H Klym, A Ingram, O Shpotyuk - Nanoporous spinel-type functional ceramics characterized by pal technique - страница 1

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ВІСНИК ЛЬВІВ. УН-ТУ

Серія фізична. 2007. Вип.40. С.200-205

VISNYKLVIV UNIV. Ser.Physic. 2007. N40. P.200-205

PACS number(s): 61.43.Gt, 78.70.Bj, 61.46.Df, 68.35.Dv, 81.05.Je

NANOPOROUS SPINEL-TYPE FUNCTIONAL CERAMICS CHARACTERIZED BY PAL TECHNIQUE

H. Klym12, A. Ingram3, O. Shpotyuk1, J. Filipecki4

'institute of Materials of Scientific Research Company "Carat", 202, Stryjska str., UA-79031 Lviv, Ukraine e-mail: shpotyuk@novas.lviv.ua 2Lviv Polytechnic National University, 12, Bandera str., UA-79013 Lviv, Ukraine

3Opole University of Technology, 75, Ozimska str., PL-45370 Opole, Poland e-mail: ingram@wszia.opole.pl 4Institute of Physics of Jan Dlugosz University of Czestochowa, 13/15, al. Armii Krajowej, PL-42201 Czestochowa, Poland e-mail: j.filipecki@wsp.czest.pl

Nanoporous spinel-type functional MgAl2O4 ceramics are characterized by positron annihilation lifetime spectroscopy. It is shown, that this method can be used to investigate both extended positron trapping defects and moisture adsorption processes in ceramics bulk.

Key words: spinel, spectroscopy, porosity, positron defects.

It is well known, that positron annihilation lifetime (PAL) spectroscopy is one of the most powerful techniques to study electron-defect structure of solids [1]. Previously, this method was successfully used for investigation of free-volume extended defects in some kinds of functional materials such as perovskite-type BaTiO3 [2, 3] and SrTiO3 [4] ceramics, nanocrystalline ferrites [5], hot-isostatic-pressed transparent MgAl2O4 ceramics [6], etc. In these materials, the main channels of positron annihilation were ascribed to individual vacancies and vacancy-like clusters, powder particle surfaces, grain-boundaries and nanostructural voids (pores), which can capture positronium atoms. By treating the obtained PAL spectra, the best results were achieved owing to a so-called three-component fitting procedure. But sometimes (as in the case of [6]), there were no numerical information on third component in the deconvoluted spectra, which corresponds to decay of ortho-positronium o-Ps.

In 2002, we tried to use PAL technique for mixed transition-metal managanite electroceramics with thermistor effect [7]. Despite strict confirmation on three-component fitting, a very small number of experimental data with poor statistics sufficiently complicated the final decision on the possibilities channels of positron annihilation in these materials. Therefore, additional attempts are needed now to clarify relations between positron trapping and positronium decay modes in these nanoporous ceramics.

© Klym H., Ingram A., Shpotyuk O. et al., 2007

In the last year, we tried to apply PAL method for magnesium aluminate MgAl2O4 ceramics - one of the most perspective materials for humidity sensors (HS) [8, 9]. In contrast to the previous research [7], it was shown that two channels of positron annihilation are character for these spinel-type ceramics - positron trapping and o-Ps decay modes, the latter process being supposed to occur via a so-called pick-off annihilation of o-Ps in the adsorbed water [10].

In this work, we shall try to confirm the above hypothesis on water-related origin of o-Ps decay modes in humidity-sensitive spinel-type magnesium aluminate MgAl2O4 ceramics.

The investigated samples of magnesium aluminate MgAl2O4 ceramics were prepared via conventional sintering route [11].

Starting MgO and Al2O3 reagents (with surface area of 17,1 and 10,7 m2/g, respectively) were taken in a molar ratio of 1:1. These oxides were weighed, mixed with a highly pure acetone, ball-milled during 96 h and dried. The obtained powder with intrinsic surface area of 89.6 m2/g was mixed with an organic binder to prepare green body disk-shaped billets. Then, these pellets were sintered in a special regime with maximal temperature Ts=1 100 °C. Heating was carried out with rate 100 °C/h from room temperature to 300 °С, next - heating with rate 200 °C/h to temperature Ts, extract of samples at this temperature during 2 hours and aftercooling in the regime of "furnace off". In the result, we obtained porous ceramic samples with humidity-sensitive properties.

PAL measurements were performed with an ORTEC spectrometer [12], the full width at half maximum being 270 ps. The 22Na isotope with 0,74 MBq activity was used as a positron source (with 9%), placed between two identical ceramic samples.

Firstly, the PAL measurements were carried out in as-prepared ceramic samples at 20 oC temperature and ~35% relative humidity. In order to verify hypothesis on water-related origin of o-Ps decay modes in these ceramics, which were well worked previously for cement pastes [13, 14], we placed these samples into distillated water for 12 h. Later, PAL measurements were repeated at the same conditions. One month later, these samples were investigated again.

The obtained PAL spectra were decomposed by LT computer program of J. Kansy [15] using a sum of a few weighted exponential functions convoluted with measured resolution function of the spectrometer. We used three measured PAL spectra for each investigated pair of samples, differing by a total number of counts. Each spectrum was multiply treated owing to slight changes in the number of final channels, background of annihilation and time shift of PAL spectrum. The best results were chosen by comparing the FITs, determined as statistically weighted least-squares deviations between experimental points and theoretical curve. In such a way, we formed a few groups of results containing different number of experimental points within each mathematical treatment procedure. Only results with FIT values close to 1,0 (the optimal FIT deviates from 0,95 up to -1,1-1,2) were left for further consideration.

At the next stage, this FIT values and determined PAL parameters were controlled in dependence on the background of annihilation and time shift of PAL spectrum, the results showing slight changes being chosen. It should be noted that source correction and spectrometer resolution function were kept unchangeably for all PAL spectra.

The normalized positron lifetime spectra for investigated MgAl2O4 ceramics obtained in as-prepared, 12 h water-immersed and one-month natural-dried after water-immersion samples was shown in fig. Each of them is a superposition of a few spectra

202 H. Klym, A. Ingram, О. Shpotyuk et al.

with different positron lifetimes. The obtained dependences are characterized by a narrow peak and region of long fluent decaying of coincidence counts in a time. The mathematical decay of such shape curve is represented by a sum of decreasing exponential functions with different values of power-like indexes inversed to positron lifetime [16].

0,0001 -h-1-1-1-r-

0 5 10 15 20

Lifetime, ns

Fig. Peak-normalized positron lifetime spectra of magnesium aluminate MgAl204 ceramics (a comparison between as-prepared, 12 h water-immersed and one-month dried after water-immersion samples)

As we shown early [8], at mathematical treatment of PAL spectra of MgAl204 ceramics by LT computer program, the best results were obtained at three-component fitting procedure.

The first channel of positron annihilation in ceramics bulk was attributed with shortest (lifetime Xj and intensity L_) and middle (lifetime т2 and intensity I2) PAL components. This channel can be attributed to positron trapping modes, the trapping defects being individual vacancies and small vacancy-like clusters within ceramics bulk (xj=0,24 ns) and positron trapping defects in the form of neutral or negatively charged clusters of charge-compensating vacancies located at grain-boundaries (x2=0,50 ns).

In addition, the following positron trapping parameters were calculated for this

+ T2I2

case: the mean positron lifetime xav

I1 +12

which reflects cumulative defect

environment prevailing in the sample [5]; bulk lifetime Tb

= ^ + I , associated with

T1 T2

positron trapping in defect-free bulk [2, 5, 6, 15] and positron trapping rate in defects [12]. In the investigated MgAl2O4 ceramics, the difference (x2-xb) can

k =-2.

be treated as size measure of trapping defect [5, 17], whereas x2/xb value represents the nature of this defect [6] (see Table).

Table

PAL characteristics of magnesium aluminate MgAl2O4 ceramics mathematically treated with three-component fitting procedure

Sample pre-history

Fitting parameters

Positron trapping modes

 

ns

I1, a.u.

ns

I2, a.u.

ns

a.u.

ns

ns

ns-

1

T2 -

Tb,

ns

T2/Tb

as-prepared

0,24

0,68

0,50

0,30

2,59

0,02

0,32

0,28

0,7

0,21

1,7

water-immersed for

12 h

0,24

0,56

0,50

0,29

1,88

0,15

0,33

0,29

0,7

0,21

1,7

one-month natural-

dried after water-

immersion

0,18

0,56

0,42

0,35

2,02

0,09

0,27

0,23

1,2

0,19

1,8

The second channel of positron annihilation described by longest PAL component (lifetime T3 and intensity I3) can be attributed to o-Ps decay modes. In as-prepared ceramics, the lifetime t3 equals 2,59 ns. But after 12 h water-immersion, this lifetime was decreased to 1,88 ns, the value close to o-Ps pick-off lifetime in water [10]. Respectively, the intensity at this component increased from 0,02 to 0,15. This increase was accompanied by the corresponding decrease in the first PAL component (see Table), while the second component left without any significant changes.

So, like to cement pastes [13], in humidity-sensitive magnesium aluminate MgAl2O4 ceramics the process of pick-off annihilation of o-Ps in volume pores filled with water occurs [18]. After one-month natural drying of water-immersed samples, the irreversible changes occur in the structure of MgAl2O4 ceramics. These changes are probably caused by separation of other phases in the pores and water remainders (I3 = 0,09). In the result, the positron-trapping defect centres are modified by drying (the positron trapping rate increases from 0,7 to 1,2 ns-1).

PAL investigation was confirmed that two channels of positron annihilation are character for nanoporous humidity-sensitive magnesium aluminate ceramics - positron

204

H. Klym, A. Ingram, O. Shpotyuk et al.

trapping and o-Ps decay modes. The first channel is connected with positron trapping defects in the form of individual vacancies, small vacancy-like clusters and vacancies located at grain-boundaries. The second channel is linked with pick-off o-Ps annihilation in the adsorbed water. The PAL method can be applied to investigate the processes of structural changes in magnesium aluminate MgAl2O4 ceramics.

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НАНОПОРИСТА ФУНКЦІОНАЛЬНА КЕРАМІКА ШПІНЕЛЬНОГО ТИПУ, ОХАРАКТЕРИЗОВАНА ЗА ДОПОМОГОЮ ТЕХНІКИ ПАС

Г. Клим12, A. Інграм3, O. Шпотюк1, Я. Філіпецькі4

1 Науково-виробнич підприємство "Карат ", вул. Стрийська, 202,79031 Львів, Україна e-mail: shpotyuk@novas.lviv.ua 2Національний університет „ Львівська політехніка " вул. Бандери, 12, 79013 Львів, Україна 3Опольський технічний університет, вул. Озімска, 75, 45370 Ополє, Польща e-mail: ingram@wszia.opole.pl 4Інститут фізики університету Яна Длугоша в Ченстохові, вул. Армії Крайової, 13/15, 42201 Ченстохова, Польща e-mail: j.filipecki@ajd.czest.pl

Охарактеризовано нанопористу функціональну кераміку шпінельного типу MgAl2O4 методом позитронної анігіляційної спектроскопії. Показано, що цей метод може бути використано для дослідження в кераміці як об'ємних дефектів, так і процесів вологопоглинання.

Ключові слова: шпінель, спектроскопія, пористість, позитронні дефекти.

Стаття надійшла до редколегії 29.05.2006 Прийнята до друку 26.02.2007

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H Klym, A Ingram, O Shpotyuk - Nanoporous spinel-type functional ceramics characterized by pal technique