ICA1-CT-2000-70005. References. [1] T.Strachowski, E.Grzanka, B.Palosz, A.Presz, L.Slusarski, W.Lojkowski, EMRS Fall Meeting 2002- in print. [2] J.Z.Jang ...
Phase Transition in Nanocrystalline ZnO E.Grzanka 1,2, S.Gierlotka 1, S.Stelmakh 1, B.Palosz1, T.Strachowski1, A.Swiderska-Sroda1, G.Kalisz1, W.Lojkowski1, F.Porsch3 1High
Pressure Research Center, Sokolowska 29/37, 01142 Warsaw, Poland
2Institiute
of Experimental Physics, Hoza 69, 00681 Warsaw, Poland
3 HASYLAB
at DESY, Notkestr. 85, D-22603 Hamburg, Germany
Nanocrystals often show novel physical and chemical properties different from those of the corresponding bulk material. Zinc Oxide powders are extensively used in a variety of applications such as varistors, opto- and acousto-electronics, semiconductors, luminescent devices, pigments and components for cosmetics industries, sunscreens and rubber, etc. This work is dedicated to examination of the effect of grain-size on transition pressure of ZnO from B1 to B4 structure by insitu synchrotron radiation X-Ray diffraction. Nanocrystalline zinc oxide powders with grain size 18, 22, 30 and mm as reference sample were examined with use of Diamond Anvil Cell up to 40 GPa at F3 beamline. The diffraction data was collected in energy dispersive geometry; gold was applied as a pressure marker. The experiments were performed under (i) hydrostatic pressure conditions with mineral oil as pressure medium and (ii) so-called isostatic pressure conditions without pressure medium. Nanocrystalline zinc oxide was synthesised by decomposition of zinc chloride in an aqueous solution under alkaline conditions or with addition of urea. The reactions were carried out in hydrothermal conditions under pressure up to 4 GPa using a microwave reactor [1].
Recently it has been reported that transition pressure of 12 nm grain size ZnO is 15.1 GPa, i.e. 50% GPa larger than of microcrystalline powder, 9.5 GPa: the phase transition of microcrystalline ZnO from B4 (wurtzite) to B1 (rock salt) begins around 9 GPa and ends approximately at 11 GPa [2,3]. We have examined similar B4 to B1 transition for 18 nm and 30 nm ZnO. This transition is illustrated in Figs. 1 to 4. Fig.1 shows plot of the intensity ratio of the I200 (B1) to I100 (B4) peaks for 18 and 30 nm powder as a function of pressure; similar plots were presented for micro- and 12 nm ZnO in [2]. The increase of the transition pressure with a decrease of grain size conforms to the previous data [2]. In Fig.2 we show estimation of the transition pressure from changes of FWHM of 101 (B4) (=111 B1) reflection assuming that the transition pressure corresponds to minimum of the reflection broadening: this is approximately 10.8 GPa for 30 nm and 12 GPa for 18 nm powder. Fig.3 shows changes of the intensity ratio of (I100 : I002) B4 Bragg reflections which show very different shape for micro- and nanocrystalline powders. This is an indication that there are different mechanisms of the transition B4 in B1 in nano- and micron-size grains We suggest that a very strong increase of the ratio I100 : I002 for large grains results from a simultaneous growth of a number of domains of B1 phase in the low pressure B4 what leads to very strong internal stresses. A single a nano-grain tend to remain one phase structure and therefore the intensity ratio I100 : I002 (B4) varies only very little. During increase of the external stress applied to the ZnO powders above the transition pressure there is no increase of strains observed in large grains but there is an increase of strain in nanograins, Fig.4; above 12 GPa one observes an increase of strain in 18 nm ZnO, a little bit smaller in 30 nm powder, while in mm grain size sample there is practically no increase of micro-strain up to 40 GPa pressure. Similar behaviour was observed for these materials under isostatic pressure conditions. We suggest that the increase of strains in nanograins is connected with a very large surface area of the grains (inter-grain interfaces) which obviously show different elastic properties than the bulk (interior of the grains) [4]. This work still remains in elaboration.
Figure 1: Changes of intensity ratio of 200 (B1) to 100 (B4) Bragg reflections with an increase of hydroststic pressure for ZnO powder with 18 and 30 nm grains.
Figure 3: Changes of intensity ratio of 100 to 002 (B4) reflections below the transition pressure for micro- and nanocrystalline ZnO powders.
Figure 2: Changes of broadening of Bragg reflections 101(B4) = 111(B1) under hydrostratic pressure.
Figure 4: Changes of broadening of 200 (B1) Bragg reflection above the transition pressure for micro- and nano-crystalline ZnO powders.
This work was supported by the Polish Committee for Scientific Research – grant PBZ/KBN013/T08/30, the Polish –German Project POL-00/009, DESY-HASYLAB Project II-99-053 and in part by the EC Grant "Support for Centers of Excellence" No. ICA1-CT-2000-70005.
References [1] T.Strachowski, E.Grzanka, B.Palosz, A.Presz, L.Slusarski, W.Lojkowski, EMRS Fall Meeting 2002in print [2] J.Z.Jang, J.S.Olsen, L.Gerward, D.Frost, D.Rubie, J. Peyronneau , Europhys. Lett. 50 (1), 48-53 (2000) [3] F.Decremps, J.Zhang, C.Lieberman, Europhys. Lett. 51 (3), 268-274 (2000) [4] B.Palosz, Mechanics of Advanced Materials (Lecture Notes 4): Proceedings AMAS Course MAM-2001, Ed.Z.Mróz, Center of Excellence for Advanced Materials and Structures, Warsaw 2002 pp. 235-306.
