The water-sensitive positron trapping modes in nanoporous MgAl2O4 ceramics with a spinel structure 
are studied. It is shown that water-sorption processes in these ceramics leads to increase in positron trapping rates of extended defects located near intergranual boundaries. The fixation of direct positron lifetime 
components allows refining the most significant changes in positron trapping rate.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3503

Filipecki, J.

Klym, H.

Electronic Sumy State University Institutional Repository

PROCEEDINGS OF THE INTERNATIONAL CONFERENCE NANOMATERIALS: APPLICATIONS AND PROPERTIES Vol. 1 No 1, 01PCN43(2pp) (2012)   2304-1862/2012/1(1)01PCN43(2) 01PCN43-1  2012 Sumy State University Positron Trapping Effects in Water-filled Nanopores of Spinel Ceramics  H. Klym1,2,*, J. Filipecki2  1 Lviv Polytechnic National University, IKTA, 12, Bandera Str., 79013 Lviv, Ukraine 2 Institute of Physics of Jan Dlugosz University, 13/15, al. Armii Krajowej, 42201 Czestochowa, Poland  (Received 01 July 2012; published online 07 August 2012)  The water-sensitive positron trapping modes in nanoporous MgAl2O4 ceramics with a spinel structure are studied. It is shown that water-sorption processes in these ceramics leads to increase in positron trap-ping rates of extended defects located near intergranual boundaries. The fixation of direct positron lifetime components allows refining the most significant changes in positron trapping rate.  Keywords: Water-sorption process, Intergranual boundaries, Positron trapping.    PACS numbers: 82.45.Хy, 92.60.Jq   ______________ * klymha@yahoo.com 1. INTRODUCTION  The spinel-structured MgAl2O4 ceramics are per-spective materials for humidity sensors mainly due to a uniform porous structure, which promotes effective adsorption of great number of water molecules [1]. Re-cently, it was shown that the amount of adsorbed water in these ceramics affects not only their electrical con-ductivity, but also positron trapping modes of extended defects tested with positron annihilation lifetime (PAL) spectroscopy [1,2]. The positrons injected in the studied MgAl2O4 ceramics underwent two positron trapping with two components in positron lifetimes and ortho-positronium o-Ps decaying, these parameters being obtained with a so-called three-component mathemati-cal fitting procedure. Within this approach, the shortest component of the deconvoluted PAL spectra with posi-tron lifetime 1 reflects mainly microstructure specifici-ty of the spinel ceramics and the middle component with positron lifetime 2 corresponds to extended de-fects located near intergranual boundaries. The third component with lifetime 3 is due to “pick-off” annihila-tion of o-Ps in the nanopores. It is established that the adsorbed water molecules act catalytically on positron trapping in MgAl2O4 ceramics, do not changing signifi-cantly o-Ps decaying modes [2].  To refine the most significant changes in positron trapping in MgAl2O4 ceramics caused by water sorp-tion, a new mathematical approach to the treatment of experimental PAL data should be developed in such a way to accumulate the catalytic effect in some non-direct trapping parameters, while other direct compo-nents (the reduced bulk and defect-related lifetimes, in the first hand) being left nearly constant.  2. EXPERIMENTAL  The studied spinel-type MgAl2O4 ceramics were sin-tered from fine-dispersive Al2O3 and MgO powders us-ing a special regime with maximal temperatures of 1200 C, the total duration being 2 h [2]. The PAL measurements were performed with an ORTEC spectrometer with 22Na source placed between two ceramic samples (see Fig. 1) at 20 oC within row of relative humidity (RH) of 25-60-98-60-25 % using hu-midistat PID+ (see Fig. 2).  3.1 3.2  ln = f(U)  LT 1 2         124.1 4.25.1 5.2  67891011anodeanodedynodestopstart  12 Fig. 1 – Block-scheme of conventional sample-source “sand-wich” arrangement for PAL measurements using the ORTEC apparatus [1]: 1 – foil-covered 22Na source, 2 – two identical samples, 3.1 and 3.2 – scintillators of γ-quanta, 4.1 and 4.2 – photomultipliers, 5.1 and 5.2 – constant fraction discrimina-tors, 6 – delay line, 7 – time-pulse height converter, 8 – pre-amplifier, 9 – amplifier, 10 – single channel analyzer, 11 – multichannel analyzer, 12 – personal computer.  The selection of corresponding values for measuring chamber permit to investigation of samples at constant values of RH in the range of 25-60 % with an accuracy of ± 0,5 % and 60-98 % з with an accuracy of ± 1 %. The obtained PAL data were mathematically treated within three-component fitting procedure with fixed first and second positron lifetimes using LT computer program. Using formalism for two-state positron trapping model [1], the following parameters can be calculated:  2b12dτ1τ1IIκ ,221121bτIτIIIτ, 212211avIIIτIττ,    (1)  where d is positron trapping rate in defect, b – positron lifetime in defect-free bulk and av – average positron lifetime. The difference ( 2 – b) can be accept-ed as a size measure of extended defects, as well as the 2/ b ratio represents the nature of these defects [2].  3. RESULTS AND DISCUSSION  It is established that the adsorbed water molecules act catalytically on positron trapping modes in MgAl2O4 ceramics, do not changing significantly o-Ps decaying  H. KLYM, J. FILIPECKI PROC. NAP 1, 01PCN43 (2012)   01PCN43-2   Fig. 2 – Schematic of humidity control instrument PID+: UВ – turn-on voltage, which  proportional to desired value of  RH; Uс – output voltage of humidity sensor, which proportion-al to desired value of RH in the chamber; ε(t) – difference be-tween the set and necessary values: ε ~ UВ – Uс; ρ(t) – regulator voltage, which  proportional to the given portion of air in humidity; R – difference input in amplification regula-tion: k1 (RK1), k2 (RK2); I – integration input in the general time regulation Ti (R6, C1), D –difference input in detection loss regulation Td (R8, C2),  –adder input from input voltage controlling by voltage amplifier:          iTdi dttdeTdttTkkttktu0121)()(1)()()(,     (2)  where k1 = RK1/RA1, k2 = RK2/R3, Ti = C1·R6 and Td = C2·R8. These values for measuring chamber were experimental se-lected based on previous calculations in accordance with Zieg-lera-Nicholsa criteria.  modes [2]. Nevertheless, refining the most considerable changes in positron trapping in the studied ceramics caused by water sorption is a difficult problem through a large quantity of arbitrary fitting parameters at treatment of PAL spectra. This task can be permitted due to the treatment of experimental PAL data at the fixed values of lifetimes 1 and 2. It is established that lifetime 1 reflects mainly microstructure specificity of MgAl2O4 ceramics [2]. The adsorption processes are not change the structure of these ceramics. The lifetime 2 corresponds to extended defects near intergranual boundaries where ceramics are more defective. It is shown that the positrons are trapped in the same ex-tended defects in MgAl2O4 independently of the content of absorbed water in their nanoporous [2]. Thus, the lifetimes of the first and the second PAL components ( 1 and 2) at the treatment of experimental PAL data can be considered nearly constant. Within this mathematical approach, all changes in the fitting parameters of these components will be reflected in their intensities (I1 and I2). The third longest compo-nent with lifetime 3 is non-fixed. The treatment of ex-perimental data was carried out at fixed lifetime values of 1=0.18 ns and 2=0.38 ns. Within this approach I1 and I2 intensities of the direct PAL components are changes dependently from amount of adsorbed water in the studied ceramics. So increasing of RH from 25 to 98 % result in decreasing of I1 intensity and increasing of I2 intensity. The changing of RH from 98 to 25 % reflects inverse to the previous direction in I1 and I2 intensities (see table 1). The positron trapping in wa-ter-filled defects reflecting the second component with I2 intensity occurs more intensive. Table 1 – PAL characteristics of MgAl2O4 ceramics  RH, % Fitting parameters 1, ns I1, a.u. 2,  ns I2, a.u. 3,  ns I3, a.u. 25 0.18 0.79 0.38 0.20 2.37 0.01 60 0.18 0.78 0.38 0.21 2.55 0.01 98 0.18 0.76 0.38 0.23 2.27 0.01 60 0.18 0.77 0.38 0.22 2.26 0.01 25 0.18 0.78 0.38 0.21 2.21 0.01 RH, % Positron trapping modes av, ns b, ns d, ns-1 τ2 - b, ns τ2/ b 25 0.22 0.20 0.59 0.18 1.89 60 0.22 0.20 0.62 0.18 1.87 98 0.22 0.20 0.67 0.18 1.86 60 0.22 0.20 0.64 0.18 1.87 25 0.22 0.20 0.61 0.18 1.88  The lifetimes 3 are closed to 2.2-2.5 ns (see Table 1). The input of this third component is not change and intensity closed to 0.01. Thus, this channel is non-significant to water sorption-desorption processes. The positron trapping modes such as the average av, defect-free bulk b and difference τ2 – b are non-changed with RH. In addition, the positron trapping centre (τ2/ b) is formed on a typical for MgAl2O4 ceramics level of 1.9 [2], which testify to the same nature of trapping sites whichever the content of absorbed water. In contrast, most significant changes in positron trapping in MgAl2O4 ceramics caused by water sorption reflect in positron trapping rate in defect d. Thus, the catalytic water-sorption effect in the studied spinel-structured ceramics is accumulated in non-direct trapping d parameter.  4. CONCLUSION The fixation of all water-dependent positron trapping inputs allow to refine the most significant changes in positron trapping rate of extended defects and nanopores located near intergranual boundaries. The water sorption processes in nanoporous humidity-sensitive spinel-type MgAl2O4 ceramics leads to corresponding increase in positron trapping rates of extended defects located near intergranual boundaries. This catalytic affect has reversible nature, being strongly dependent on sorption water fluxes in ceramics.  AKNOWLEDGEMENTS  Halyna Klym is kindly grateful for assistance of-fered by International Visegrad Fund.  REFERENCES  1. R. Krause-Rehberg, H.S. Leipner, Positron Annihilation in Semiconductors. Defect Studies (Springer-Verlag, Berlin-Heidelberg-New York: 1999). 2. H. Klym, A. Ingram, O. Shpotyuk, J. Filipecki, I. Hadzaman, phys. status soidi C 4 No3, 715 (2007). 

Positron Trapping Effects in Water-filled Nanopores of Spinel Ceramics

SocioEconomic Challenges Journal (Sumy State University)

PROCEEDINGS OF THE INTERNATIONAL CONFERENCE NANOMATERIALS: APPLICATIONS AND PROPERTIES 
Vol. 1 No 1, 01PCN43(2pp) (2012) 
 
 
2304-1862/2012/1(1)01PCN43(2) 01PCN43-1  2012 Sumy State University 
Positron Trapping Effects in Water-filled Nanopores of Spinel Ceramics 
 
H. Klym1,2,*, J. Filipecki2 
 
1 Lviv Polytechnic National University, IKTA, 12, Bandera Str., 79013 Lviv, Ukraine 
2 Institute of Physics of Jan Dlugosz University, 13/15, al. Armii Krajowej, 42201 Czestochowa, Poland 
 
(Received 01 July 2012; published online 07 August 2012) 
 
The water-sensitive positron trapping modes in nanoporous MgAl2O4 ceramics with a spinel structure 
are studied. It is shown that water-sorption processes in these ceramics leads to increase in positron trap-
ping rates of extended defects located near intergranual boundaries. The fixation of direct positron lifetime 
components allows refining the most significant changes in positron trapping rate. 
 
Keywords: Water-sorption process, Intergranual boundaries, Positron trapping. 
 
  PACS numbers: 82.45.Хy, 92.60.Jq 
 
 
______________ 
* klymha@yahoo.com 
1. INTRODUCTION 
 
The spinel-structured MgAl2O4 ceramics are per-
spective materials for humidity sensors mainly due to a 
uniform porous structure, which promotes effective 
adsorption of great number of water molecules [1]. Re-
cently, it was shown that the amount of adsorbed water 
in these ceramics affects not only their electrical con-
ductivity, but also positron trapping modes of extended 
defects tested with positron annihilation lifetime (PAL) 
spectroscopy [1,2]. The positrons injected in the studied 
MgAl2O4 ceramics underwent two positron trapping 
with two components in positron lifetimes and ortho-
positronium o-Ps decaying, these parameters being 
obtained with a so-called three-component mathemati-
cal fitting procedure. Within this approach, the shortest 
component of the deconvoluted PAL spectra with posi-
tron lifetime 1 reflects mainly microstructure specifici-
ty of the spinel ceramics and the middle component 
with positron lifetime 2 corresponds to extended de-
fects located near intergranual boundaries. The third 
component with lifetime 3 is due to “pick-off” annihila-
tion of o-Ps in the nanopores. It is established that the 
adsorbed water molecules act catalytically on positron 
trapping in MgAl2O4 ceramics, do not changing signifi-
cantly o-Ps decaying modes [2].  
To refine the most significant changes in positron 
trapping in MgAl2O4 ceramics caused by water sorp-
tion, a new mathematical approach to the treatment of 
experimental PAL data should be developed in such a 
way to accumulate the catalytic effect in some non-
direct trapping parameters, while other direct compo-
nents (the reduced bulk and defect-related lifetimes, in 
the first hand) being left nearly constant. 
 
2. EXPERIMENTAL 
 
The studied spinel-type MgAl2O4 ceramics were sin-
tered from fine-dispersive Al2O3 and MgO powders us-
ing a special regime with maximal temperatures of 
1200 C, the total duration being 2 h [2]. 
The PAL measurements were performed with an 
ORTEC spectrometer with 22Na source placed between 
two ceramic samples (see Fig. 1) at 20 oC within row of 
relative humidity (RH) of 25-60-98-60-25 % using hu-
midistat PID+ (see Fig. 2). 
 
3.1
 
3.2
 
 ln = f(U) 
 
LT 
1 2 
 
  
  
 
 
 
1
2
4.1 4.2
5.1
 
5.2
  
67
8910
11
anode
anode
dynode
stopstart
  
12
 
Fig. 1 – Block-scheme of conventional sample-source “sand-
wich” arrangement for PAL measurements using the ORTEC 
apparatus [1]: 1 – foil-covered 22Na source, 2 – two identical 
samples, 3.1 and 3.2 – scintillators of γ-quanta, 4.1 and 4.2 – 
photomultipliers, 5.1 and 5.2 – constant fraction discrimina-
tors, 6 – delay line, 7 – time-pulse height converter, 8 – pre-
amplifier, 9 – amplifier, 10 – single channel analyzer, 11 – 
multichannel analyzer, 12 – personal computer. 
 
The selection of corresponding values for measuring 
chamber permit to investigation of samples at constant 
values of RH in the range of 25-60 % with an accuracy 
of ± 0,5 % and 60-98 % з with an accuracy of ± 1 %. The 
obtained PAL data were mathematically treated within 
three-component fitting procedure with fixed first and 
second positron lifetimes using LT computer program. 
Using formalism for two-state positron trapping model 
[1], the following parameters can be calculated: 
 
2b1
2
d
τ
1
τ
1
I
I
κ ,
2
2
1
1
21
b
τ
I
τ
I
II
τ
, 
21
2211
av
II
IτIτ
τ
,    (1) 
 
where d is positron trapping rate in defect, b –
 positron lifetime in defect-free bulk and av – average 
positron lifetime. The difference ( 2 – b) can be accept-
ed as a size measure of extended defects, as well as the 
2/ b ratio represents the nature of these defects [2]. 
 
3. RESULTS AND DISCUSSION 
 
It is established that the adsorbed water molecules act 
catalytically on positron trapping modes in MgAl2O4 
ceramics, do not changing significantly o-Ps decaying 
 
H. KLYM, J. FILIPECKI PROC. NAP 1, 01PCN43 (2012) 
 
 
01PCN43-2 
 
 
Fig. 2 – Schematic of humidity control instrument PID+: 
UВ – turn-on voltage, which  proportional to desired value of  
RH; Uс – output voltage of humidity sensor, which proportion-
al to desired value of RH in the chamber; ε(t) – difference be-
tween the set and necessary values: ε ~ UВ – Uс; ρ(t) –
 regulator voltage, which  proportional to the given portion of 
air in humidity; R – difference input in amplification regula-
tion: k1 (RK1), k2 (RK2); I – integration input in the general 
time regulation Ti (R6, C1), D –difference input in detection 
loss regulation Td (R8, C2),  –adder input from input voltage 
controlling by voltage amplifier: 
 
        
iT
d
i dt
tde
Tdtt
Tk
k
ttktu
01
2
1
)(
)(
1
)()()(
,     (2) 
 
where k1 = RK1/RA1, k2 = RK2/R3, Ti = C1·R6 and Td = C2·R8. 
These values for measuring chamber were experimental se-
lected based on previous calculations in accordance with Zieg-
lera-Nicholsa criteria. 
 
modes [2]. Nevertheless, refining the most considerable 
changes in positron trapping in the studied ceramics 
caused by water sorption is a difficult problem through 
a large quantity of arbitrary fitting parameters at 
treatment of PAL spectra. This task can be permitted 
due to the treatment of experimental PAL data at the 
fixed values of lifetimes 1 and 2. It is established that 
lifetime 1 reflects mainly microstructure specificity of 
MgAl2O4 ceramics [2]. The adsorption processes are not 
change the structure of these ceramics. The lifetime 2 
corresponds to extended defects near intergranual 
boundaries where ceramics are more defective. It is 
shown that the positrons are trapped in the same ex-
tended defects in MgAl2O4 independently of the content 
of absorbed water in their nanoporous [2]. 
Thus, the lifetimes of the first and the second PAL 
components ( 1 and 2) at the treatment of experimental 
PAL data can be considered nearly constant. Within 
this mathematical approach, all changes in the fitting 
parameters of these components will be reflected in 
their intensities (I1 and I2). The third longest compo-
nent with lifetime 3 is non-fixed. The treatment of ex-
perimental data was carried out at fixed lifetime values 
of 1=0.18 ns and 2=0.38 ns. Within this approach I1 
and I2 intensities of the direct PAL components are 
changes dependently from amount of adsorbed water in 
the studied ceramics. So increasing of RH from 25 to 
98 % result in decreasing of I1 intensity and increasing 
of I2 intensity. The changing of RH from 98 to 25 % 
reflects inverse to the previous direction in I1 and I2 
intensities (see table 1). The positron trapping in wa-
ter-filled defects reflecting the second component with 
I2 intensity occurs more intensive. 
Table 1 – PAL characteristics of MgAl2O4 ceramics 
 
RH, 
% 
Fitting parameters 
1, 
ns 
I1, 
a.u. 
2,  
ns 
I2, 
a.u. 
3,  
ns 
I3, 
a.u. 
25 0.18 0.79 0.38 0.20 2.37 0.01 
60 0.18 0.78 0.38 0.21 2.55 0.01 
98 0.18 0.76 0.38 0.23 2.27 0.01 
60 0.18 0.77 0.38 0.22 2.26 0.01 
25 0.18 0.78 0.38 0.21 2.21 0.01 
RH, 
% 
Positron trapping modes 
av, ns b, ns d, ns-1 τ2 - b, ns τ2/ b 
25 0.22 0.20 0.59 0.18 1.89 
60 0.22 0.20 0.62 0.18 1.87 
98 0.22 0.20 0.67 0.18 1.86 
60 0.22 0.20 0.64 0.18 1.87 
25 0.22 0.20 0.61 0.18 1.88 
 
The lifetimes 3 are closed to 2.2-2.5 ns (see Table 1). 
The input of this third component is not change and 
intensity closed to 0.01. Thus, this channel is non-
significant to water sorption-desorption processes. The 
positron trapping modes such as the average av, defect-
free bulk b and difference τ2 – b are non-changed with 
RH. In addition, the positron trapping centre (τ2/ b) is 
formed on a typical for MgAl2O4 ceramics level of 1.9 [2], 
which testify to the same nature of trapping sites 
whichever the content of absorbed water. In contrast, 
most significant changes in positron trapping in MgAl2O4 
ceramics caused by water sorption reflect in positron 
trapping rate in defect d. Thus, the catalytic water-
sorption effect in the studied spinel-structured ceramics is 
accumulated in non-direct trapping d parameter. 
 
4. CONCLUSION 
The fixation of all water-dependent positron trapping 
inputs allow to refine the most significant changes in 
positron trapping rate of extended defects and nanopores 
located near intergranual boundaries. The water 
sorption processes in nanoporous humidity-sensitive 
spinel-type MgAl2O4 ceramics leads to corresponding 
increase in positron trapping rates of extended defects 
located near intergranual boundaries. This catalytic 
affect has reversible nature, being strongly dependent on 
sorption water fluxes in ceramics. 
 
AKNOWLEDGEMENTS 
 
Halyna Klym is kindly grateful for assistance of-
fered by International Visegrad Fund. 
 
REFERENCES 
 
1. R. Krause-Rehberg, H.S. Leipner, Positron Annihilation in 
Semiconductors. Defect Studies (Springer-Verlag, Berlin-
Heidelberg-New York: 1999). 
2. H. Klym, A. Ingram, O. Shpotyuk, J. Filipecki, I. Hadzaman, 
phys. status soidi C 4 No3, 715 (2007). 


https://essuir.sumdu.edu.ua/bitstream/123456789/35038/1/princon_2012_1_1_45.pdf

Positron Trapping Effects in Water-filled Nanopores of Spinel Ceramics

Abstract

Similar works

Full text

Available Versions

Electronic Sumy State University Institutional Repository

SocioEconomic Challenges Journal (Sumy State University)