CO 2 emissions. In addition, amon the diffeent measues, themal insulation has the potential to povide the hihest etun on investment. Fo a lon time, 10 cm of insulation wee consideed as ood insulation. But eney specialists calculated that the economically optimised thickness should be aound 50 cm. Today, many existin buildin eulations demand, fo oofs and walls, insulation layes of about 20 cm thick but such equiements ae often not appopiate in paticula fo etofittin [1]. The most laely used insulation mateials, such as lass wool o mineal wool and expanded polystyene, entap ai as the isolato as. Thus, the themal conductivity of ai, which is appoximately W/m K, sets the limit of pefomance fo such mateials which each today the minimum value of W/mK fo commecial poducts. Aithiht Foil Coe Mateial Fumed Silica Souce : va-q-tec AG, Wüzbu Fiue 1 : Vacuum Insulation Panel VIP To ovecome this, new poducts, named VIP, aconym fo Vacuum Insulation Panel (fiue 1), ae bein developed based on mico-nano-poous coe mateials wapped in aitiht foil at low pessue. The coupled effect of "confinement" and "low pessue" educes the mobility of the ai and, as a esult, a vey low themal conductivity of about W/(mK) can be eached fo VIP. Howeve, hih poosity ( > 90 %) and tiny poes ( < 0.1 µm ) means hih specific aea and consequently, a hih potential of as adsoption, especially wate vapou which can eatly damae the themal popeties. 2. CORE MATERIALS PHYSICAL PROPERTIES In pinciple, VIP can be poduced usin any poous mateial with a hih poosity but silica powdes have been shown to be the best candidates to make coe mateials fo VIP's due thei mico-nano-poous stuctue [2,3]. Actually, this mateials pesents simultaneously a hih poosity (> 90 %) and a vey fine poe-size-distibution with an impotant family of poes with a size below 0.1 µm. In the pesent pape, two coe mateials have been investiated, they ae made with fumed silica, oanic o inoanic fibes and opacifies. They have been efeenced as follows : - SIL 1 (o Fumed Silica 1) and SIL 2 (o Fumed Silica 2) in the text.

3 2.1 Density, Poosity & Specific Aea The appaent density mesuement was caied out by weihin boads accodin to the standad EN 1602 [4]. As fo the poosity, it was measued usin a pycnomete which povide also the bulk and skeleton density. The specific aea was analysed usin the BET (Bunaue- Emmet-Telle) method [5,6] which is based on the adsoption of N 2 molecules consideed as a pobe to cove the suface and then to estimate the specific suface (m²/). The esults ae pesented in Table 1. Table 1 : Density, Poosity & Specific aea Mateial Refeence Total Poosity (Pycnomete) % Bulk Density (Pycnomete) k/m 3 Skeleton Density (Pycnomete) k/m 3 Appaent Density (EN 1602) k/m 3 Specific Aea m 2 / SIL1 93 ± ± ± SIL2 94 ± ± ± It s impotant to note that the aveae densities of both mateials ae one ode of manitude hihe than that of taditional insulatin mateials, such as Mineal Wool o Cellula Foams (13-20 k/m 3 ). The bulk densities obtained with the pycnomete ae in ood aeement with the appaent densities iven by the weihin method. The skeleton density is almost the same as the density of silica, between 2500 and 2800 k/m 3. Moeove, in spite of the hih density of both mateials, thei poosity is still hihe than 90 %. Finally, the specific sufaces of both mateials ae about 200 m²/, in aeement with data iven fo commecialized poducts (between 100 & 400 m²/). In compaison to cabon black ( m²/) o ypsum (~ 2 m²/), it is a hih value. 2.2 Poe Size Distibution The Poe Size Distibution (PSD) was investiated usin the well known Mecuy Intusion Poosimety (MIP) method. The mecuy intuded volume is plotted aainst the poe adius (Fiue 2) and the PSD is computed fom the deivative of this cuve. Two mains peaks will appea, one aound 2 µm and 0.01 µm, as shown by the dotted lines. 6 Cumulative poe volume ml/ SIL 1 SIL E-3 Mean Diamete µm Fiue 2 : MIP cuves : Cumulative intuded poe volume vesus mean poe diamete.

4 The density and poosity obtained fom MIP cuves ae pesented in Table 2. Table 2: Density & Poosity fom MIP. Mateial Refeence Density k/m 3 Poosity % SIL SIL The fou methods (avimety, pycnomete, BET & MIP) confim the hih poosity and the hih specific aea of silica coe mateials.. The discepancy between the thee methods ae due to seveal easons : - the diffeent sizes of the sample : 300 x 300 x 25 mm fo the weihin method, 60 x 40 x 25 mm fo the pycnomete and less than 10 x10 x 10 mm fo the MIP. - the fiability of the mateials and the hih pessue (400 MPa), needed to each tiny poes (< 10 nm) with the MIP method, could damae the mateial [7]. 3. HYGRO-THERMAL PROPERTIES 3.1 Wate Vapou Adsoption The wate vapo adsoption test was pefomed accodin to the standad EN ISO [8]. Befoe testin at contolled tempeatue of 23 C, the samples wee died at 105 C to constant weiht in ode to define the efeence "dy" mass. Then, the samples wee stoed and eulaly weihed in climatic chambe at espectively : 30, 50, 75 and 95 % of elative humidity. The inceasin of sample weiht, at each step, allows the calculation of wate content by mass (u - k/k). The esults ae plotted in fiue 3 with additional data fom two othe laboatoies NRC (Canada) and ZAE (Gemany) [9]. This fiue shows that samples ae vey simila and esults fom the thee laboatoies ae vey close to each othe Fumed Silica 1 CSTB Fumed Silica NRC Fumed Silica ZAE Wate content k/k Relative humidity Fiue 3: Compaison of expeimental esults and fittin cuve.

5 On one hand, fo low humidity (RH<60 %), the amount of adsobed wate is low ( u < 5 % in k/k). In this ane, wate molecules (aveae size 0.3 nm) only cove the suface of the silica ains by molecula adsoption [10,11]. On the othe hand, fo hih humidity, fom 60 up to 95 %, thee is an exponential incease mainly due to the capillay condensation in the small poes. 3.2 Themal Conductivity vesus Pessue and Tempeatue In mico-nano poous mateials, heat is tansfeed thouh thee distinct mechanisms which ae assumed to act in paallel : conduction thouh solid ( s ) and as phases ( ) and adiation ( ) [12]. The equivalent oveall themal conductivity e is then consideed to be the sum of these thee distinct modes : = + + = + + (1) e s s Considein the hih poosity ε of the fumed silica, the paallel model is applied as an appoximation to descibe the as & solid conduction usin the followin linea combination : s + = ε + ( 1 ε) s ε : poosity (2) In mico-nano poous silica, the majo effect of the naow poes is to educe the fee ai conduction 0. This phenomena is called the 'Knudsen effect' and the as conduction is witten as follows [13, 14] : l 0 K = = m n : Knudsen numbe δ (3) A K n A : constant close to 1 The Knudsen numbe is the atio between l m (mean fee path of ai molecules) and δ (poe size). The mean fee path can be estimated usin the followin equation : k T P : as pessue (Pa) lm = 2 d 2 π d P : diamete of the as molecule (3,53x10-10 m) (4) k : Boltzman s constant (1, J/K). In nomal conditions, (23 C, 1 atm.), the mean fee path of ai is aound 74 nm. Fom equations 3 and 4, the themal conductivity of ai in confined poous media can be witten as follows: 0 = C :constant T 1 + C. 0 : fee ai conduction (5) δ P δ : poe size. The equation 5 emphasizes the eat ole of the poduct δ.p on the as conduction. Pactically, it means that the use of mico o nanopoous mateials will equie only a modeate vacuum to each a low themal conductivity. Assumin that the silica powde is an optically thick mateial, the effect of tempeatue is descibed usin the followin equation based on the Rosseland appoximation [15, 16] : 3 16 σt E (m -1 ) : extinction coefficient is called = (6) 3 E σ : Stefan-Boltzman constant W/(m 2 K 4 ) The extinction coefficient E is estimated by fittin expeimental data.

6 In pactice, the combination of equations 2, 5 & 6 is often simplified as follows [2,3] : e = s+ + P / 2 P P 1/2 : "fitted" as pessue (Pa) (7) The themal conductivity vesus pessue and tempeatue was measued usin the hotwie pobe method [18] descibed in the Euopean Standad EN [19]. The thin hotwie pobe is placed between two samples of size 80 mm x 150 mm. Then, the sandwich is put in a contolled climatic chambe (23 C, 50% RH) and the pessue was measued usin a Baaton Gaue. 8 Themal conductivity 10-3 W/m.K Fumed Silica 1 CSTB Pecipitated Silica CSTB Fumed Silica ZAE PU Foam Themal conductivity 10-3 W/m.K Fumed Silica 1 CSTB Fumed Silica ZAE Pecipitated Silica ZAE Fumed Silica 1 NRC 4 1E Pessue hpa Fiue 4 : Themal Conductivity vesus pessue at ambient tempeatue (23 C) Tempeatue K Fiue 5 : Themal conductivity vesus tempeatue at low pessue (0.01 hpa) In fiue 4, the expeimental data fo both fumed silica ae compaed with the esults published by ZAE [3,20]. At ZAE, the measuements wee pefomed at 10 C with a uaded hot plate. Additional esults fo PU foam, with cell size aound 100 µm and pecipitated silica ae plotted fo compaison. Applyin equation 8 fo the investiated samples (fiue 4), the fittin paamete P 1/2 has been obtained. The values ae pesented in Table 3. Table 3: Fittin paametes P 1/2 & E (equations 8 & 7) fo silica based mateials (fiues 5 & 4) Mateial efeence s+ P 1/2 s+ E Fumed silica 1 CSTB Fumed silica 2 CSTB Fumed silica ZAE Pecipitated silica CSTB Polyuethane Foam To evaluate the effect of tempeatue, the measuements wee pefomed at a vey low pessue (0.01 hpa). Thus, the sample is consideed as "dy". In fiue 5, the expeimental

7 points and the fitted cuves (coefficeint E in Table 3), obtained usin equations 1 and 6, ae plotted and compaed with esults fom ZAE obtained fo simila fumed silica and one pecipitated silica [2, 21]. The small diffeence between SIL 2 and ZAE fumed silica may be due to the measuement technique (hot wie vesus uaded hot plate) and the testin tempeatue (23 C and 10 C). 3.3 Themal Conductivity vesus Moistue Content. The measuements of themal conductivity vesus moistue content u (k/k) wee pefomed usin the hot wie pobe, at ambient conditions (23 C and nomal pessue). Only the adsoption pocess was investiated. The expeimental points ae plotted in fiue 6 and fitted usin the followin linea elationship. = + A u u : mass wate content k/k (%) (8) u e Usin the soption isothem (fiue 3) and the elation P v =RH.P vs (P vs : satuated vapo pessue) to convet the mass wate content u (k/k) into the patial vapou pessue P v, it is possible to plot the themal conductivity vesus the wate vapou pessue P v (fiue 7). 60 Fumed Silica 1 CSTB 60 Fumed Silica 1 CSTB Themal Conductivity 10-3 W/m.K Themal Conductivity 10-3 W/m.K Wate Content % Vapo Pessue hpa Fiue 6 : Themal conductivity vesus mass wate content at ambient pessue Fiue 7 : Themal conductivity vesus vapou pessue P v at ambient pessue At ambient pessue, the cuves in fiue 7 can be fitted with the followin fomula : C Pv u = e + B e (9) The coefficients obtained usin the equations 8 and 9 to fit data fom fiues 6 and 7 ae pesented in Table 4. Table 4 : Fittin Coefficients Equations 8 & 9 Fiues 6 & 7 Mateial efeence e A e B C Fumed Silica 1 - SIL Fumed Silica 2 SIL

8 The fiue 8 shows that the incease of the themal conductivity of the poous silica emains easonable if the elative humidity is kept below 60 % (u<5 k/k o P v <16 hpa ). But a hih elative humidity, above 80 % (P v >23 hpa), stonly affects the themal conductivity which can each moe than 50 mw/mk aound 95 % RH (P v =28 hpa). 3.4 Themal Conductivity vesus Moistue Content and Pessue A second set of expeiments have been pefomed with a simultaneous contol of the tempeatue (23 C), the total pessue P and the elative humidity RH (o patial vapou pessue P v ) usin salt solutions. At ambient condition, thee ae thee main as in the contolled chambe: H 2 O, N 2 and O 2 and the total pessue P is equal to P = Pv + PN 2 + P0. (P 2 v =P H2O ) In a fist phase, the total pessue is educed by keepin the vapou pessue P v constant usin diffeent salt solutions (30, 54, 70, 80 %). Thus, only N 2 & O 2 ae emoved fom the chambe (and the mateial) and the as conductivity deceases wheeas the solid conductivity modified by the adsobed wate emains at a constant value. When the total pessue deceases beyond the satuated vapou pessue, the salt solution which foces the elative humidity must be emoved to avoid "boilin". Then, the total pessue can be deceased below the satuated vapou pessue and moe and moe wate molecules ae emoved as the same time as N 2 & O 2, theefoe the mateial stats to dy. Consequently, the themal conductivity will stonly decease to meet the cuve of the "dy" mateial aound 1 hpa. Themal Conductivity 10-3 W/m.K dy 30% RH 54% RH 70% RH 80% RH Pessue hpa Fiue 8 : Themal conductivity vesus vapou pessue P v, at ambient tempeatue The expeimental esults pesented in fiue 8 can be analysed as follows :. - At vey low pessue (0.01 hpa), the coe mateial can be consideed as dy and the measued themal conductivity epesents mainly the solid conductivity, plus the adiation contibution.

9 - At ambient tempeatue (23 C) and pessue (1000 hpa) but with a vey low elative humidity (RH~4 %), the measued themal conductivity takes into account the solid conduction without adsobed wate, the adiative pat and the ai contibution (N 2 & O 2 ) with vey few wate molecules. - At ambient tempeatue and pessue with contolled elative humidity (30, 54, 70, 80 %), the measued themal conductivity includes the effect of adsobed wate on the solid conductivity and the additional wate molecules in the ai. - When the total pessue deceases fom 1000 to 0.01 hpa, thee ae two main phases. Fist, above the citical wate vapou pessue coespondin with each imposed elative humidity (30, 54, 70, 80 %), the deceasin of the themal conductivity is due to the eduction of the as content (N 2 & O 2 ). Second, below the citical wate vapou pessue, the faste deceasin esults fom the coupled phenomena of the sample dyin (fiue 7) and the as content eduction (N 2 & O 2 + H 2 0). At T=23 C, the wate vapou pessue anes fom 1 Pa (at 4 %RH) to 28 Pa (at 100 %RH), as illustated by the dotted line in fiue 8. Accodin to the esults pesented in fiue 8, a simplified model is suested to descibe the evolution of the themal conductivity of mico-nano poous mateials placed unde low pessue with moistue. e = + B Pv ( 1 ε) ( + A e ) 4. CONCLUSIONS s 0 + ε T 1 + C δ P Both poous mateials investiated in this study ae compaable fumed silica with mino diffeences, especially the natue of fibes fo einfocement. Fo both mateials, the poosity is hihe than 90 % and the specific aea hihe than 200 m²/k. The densities ae slihtly diffeent 160 and 190 k/m 3 espectively. At low pessue (0.01 hpa), the effective themal conductivity that is mostly due to the solid and adiative contibutions is close to W/mK wheeas this value ise up to W/mK at ambient pessue in dy condition. Finally, these mico and nanopoous mateials ae vey ood canditates fo bein coe mateials of VIP. Nevetehless, they ae vey sensitive to moistue. In paticula due to the hih specific aea and the small size of the poes, these mateials ae nice ettes fo ases, especially fo wate vapou. Fom this, a sinificant influence of adsobed wate and of wate vapou on the themal conductivity was found, that aises to an offset of 1 to W/mK at ambient conditions. This isk is not acceptable fo hih pefomance themal insulation an the next challene will be to develop a totally aitiht wappin, takin into account the foil and the weldin. ACKNOWLEDGEMENTS This wok has been pefomed within the famewok of a poject patially by the Fench Eney Aency (ADEME) and the Fench Electicity Company (EDF). The authos ae vey ateful to Samia Kheouf fom ADEME and Benad Yieix fom EDF fo thei suppot. (10)

10 REFERENCES [1] Hih Pefomance Themal Insulation Systems - Vacuum Insulated Poducts (VIP) Poceedins of the Intenational Confeence and Wokshop EMPA Duebendof, (januay 22-24, 2001) [2] R. Caps, J. Ficke. 'Themal Conductivity of Opacified Powde Fille Mateials fo Vacuum Insulations', Intenational Jounal of Themophysics, 21, 2, (2000), [3] R. Caps, U. Heinemann, M. Ehmanntaut, J. Ficke, 'Evacuated Insulation Panels Filled with Pyoenic Silica Powdes: Popeties and Application', Hih-Tempeatues-Hih Pessues, 33, (2001), [4] Euopean Standad EN 1602, 'Themal insulatin poducts fo buildin applications - Detemination of the appaent density', [5] S.J. Ge & K.S.W Sin : Adsoption, Suface Aea and Poosity, Academic Pess (1982) [6] ISO Standad 9277, 'Detemination of the specific suface aea of solids by as adsoption usin the BET method, [7] R. Piad, A. Riacci, J.C. Maéchal, D. Quenad, B. Chevalie, P. Achad, J.P. Piad 'Chaacteization of hypepoous polyuethane-based els by non-intusive mecuy poosimety' Polyme, 44, (2003), [8] Euopean Standad EN ISO 12571, 'Hyothemal pefomance of buildin mateials and poducts Detemination of hyoscopic soption popeties' [9] IEA Annex 39, 'HIPTI : Hih Pefomance Themal Insulation', [10] S.J. Ge & K.S.W. Sin, Adsoption, Suface Aea and Poosity, (Academic Pess, 1982) [11] A.W. Adamson & A.P. Gast, Physical Chemisty of Suface, 6 th Ed (John Wiley & Sons, 1997). [12] D. Quenad, D. Giaud, F.D. Menneteau, H. Sallée, 'Heat tansfe in packin of cellula pellets : micostuctue and appaent themal conductivity', Hih Tempeatus-Hih Pessues, (1998), 30, 1998, , Pesented at the 14 th Euopean Confeence on Themophysical Popeties, Septembe 16-19, 1996 Lyon Villeubanne, Fance [13] E.H. Kennad, Kinetic Theoy of ases, Mc-Gaw-Hill, New Yok and London, (1938) [14] A. Chistefeund & al., 'Vacuum Based supe insulation panels fo appliances', ICI Polyuethane SPI 96, Las Veas USA, (octobe 1996), [15] S.Q. Zen, A. Hunt, R. Geif, 'Tanspot popeties of as in silica aeoel', Jounal of Non- Cystalline Solids, 186, (1995), [16] M. Quinn Bewte : Themal Radiative Tansfe and Popeties, A Wiley-Intescience Publication, John Wiley & Sons, Inc, (1992) [17] R. Sieel & J.R. Howel, 'Themal Radiation Heat Tansfe, Hemisphee Publishin Copoation, Taylo & Fancis Goup, (1981) [18] A. Riacci, B. Ladevie, H. Sallee, B. Chevalie, P. Achad, O. Fudym, 'Measuements of compaative appaent themal conductivity of lae monolithic silica aeoels fo tanspaent supeinsulation applications', Hih Tempeatues-Hih Pessues, 34, 5, (2002), [19] Euopean Standad EN , Methods of test fo dense shaped efactoy poducts - Pat 15: Detemination of themal conductivity by the hot-wie (paallel) method [20] R. Caps, U.Heinemann, J.Ficke, P. Randel, 'Application of Vacuum Insulation in Buildins, VIA Symposium : Poess in vacuum insulation, Vancouve, (june 2000). [21] U. Heinemann, R. Caps, J. Ficke, 'Chaacteization and Optimisation of fille mateials fo vacuum s upe insulation', VUOTO, vol XXVIII, N. 1-2 Gennaio-Giuno, (1999).