Bi 2 O 3 -ZnO nanocomposites : a new electrolyte?

Transcription

1 Bi 2 -ZnO nanocomposites : a new electrolyte? PREPARATION BY SOLAR PHYSICAL VAPOR DEPOSITION (SPVD) AND NANOSTRUCTURAL STUDY OF PURE AND Bi DOPE ZnO NANOPOWDERS T.AIT AHCENE, C.MONTY, J.KOUAM A.THOREL, G.PETOT-ERVAS, A.DJEMEL ROLE PLAYED BY THE GRAINBOUNDARIES ON MATTER TRANSPORT IN NANOMATERIALS THE BRICK BOUNDARY MODEL Claude MONTY

2 INTRODUCTION General Research Topic : PREPARATION, CHARACTERISATION, PROPERTIES AND APPLICATIONS OF NANOMATERIALS Preparation of nanopowders by Solar Physical Vapor Deposition (SPVD). Preparation of massive nanomaterials and nanostructured coatings. Examples of nanopowders prepared by SPVD : Pure ZnO, Zn 1-x In x O, Zn 1-x Sb x O, Zn 1-x Co x O, Zn 1-x Al x O, Zn 1-x Bi x O TiO 2, Ti 1-x Fe x O 2, CeO 2,, Ce 1-x Gd x O 2, Zr 1-x Y x O 2

3 Properties luminescence : (blue light emission from Zn 1-x Al x O SPVD nanopowders) optical properties : transparency of Zn 1-x Al x O films magnetic properties : (Zn 1-x Co x O ferromagnetic nanophases) chemical reactivity and photoreactivity : TiO 2, ZnO electrical properties : Zn 1-x Bi x O varistor effect, electrolytes : Ce 1-x Gd x O 2, Zr 1-x Y x O 2

4 Preparation of nanopowders by SPVD SPVD (Solar Physical Vapor Deposition) NANOSOL SPVD process Vaporisation-condensation or sublimation-condensation in a solar reactor. This process is efficient when the vapor pressure over the material-target is high (ZnO, CeO 2, SnO 2, In 2, TiO 2 ) NANOSOL Vaporisation-condensation sur doigt froid et piégeage sur filtre SOLFACE

5 HELIOTRON Preparation of nanopowders by SPVD The HELIOTRON reactor combines two vapor (or smoke) trapping modes : condensation on a cold finger and pumping through a filter. HELIOTRON

6 Preparation of nanopowders by SPVD HELIOTRON

7 Pure ZnO nanophases prepared by SPVD Pure ZnO nanophases prepared by vaporisation-condensation in a solar reactor (SPVD : Solar Physical Vapor Deposition process). The nanophases are whiskers of several hundred nm length, 3 à 4 nm in diameter, in a tetraedral arrangement. Such nanophases are obtained when the pressure in the reactor is of the order of 7-1Torr.

8 XRD is a powerful tool to determine the thermodynamic state of the nanopowders or massive nanomaterials : phase stability (phase diagram), nonstoichiometry, lattice parameters (Bragg law) and their correlation with the thermodynamical parameters (composition, T, po2 ) to determine the average coherency domain size («grain size») and the mechanical strains and stresses inside the material (Scherrer and Langford relations). 3 (11) ZnO annealed at 14 C (48h) under air VC ZnO undoped P=35Torr 25 2 (1) Cps/Sec 15 (2) 1 (12) (11) (13) (112) 5 (2) (21) θ ( ) C u K α

9 XRD is a powerful tool - to determine the thermodynamic state of the nanopowders or massive nanomaterials : phase stability (phase diagram), nonstoichiometry, lattice parameters (Bragg law) and their correlation with the thermodynamical parameters (composition, T, po2 ) - to determine the average coherency domain size («grain size») and the mechanical strains and stresses inside the material (Scherrer and Langford relations) Z n O n a n o p h a s e s 7 Intensity Θ [d e g ]

10 It is the Full Width at Half Maximum which gives access to the coherency domain size (average size of the substructure). To determine FWHM (without monochromator) it is necessary to decompose the XRD peaks to separate the components related to the wavelengths used and to fit the experimental profile. A Lorentz function provides a good fit. ZnO powder annealed at 14 C/48H peak(2) Cps/sec Data: Data1_B Model: Lorentz Chi^2 = R^2 = y ± xc ±.8 w ±.249 A ± xc ±.14 w ±.473 A ± Intensity /cts/ Model : Lorentz y 174 ± xc ±.491 w ±.79 A ± xc ±.816 w ±.278 A ± vc (ZnO pur, P air = 35Torr) Θ( ) Cu Kα Q /deg/ Fig.6 : Pure ZnO annealed at 14 C/48h/air Deconvolution 2 peak into two Lorentz functions, corresponding to two wavelength λ Kα1 et λ Kα2 of cupper. Fig.11 : Pure vapo-condensed ZnO (SPVD) The two superposed components of 2 peak are more widened than the one of annealed ZnO

11 To determine FWHM (Full Width at Half Maximum) corresponding to the size effect coherency domain size, it is necessary to take into account for the instrumental function which is the value FWHM (2θ) measured on a reference material with a large grain size. ZnO powder annealed at 14 C/48H peak(2) 25.2 Cps/sec Data: Data1_B Model: Lorentz Chi^2 = R^2 = y ± xc ±.8 w ±.249 A ± xc ±.14 w ±.473 A ± FWHM( ).15.1 ZnO annealed 14 C/48H/air Y = X X Θ( ) Cu Kα Fig.6 : Pure ZnO annealed at 14 C/48h/air Deconvolution 2 peak into two Lorentz functions, corresponding to two wavelength λ Kα1 et λ Kα2 of cupper. Langford (1986) ZnO annealed at 14 C/48H/air Langford Y =, X X Θ ( )

12 Influence of the air pressure in the reactor on the «average «grain size» of pure ZnO nanopowders obtained by SPVD. Grain size (nm) VC [undoped ZnO] Note : A shape anisotropy appears in this diagram, it corresponds to the tendency of ZnO nanophases to form whiskers elongated along the [1] direction (c-axis). a Air pressure (Torr) (1) (2) (11) x yz c b

13 Influence of the air pressure in the reactor on the crystallographic cell of pure ZnO nanopowders prepared by SPVD Parameter "c" 5.2 Parameter "a" VC undoped ZnO ZnO annealed at 14 C/48h/air ZnO JCPDS JCPDS JCPDS JCPDS Air pressure (Torr) 3.24 VC undoped ZnO ZnO annealed at 14 C /48h/air ZnO JCPDS JCPDS JCPDS JCPDS Air pressure (Torr) The cristallographic cell is elongated along the c-axis when the pressure inside the reactor increases. Volume of the unit cell of ZnO VC undoped ZnO ZnO annealed at 14 C/48h/air ZnO JCPDS JCPDS JCPDS JCPDS Air pressure (Torr)

14 XRD : ZnO nanopowders compared to theoretical powders Intensity Z n O n a n o p h a s e s Θ [d e g ] Comparing to a theoretical spectrum (CaRine), there is an inversion of the XRD peaks intensities : the 2 line has not the maximum intensity, 21 is not the second highest peak This is probably due to a texture effect. Intensity (% ) 1,,2 (34.43,1.) 9 8 2,-1, (56.61,74.3) 7 1,,1 (36.26,66.4) 6 5 2,-1,2 (67.97,51.3) 4 1,, (31.78,34.5) 3 1,,3 2 1,,2 (62.88,16.7) (47.55,13.1) 2,,1 1 (69.1,6.8),,4 2,, (72.59,4.3) 2,,2 (66.39,2.4) (76.98,1.9) 2 θ ( )

15 ZnO XRD : normal powders compared to theoretical powders Compared to a theoretical spectrum (CaRine), there is an inversion of the XRD peaks intensities : the 2 line has not the maximum intensity, 21 is not the second highest peak This is probably due to a texture effect P u r e Z n O p o w d e r a n n e a l e d a t 1 4 C / 4 8 H / a i r Cps/sec Θ ( ) C u K α Intensity (% ) 1,,2 (34.43,1.) 9 8 2,-1, (56.61,74.3) 7 1,,1 (36.26,66.4) 6 5 2,-1,2 (67.97,51.3) 4 1,, (31.78,34.5) 3 1,,3 2 1,,2 (62.88,16.7) (47.55,13.1) 2,,1 1 (6 9.1,6.8 ),,4 2,, (72.59,4.3) 2,,2 ( ,2.4 ) (7 6.98,1.9 ) 2 θ ( )

16 ZnO-Bi 2 Among the elements added to ZnO to obtain controlled properties, Bi is well known. The electrical properties resulting from Bi additions are used in varistors where the intensity-voltage curves (I-V) are the signature of a given composition and microstructure. The initial idea was to study the changes of I-V characteristics as a function of the grain size and to look specially at nanomaterials. Using the expertise resulting from other systems ZnO-M a O b prepare Bi doped ZnO nanopowders... we have tried to

17 ZnO-Bi 2 phase diagram 11 (a) (b) Temperature ( C) ZnO(ss ) ZnO(ss)+liquid ZnO(ss)+24Bi 2.ZnO ZnO+liq Bi 38 ZnO 58 +ZnO 74 liquid γ-bi 2 +liq Bi 38 ZnO 58 +liq 753 γ γ-bi 2 +Bi 38 ZnO α- Bi 2 + Bi 38 ZnO α+ γ 7 α mol % Bi 2

18 XRD diagram of (ZnO) a (Bi 2 ) a powder mixtures annealed in air at 7 C 5 ZnO+ 5% wt Bi 2 annealed at 7 C during 17h under air ZnO+ 12% wt Bi 2 annealed at 7 C during 17h under air ZnO+ 2% wt Bi 2 annealed at 7 C during 8h under air (11) Z 4 Z: ZnO ZBO : Bi 38 ZnO 58 Cps/Sec 3 2 (1) Z (2) Z 1 (22) ZBO (31) ZBO (222) ZBO (321) ZBO θ ( ) CuKα

19 Synthesis of the Bi 38 ZnO 58+δ phase (cubic) Mixing 19 moles Bi 2 and 1 mole ZnO, annealing in air 12h at 7 C and 5h at 73 C (31) (321) New phase Bi 38 ZnO 58+δ (annealed at 73 C /air) ZnB i 38 O 6 JCPDS α -Bi 2 JCPDS δ -B i 2 JCPDS ZnO JCPDS Cps/sec (222) θ ( ) C u Kα

20 XRD nanopowders obtained by SPVD from (ZnO) a (Bi 2 ) b targets Two solid solutions are formed : Zn 1-x Bi x O and β-(bi 1-y Zn y ) 2 15 (11) (13) VC [ZnO + 5 % wt Bi 2 ] VC [ZnO + 12 % wt Bi 2 ] VC [ZnO + 2 % wt Bi 2 ] Zn 1-x Bi x O y β -(Bi 1-x Zn x ) 2 -y ZnO JCPDS 8-75 Bi 7.65 Zn JCPDS (112) 5 (23) Cps/Sec (421) (213) (42) (2) (21) θ ( ) C u K α

21 Influence of the target s initial composition on the XRD spectra performed on the SPVD nanopowders The Zn 1-x Bi x O solid solution Cps/Sec (13) (13) peak is shifted compared to pure ZnO and is widened when the Bi content increases. (ZnO)+ 5%wt(Bi 2 ) 2 (ZnO)+12%wt(Bi 2 ) θ( ) CuKα (ZnO)+ 2%wt(Bi 2 )

22 Zn 1-x Bi x O soilid solution Lattice parameters : When Bi is added to ZnO, the hexagonal cell is contracted along the c-axis Parameter "c" (3) (2) (1) Target Composition (at % Bi) VC [Bi doped ZnO] ZnO annealed at 14 C/48h/air ZnO JCPDS VC [ZnO] (1) P = 5 Torr, (2) P = 3 Torr, (3) P = 7, 1 Torr Parameter "a" Target composition (at % Bi) VC [Bi doped ZnO] ZnO annealed at 14 C/48h/air ZnO JCPDS VC [ZnO] (1) P = 5 Torr, (2) P = 3 Torr, (3) P = 7, 1 Torr Target composition (wt %Bi 2 in ZnO) Target composition (wt %Bi 2 in ZnO) 48. Target composition (at % Bi) Cell volume of ZnO (3) (2) (1) VC [Bi doped ZnO] ZnO annealed at 14 C/48h/air ZnO JCPDS VC [ZnO] (1) P = 5 Torr, (2) P = 3 Torr, (3) P = 7, 1 Torr Target composition ( wt % Bi 2 in ZnO)

23 5 4 VC [undoped ZnO] Grain size (nm) (1) (2) (11) Grain size (nm) (1) (2) (11) Air pressure (Torr) Target composition ( wt % Bi 2 in ZnO) Air pressure influence on the «grain size» of SPVD ZnO nanophases. Influence of the Bi content on the «grain size»of SPVD ZnO 1-x Bi x O nanophases

24 HRTEM of SPVD Nanopowders ZnO Zn 1-x Bi x O et β-(bi 1-x Zn x ) 2 ZnO+5%wtBi 2 Hexagonal Tetragonal ZnO+1%wtBi 2

25 ZnO+2%wtBi 2 Zn 1-x Bi x O and β-(bi 1-x Zn x ) 2 Hexagonal Tetragonal

26 SPVD (Solar Physical Vapor Deposition) (ZnO) a (Bi 2 ) b nanopowders : Average Bi content as a function of the target composition 5 XPS analysis (for SPVD nanopowder obtained from annealed target at 7 C/air) X-Ray Fluorescence (for SPVD nanopowder obtained from non-annealed target) (ZnO) a (Bi 2 ) b Average composition of the nanopowders ( wt % Bi O ) Initial composition of the target ( wt % Bi 2 ) The XPS or fluorescence analysis of the targets and of the SPVD nanopowders shows there is an enrichment of the nanopowders. If an anealing of the initial mixture has been performed, the increase is less important : that means a Bi loss during the annealing (7 C / air / 8-17h).

27 Sintered sample ( air 82 C/2h and 1 C/2h ) The used nanopowders (Φ g =35nm) were obtained by SPVD from ZnO+15%wtBi 2 targets. grain size of the sample : 13 nm the β-phase is now an α-phase The sample is nanostructured with two phases : Zn 1-x Bi x O and α-(bi 1-x Zn x ) 2 +δ ) XRD spectrum 6 5 V C [Z n O + 1 w t B i 2 ] sintered at 1 C/2h in air V C [Z n O + 1 w t B i 2 ] sintered at 82 C/2h in air ZnO 8-75 α -B i 2 ( ) Zn 1-z Bi z O (1) (11) α -(B i 1-x Zn x ) 2 +γ 4 (2) Cps/Sec (111) (12) (12) (121) (2) (21) (13) (112) θ( ) C u K α

28 ZnO-Bi 2 phase diagram 11 (a) (b) Temperature ( C) ZnO(ss ) ZnO(ss)+liquid ZnO+liq Bi 38 ZnO 58 +ZnO 74 liquid γ-bi 2 +liq Bi 38 ZnO 58 +liq γ-bi 2 +Bi 38 ZnO γ. 9 8 α+ γ 7 ZnO(ss)+24Bi 2.ZnO α- Bi 2 + Bi 38 ZnO 58 α mol % Bi 2

29 Electrical conductivity 4 2 δ-bi 2 β-bi 2 log σt (S.cm -1.K) -2-4 α-bi 2 α-bi 2 Nanos (ZnO) 95.2 (Bi 2 ) 4.8 Φ g =13nm CeO 2 +1% mol Ge 2-6 Zr.81 Y.19 O 2 Bi 38 ZnO 58+δ Nanos (ZnO) 98.7 (Bi 2 ) /T (1/K)

30 Ionic conductivity measurements Impedance spectroscopy R (single ionic crystal) C -Z ( Ω x 1 4 ) θ = ω ω M 1 monocristal 5 63 KHz 112 KHz ZrO 2 + Y 2 (9.5%mol) 28 KHz 3 C ω M = 1/RC R v Z ( Ω x 1 4 ) V(ωt) I(ωt+φ) = R V Z(ω) : résistance = Z (ω) du j Z (ω) volume = R (monocristal) 1-jθ 1+θ 2

31 R 1 C 1 R 2 C 2 Ionic conductivity measurements Impedance spectroscopy (ionic polycristals) V(ωt) I(ωt+φ 1 ) = Z 1 (ω) = Z 1 (ω) j Z 1 (ω) V(ωt) I(ωt+φ 2 ) = Z 2 (ω) = Z 2 (ω) j Z 2 (ω) θ 1 = ω ω M1 θ 2 = ω ω M2 ω M1 = 1/R 1 C 1 ω M2 = 1/R 2 C Z²( Ωx 1 3 ) 45 KHz (ZrO2 + 4 %moly2o3) 3 C 4.5 KHz R1 R Hz Z ( Ωx 1 4 )

32 -Z²( Ωx 1 3 ) 2 15 (ZrO2 + 4 %moly2o3) 3 C R V R KHz 4.5 KHz R1 R Hz Z (Ωx 1 4 ) C V R // C C //

33 Brick Boundary Model c a δ Bricks = cubes of size a Grain bondaries, thickness δ a = α δ σ V σ // σ V σ σ V σv

34 Effective bulk conductivity of the i-species σ 1 = σ v Θ Θ = α2 +(2α+1)ν α(α+1) Effective total conductivity of the i-species σ 2 = σ v Θ 1+ μθ ν = μ = σ J // σ v σ v σ J

35 «International workshop on Mechanical Properties in Advanced Materials : recent insights» Fuenteherridos SEVILLE 7-11 june 26 Matter transport in iono-covalent materials J i = - (c i D i / kt) grad η i J i = - (c i D i / kt) grad (μ i +q i V) Diffusionnal creep : V= J i = - (c i D i / kt) grad μ i η i = μ i +q i V = μ i + kt log c i + q i V Applied electric field : J i = -c i D i [(grad c i / c i ) -(q i E / kt)] constant composition electric stationary current J i = (c i D i q i E )/ kt) I i =J i q i = c i D i q i2 E / kt = σ i E σ i = c i D i q i 2 / kt

36 Brick Boundary Model log 1 χ 1 2 χ 1 = σ 1 /σ v =D 1 /D v ν= σ sc /σ v = D J / Dv // ν=1 ν=1 1 ν=1 2 ν=1 3 ν=1 4 ν=1 5 ν= log 1 χ 2-3 χ 2 = σ 2 /σ v = D 2 /D v 1 Brick Boundary Model (C.Monty 22) log 1 α ν=σ sc /σ v =D J /D μ=σ // v b /σ v =D J /D L v ν=1-4 μ=1 ν=1 1 μ=1 1 ν=1 2 μ=1 2 ν=1 3 μ=1 3-5 ν=1 4 μ=1 4 ν=1 5 μ=1 5 ν=1 6 μ= log 1 α 4 χ 2 =σ 2 /σ v =D 2 /D v ν=σ sc /σ sv =D J // /D v μ=s v /s b =D v /D J L ν=1 4 μ=1 4 ν=1 4 μ=1 3 ν=1 4 μ=1 2 ν=1 4 μ=1 1 ν=1 4 μ=1 log 1 χ log 1 α

37 Conclusions The (ZnO) a -(Bi 2 ) b nanocomposites prepared from SPVD nanopowders with 15%wt mol - Bi 2 are highly conducting at 3 C. The conductivity measured is ionic and higher than that of β Bi 2 or gadolinum doped ceria until 7 C. If any electronic conductivity appears at temperatures below 7 C and if (as the most probably is) their conductivity is associated to the oxygen ions diffusion, these compounds could be one of the best electrolyte material for SOFC. Prospects - To control the preparation of nanocomposites (ZnO) 78 -(Bi 2 ) 22 [SPVD, Optimisation of the final composition, reproducibility, anisotropic nanostructures (colonnar coatings) ] - Nanostructural studies of the nanocomposites. Thermodynamic stability. - Transport properties (electrical conductivity, oxygen diffusion). - Ageing studies. - Modelling of transport properties (composite effect)

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