# Effect ofγ-Irradiation on the Dielectric and Conductivity Properties of Nano-Wollastonite

Authors: Sandhya Kulkarni; B. M. Nagabhushana; Narsimha Parvatikar; C. Shivakumara; R. Damle
Journal: ISRN Materials Science
 (2011)

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## Abstract

Nanocrystalline porous CaSiO3 ceramic powders have been synthesized by a novel low-temperature initiated self-propagating, gas-producing solution combustion method. Single phase β-CaSiO3 (Wollastonite) is formed by calcination at 900°C for 3 h. The crystallization and phase formation temperatures in this method are found to be lower compared to the powder obtained via solid state reaction method. The powder is characterized by powder XRD (X-Ray Diffraction), and crystallite sizes are evaluated using Scherrer's formula as well as from TEM (Transmission Electron Microscopy). The Wollastonite powder is exposed to 60Co γ-radiation to accumulated doses of 1, 3, and 5 KGy and low-frequency (102–106 Hz) dielectric measurements are carried out before and after irradiation. The dielectric conductivity is estimated from the dielectric constant and loss tangent. Exposure to γ-radiation results in substantial modification in the properties of the ceramic powder due to changes in the porosity of the material. The correlation between dielectric and conductivity properties is discussed in relation to porosity effect.


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## Body

## 1. Introduction

Materials  with  porous  architecture  and  high  surface  area  are  known to have   numerous  potential  applications  in  nanotechnology.  Several attempts are being made to develop these materials for specific applications such as catalysis and separation  science,  molecular  sieves,  zeolites,  hybrid  optics,  bioceramics  and  many  more. Ceramic materials with porous structure like silicates of calcium can combine the advantages of both porous materials and ceramics as they have a combination of thermal, electrical, and dielectric properties which make them useful for technological applications. CaSiO3  is  one such   material  that   can  be  used  for the  production  of special  radio  ceramics,  sanitary  components,  porcelain  materials,  lining  bricks,  glaze,  and  flux [1]. CaSiO3  ceramics  are  also  used  as  a  biomaterial  in  medical  industry [2]. CaSiO3 in different phases have been synthesized by different routes and characterized earlier [3]. However, a complete understanding of the dielectric and electrical properties of these materials has not yet emerged. An understanding of the dielectric and electrical properties is important from the point of applications. Many times these materials are used for applications in radiation rich environments such as space, nuclear reactors and nuclear waste containers.   Exposure  to  radiation  can bring in modification  in  their  physical  properties  due  to  structural  changes. Thus, it is important to understand the effect of ionizing radiation on the properties of these materials.Enhanced dielectric properties of CaSiO3 ceramics at microwave frequency  range  have been reported  earlier by  Cai et al., Wang et al., and  Sun  et al. [4–6]. Cai et al. [4] have reported that CaSiO3  ceramics possess excellent dielectric properties with  relative dielectric constant εr = 7.0–9.0 and loss tangent  tanδ = (1.0–5.0) × 10−4.  Wang  et al. [5]  have showed that,  at  the  sintering  temperature  of  1320°C,  the  CaSiO3  nanopowders   prepared  by  sol-gel  method  exhibit  excellent  dielectric  properties with ε = 6.69 and quality factor  Qf = 25398 GHz. Sun et al. [6] have investigated a low-permittivity  microwave  dielectric  ceramic  system (Ca1−xMgx) SiO3.  Substitution  in  the  A-site  occurred  in  the  composition  range  of  0.1 ≤ x ≤ 0.9.  When  x = 0.1,  the  specimen  sintered  at  1290°C  for  2 h  exhibits  excellent  dielectric properties  with   ε = 6.49 and Qf = 62420 GHz. Although dielectric properties of CaSiO3 at microwave frequencies have  been investigated earlier, there appears to be hardly any studies on low-frequency  dielectric and conductivity properties of CaSiO3 ceramics and their structural modification upon exposure to ionizing radiation. The  present  investigation  was  carried  out  in  an  attempt  to  understand   the  effect  of  ionizing  radiation (γ-radiation) on the structural and low-frequency (102–106 Hz)  dielectric and conductivity  properties. To the best of our knowledge, this is the first ever attempt made to study the γ-irradiation effects on dielectric and conductivity properties of porous nano-β-CaSiO3 at low frequencies (audio-radio frequencies).


## 2. Experimental

The  CaSiO3  ceramic  powder is prepared  by   dissolving  stoichiometric  quantities of Analar grade calcium  nitrate (Ca(NO3)2·4H2O),  fumed  silica (SiO2 of   99.9% purity and  surface  area of 200 m2/gm),  and  citric  acid (C6H8O7,  which  acts  as  a fuel for redox reaction)  in  a  minimum  quantity of  water  and  dispersed  well  using  magnetic  stirrer  for  half  an  hour.  The  stoichiometry   of  the  redox  mixture  used  for  combustion  process  was  calculated  using  total  oxidizing  and  reducing  valencies  of  the ingredients  which  serve as numerical coefficients  for  the  stoichiometric  balance  so  that  the  equivalence  ratio  is unity  and  the  energy  released  by  the  combustion   is  maximum [7]. The  heterogeneous  redox  mixture  was  rapidly  heated  in  a  muffle  furnace  maintained  at   500 ± 10°C.  The  redox  mixture  when  heated  at  500°C  boils  and  thermally  dehydrates  forming  a  honeycomb-like  gel,  which  ignites  to  yield  voluminous  CaSiO3  powder.   Assuming  complete  combustion  of  the  mixture used, the  synthesis  of  CaSiO3  may  be  represented by reaction equation(1)9Ca(NO3)2(aq.)+9SiO2(S)+5C6H8O7(aq.)⟶9CaSiO3(S)+20H2O(g)+30CO2(g)+9N2(g).
X-ray  powder  diffractometer  (Phillips X’pert)  using  CuKα  radiation  with  a  Ni-filter  was  used  to  record diffractograms of the ceramic powders.  The  surface  morphology,  size  distribution  of  the  grains  were  examined  by  Scanning  Electron  Microscopy (SEM)  and  Transmission  Electron  Microscopy (TEM).  SEM photographs were taken with a Philips (XL30-ESEM) instrument.  A JEM 2000 Ex  instrument  was  used  to take   TEM photographs. The combustion  product  of  “as  formed” CaSiO3 is  ground  well to form a fine powder. The calcined  powder (at  900°C,  where  it  has  attained  the  single  phase of β-CaSiO3)  is  ground  well,  and  it  is pressed  into  pellets  of  2 mm  thickness  and  5 mm  diameter.   The ceramic samples were irradiated at room temperature by γ-radiation using 60Co γ-irradiator (Blood Irradiator-2000) to three different accumulated doses of 1, 3, and 5 KGy. The  dielectric  measurements  were  carried out  using  Hioki  impedance  analyzer  3532-50 (Japan)  in  the  frequency  range  102–106 Hz  at  room  temperature.  AC conductivity as a function of frequency is estimated from the dielectric measurement data.


## 3. Results and Discussion

### 3.1. Powder XRD Studies

The XRD patterns of “as formed”, sintered (at 900°C for 3 hours), andγ-irradiated CaSiO3  powders are shown in Figure 1. The XRD patterns reveal that “as formed”  CaSiO3 powder exhibits amorphous phase. The powder gives rise to a single β-CaSiO3  phase when calcined at 900°C for 3 h. The  peak  positions of β-CaSiO3  phase and  their hkl values  are  in  good  agreement  with  JCPDS  file no. 84-0655. It is found that the crystal system of single phase β-CaSiO3 is  monoclinic with  space group  P21/a.  The  particle  size  of  the  ceramic  powders  is  calculated  from  the  full  width  at  half  maximum  (FWHM)  of  the  diffraction  peaks  and  line  broadening  Scherrer’s  formula D=kλ/FWHM·cosθ,  where  λ  is  the  wavelength  of  X-ray,  θ is the  Bragg  angle, and k  is  the  constant  which  depends  on  the  grain  shape (0.89 Å  for  circular  grains). It  was  found  that  the  average  particle  size  of  the single phase β-CaSiO3 powder is  in the range of 20–30 nm.  After γ-irradiation to an accumulated dose of 1 KGy, extra peaks are observed in the diffractogram. Further, for higher dose of 5 KGy, still more peaks are observed. This observation appears to be unusual. It is normally believed that exposure of ceramic powders to ionizing radiation would result in decrease in crystallinity and increase in amorphous content as a result of disorder introduced due to generation of defects. In the present case, additional peaks are observed after γ-irradiation. This may be attributed to the sample going into a mixed phase (α- and β-CaSiO3) after irradiation.Figure 1
XRD patterns  of  CaSiO3 (i) as formed, (ii)  calcined at 900°C (β-CaSiO3), (iii) exposed to γ-radiation to an accumulated dose of 1 KGy, and  (iv) exposed to 5 KGy.


### 3.2. Transmission Electron Microscopy

Figure2 shows the TEM  image  of  β-CaSiO3.  It can be seen that  the  particles are agglomerated. The mean particle size is observed to be about 30 nm in agreement with the estimation from XRD measurements. The SAED (Selected Area Electron  Diffraction) pattern  showed in the inset displays bright  spots. This shows single crystal  nature of  β-CaSiO3.Figure 2
TEM image ofβ-CaSiO3 calcined at 900°C with SAED inset.


### 3.3. Scanning Electron Microscopy

The morphology  and  size distribution  of  calcinedβ-CaSiO3 before γ-irradiation and  β-CaSiO3 exposed to 5 KGy dose were examined using SEM images and are  shown  in  Figure 3. The image shows that the sintered sample before irradiation is porous. On the other hand, the irradiated sample exhibits sheet morphology consisting of regularly arranged nanorods due to chain like structure of (SiO4)−4 ions increasing the porosity [8]. Similar type of sheet morphology in SEM image of CaSiO3 has been reported earlier by  Zhou and Yan [9].SEM photographs ofβ-CaSiO3 (a) before  γ-irradiation and  (b) after γ-irradiation to an accumulated  dose of 5 KGy.
(a)(b)


### 3.4. Dielectric Measurements

Ceramic materials possess high values for static dielectric constant (ε) and are widely used in technological applications.  When used for making electronic components, the value of ε determines the degree of miniaturization of electronic circuits. The  dielectric  constant of a material is related to the polarizability α, in particular to the dipole  polarizability,  which  arises  from  structures  with  a  permanent  electric  dipole  that  can  change  orientation  in  an  applied  electric  field.   In metals, the electronic charge  is  delocalized  and  ε≪0.  In insulators, the charge is localized and  ε>0.  Materials  with  a  dielectric  constant  greater  than  that  of  silicon  nitride   (ε>7)  are  classified  as  high dielectric constant materials [10].  In general, polarization of a dielectric in the presence of an electric field comprises two factors: relaxation polarization and  resonance  polarization.  The  former  plays a major  role at lower frequencies and  responds  to  the  message  of  the sample  structure.  The latter plays an important part at higher frequencies. Microstructure difference, morphotropic phase boundary, pores,  defects, and so forth are the main sources  of  the  relaxation  polarization,  but  they  do  not  work  at  higher  frequencies [11].The static dielectric  constant  is  taken  asε=ε′(ω→0),  where  the  frequency dependent  complex  dielectric  function  is  ε*(ω)=ε′(ω)+iε′′(ω),  where  ε′(ω)   and  ε′′(ω) are the real and imaginary  parts, respectively.  The  dissipation  factor  is  referred  to as  the  loss  tangent  tanδ=ε′′/ε′,  where  the angle δ is  the  phase  difference  between  the  applied  electric  field  and  the  induced  current.  The dielectric loss is a measure  of the imaginary part (ε′′) of the complex dielectric function [12].Figure4 shows the variation of relative or static dielectric constant with frequency for β-CaSiO3 powders before and after γ-irradiation. Dielectric measurements (in the frequency range 102–106 Hz) before irradiation of β-CaSiO3 reveal that the dielectric constant decreases sharply in the KHz region and more slowly in the MHz region with increasing frequency.  This suggests that at low frequencies the electronic, ionic, dipolar,  and  space charge (surface charge)  polarizations  contribute  to  the  dielectric  constant.  However, in the MHz frequency range, the  contribution  from  the  space charge  polarization  is  minimized and  the  dielectric  constant  almost remain constant.  This  feature  is  similar  to  those  observed  in  some  dielectric  oxides,  such  as  Ba4LaMNb3O15 (M = Ti and  Sn) [13].  For an accumulated dose of 1 KGy, the dielectric constant increases, while for higher doses, it decreases. The variation of loss tangent with frequency shown in Figure 5 also behaves similar to dielectric constant. Increase in dielectric constant and loss tangent after exposure to a dose of 1 KGy γ-radiation may be attributed to  atomic displacements. The γ-radiation is known to interact with atoms of the solids by first generating secondary electrons which in turn produce atomic displacements. Atomic displacements produce various types of lattice defects. The defects influence macroscopic physical properties. In the present case, the off-centre displacement of Ca and Si ions from normal sites would lead to distortion of the structure. This may result in increase in polarization and hence increase in dielectric constant and loss tangent. Similar effects have been observed previousely by Darwish et al. in crystalline Co-Zn ferrites [14].  However, the decrease in dielectric constant and loss tangent after exposure to higher doses appears to be due to a different mechanism. Higher doses of γ-radiation may cause rearrangement of atoms and primary defects. The rearrangement would produce stable defects due to redistribution of γ-radiation energy over the sample. This would suppress the polarization properties and migration of defects. Such effects are more pronounced in binary (such as CaSiO3) and ternary ceramic materials as proposed by Mektieva [15]. However, the γ-radiation effects on the dielectric properties of ceramics would depend strongly not only on the γ-dose but also on the prior history of the material.Figure 4
Variation of dielectric constant with frequency forβ-CaSiO3.Figure 5
Variation of loss tangent with frequency forβ-CaSiO3.


### 3.5. Electrical Conductivity Measurements

Electrical conductivity should be low for dielectrics to show a good response.  In  general,  the  defect  centers   and  impurities  that  could  contribute  to  the  conductivity  are  randomly  distributed  in  dielectrics. Conduction  is  basically  due  to  a  hopping  of  electrons in  the  dielectric  materials.  In  the  case  of  ionic  solid such as CaSiO3,  the  excess  electrons  in  a  narrow  conduction  band  (or  excess  holes  in  a  narrow valence band)  due  to  their  interaction  with  lattice  ions  can distort  the  surroundings  in  such  a  way  that  a  potential  well   generated  is  deep  enough  to  introduce  localization  leading to the conductivity. Although conduction does occur in dielectrics, it is rather low. The  dielectric  conductivity  (σ)  can be  obtained  from  the  dielectric  constant  and  loss  tangent  using  the  relation [16](2)σ=ε′ε0ωtanδ=ε′′ε0ω,
where “ ε0”  is  the  dielectric  constant  of  free space,  and  ω=2πf  is  the  angular  velocity. Figure 6 shows the variation of conductivity as a function of frequency for  β-CaSiO3  before  and after γ-irradiation.  Both unirradiated and irradiated samples exhibit the same behavior, that is, high frequency dispersion and flat frequency insensitive conductivities at low frequencies. The nature of conductivity behavior can be explained using Almond-West type power law using single exponent [17](3)σ(ω)=σ(0)+Aωs,
where σ(0) is the DC conductivity which is the frequency independent part of σ versus ω plot, A is a constant,  and “s” is the power law exponent. The data were computer-fitted to the above equation. The value of exponent lies in the range 0.7 to 0.9. Figure 6 shows the fit of the conductivity data to power law. The DC conductivity (frequency independent plateau in the low frequency region) evaluated from the fit for the unirradiated sample is of the order of 10−9 mho cm−1. The frequency dependent conductivity can be explained as follows: in a hopping model, it is possible to distinguish different characteristic regions of frequency. In the low frequency region where the conductivity almost remains constant, the transport would take place on infinite paths. In the region of frequencies where the conductivity increases with frequency, the transport is dominated by contributions from hopping of infinite clusters [18]. The DC conductivity increases by a factor of 3 after exposure to 1 KGy, while for higher doses of exposure, the conductivity decreases. Since the dielectric constant and dielectric conductivity are directly related, both exhibit similar variation after exposure to γ-radiation. Similarly, the decrease in dielectric constant and hence dielectric conductivity at higher dose may be attributed to the porosity of the CaSiO3 powder. However, the effect of porosity on the dielectric conductivity needs further investigation. In semiconducting SrTiO3 doped with Y2O3, it is reported that increase in porosity has resulted in decrease in conductivity [19]. However, in polycrystalline NaNbO3 ceramic, porous counterpart is known to exhibit higher bulk conductivity than dense ones [20]. In any case, exposure of ceramic powders  to γ-radiation has resulted in modifications in the dielectric and electrical properties of nano-β-CaSiO3. The investigation thus reveals the possibility of employing γ-radiation in controlling the properties of ceramic powders for specific technological applications.Figure 6
Variation of  conductivity with frequency forβ-CaSiO3. The solid lines  are fit to power law equation.


## 3.1. Powder XRD Studies

The XRD patterns of “as formed”, sintered (at 900°C for 3 hours), andγ-irradiated CaSiO3  powders are shown in Figure 1. The XRD patterns reveal that “as formed”  CaSiO3 powder exhibits amorphous phase. The powder gives rise to a single β-CaSiO3  phase when calcined at 900°C for 3 h. The  peak  positions of β-CaSiO3  phase and  their hkl values  are  in  good  agreement  with  JCPDS  file no. 84-0655. It is found that the crystal system of single phase β-CaSiO3 is  monoclinic with  space group  P21/a.  The  particle  size  of  the  ceramic  powders  is  calculated  from  the  full  width  at  half  maximum  (FWHM)  of  the  diffraction  peaks  and  line  broadening  Scherrer’s  formula D=kλ/FWHM·cosθ,  where  λ  is  the  wavelength  of  X-ray,  θ is the  Bragg  angle, and k  is  the  constant  which  depends  on  the  grain  shape (0.89 Å  for  circular  grains). It  was  found  that  the  average  particle  size  of  the single phase β-CaSiO3 powder is  in the range of 20–30 nm.  After γ-irradiation to an accumulated dose of 1 KGy, extra peaks are observed in the diffractogram. Further, for higher dose of 5 KGy, still more peaks are observed. This observation appears to be unusual. It is normally believed that exposure of ceramic powders to ionizing radiation would result in decrease in crystallinity and increase in amorphous content as a result of disorder introduced due to generation of defects. In the present case, additional peaks are observed after γ-irradiation. This may be attributed to the sample going into a mixed phase (α- and β-CaSiO3) after irradiation.Figure 1
XRD patterns  of  CaSiO3 (i) as formed, (ii)  calcined at 900°C (β-CaSiO3), (iii) exposed to γ-radiation to an accumulated dose of 1 KGy, and  (iv) exposed to 5 KGy.


## 3.2. Transmission Electron Microscopy

Figure2 shows the TEM  image  of  β-CaSiO3.  It can be seen that  the  particles are agglomerated. The mean particle size is observed to be about 30 nm in agreement with the estimation from XRD measurements. The SAED (Selected Area Electron  Diffraction) pattern  showed in the inset displays bright  spots. This shows single crystal  nature of  β-CaSiO3.Figure 2
TEM image ofβ-CaSiO3 calcined at 900°C with SAED inset.


## 3.3. Scanning Electron Microscopy

The morphology  and  size distribution  of  calcinedβ-CaSiO3 before γ-irradiation and  β-CaSiO3 exposed to 5 KGy dose were examined using SEM images and are  shown  in  Figure 3. The image shows that the sintered sample before irradiation is porous. On the other hand, the irradiated sample exhibits sheet morphology consisting of regularly arranged nanorods due to chain like structure of (SiO4)−4 ions increasing the porosity [8]. Similar type of sheet morphology in SEM image of CaSiO3 has been reported earlier by  Zhou and Yan [9].SEM photographs ofβ-CaSiO3 (a) before  γ-irradiation and  (b) after γ-irradiation to an accumulated  dose of 5 KGy.
(a)(b)


## 3.4. Dielectric Measurements

Ceramic materials possess high values for static dielectric constant (ε) and are widely used in technological applications.  When used for making electronic components, the value of ε determines the degree of miniaturization of electronic circuits. The  dielectric  constant of a material is related to the polarizability α, in particular to the dipole  polarizability,  which  arises  from  structures  with  a  permanent  electric  dipole  that  can  change  orientation  in  an  applied  electric  field.   In metals, the electronic charge  is  delocalized  and  ε≪0.  In insulators, the charge is localized and  ε>0.  Materials  with  a  dielectric  constant  greater  than  that  of  silicon  nitride   (ε>7)  are  classified  as  high dielectric constant materials [10].  In general, polarization of a dielectric in the presence of an electric field comprises two factors: relaxation polarization and  resonance  polarization.  The  former  plays a major  role at lower frequencies and  responds  to  the  message  of  the sample  structure.  The latter plays an important part at higher frequencies. Microstructure difference, morphotropic phase boundary, pores,  defects, and so forth are the main sources  of  the  relaxation  polarization,  but  they  do  not  work  at  higher  frequencies [11].The static dielectric  constant  is  taken  asε=ε′(ω→0),  where  the  frequency dependent  complex  dielectric  function  is  ε*(ω)=ε′(ω)+iε′′(ω),  where  ε′(ω)   and  ε′′(ω) are the real and imaginary  parts, respectively.  The  dissipation  factor  is  referred  to as  the  loss  tangent  tanδ=ε′′/ε′,  where  the angle δ is  the  phase  difference  between  the  applied  electric  field  and  the  induced  current.  The dielectric loss is a measure  of the imaginary part (ε′′) of the complex dielectric function [12].Figure4 shows the variation of relative or static dielectric constant with frequency for β-CaSiO3 powders before and after γ-irradiation. Dielectric measurements (in the frequency range 102–106 Hz) before irradiation of β-CaSiO3 reveal that the dielectric constant decreases sharply in the KHz region and more slowly in the MHz region with increasing frequency.  This suggests that at low frequencies the electronic, ionic, dipolar,  and  space charge (surface charge)  polarizations  contribute  to  the  dielectric  constant.  However, in the MHz frequency range, the  contribution  from  the  space charge  polarization  is  minimized and  the  dielectric  constant  almost remain constant.  This  feature  is  similar  to  those  observed  in  some  dielectric  oxides,  such  as  Ba4LaMNb3O15 (M = Ti and  Sn) [13].  For an accumulated dose of 1 KGy, the dielectric constant increases, while for higher doses, it decreases. The variation of loss tangent with frequency shown in Figure 5 also behaves similar to dielectric constant. Increase in dielectric constant and loss tangent after exposure to a dose of 1 KGy γ-radiation may be attributed to  atomic displacements. The γ-radiation is known to interact with atoms of the solids by first generating secondary electrons which in turn produce atomic displacements. Atomic displacements produce various types of lattice defects. The defects influence macroscopic physical properties. In the present case, the off-centre displacement of Ca and Si ions from normal sites would lead to distortion of the structure. This may result in increase in polarization and hence increase in dielectric constant and loss tangent. Similar effects have been observed previousely by Darwish et al. in crystalline Co-Zn ferrites [14].  However, the decrease in dielectric constant and loss tangent after exposure to higher doses appears to be due to a different mechanism. Higher doses of γ-radiation may cause rearrangement of atoms and primary defects. The rearrangement would produce stable defects due to redistribution of γ-radiation energy over the sample. This would suppress the polarization properties and migration of defects. Such effects are more pronounced in binary (such as CaSiO3) and ternary ceramic materials as proposed by Mektieva [15]. However, the γ-radiation effects on the dielectric properties of ceramics would depend strongly not only on the γ-dose but also on the prior history of the material.Figure 4
Variation of dielectric constant with frequency forβ-CaSiO3.Figure 5
Variation of loss tangent with frequency forβ-CaSiO3.


## 3.5. Electrical Conductivity Measurements

Electrical conductivity should be low for dielectrics to show a good response.  In  general,  the  defect  centers   and  impurities  that  could  contribute  to  the  conductivity  are  randomly  distributed  in  dielectrics. Conduction  is  basically  due  to  a  hopping  of  electrons in  the  dielectric  materials.  In  the  case  of  ionic  solid such as CaSiO3,  the  excess  electrons  in  a  narrow  conduction  band  (or  excess  holes  in  a  narrow valence band)  due  to  their  interaction  with  lattice  ions  can distort  the  surroundings  in  such  a  way  that  a  potential  well   generated  is  deep  enough  to  introduce  localization  leading to the conductivity. Although conduction does occur in dielectrics, it is rather low. The  dielectric  conductivity  (σ)  can be  obtained  from  the  dielectric  constant  and  loss  tangent  using  the  relation [16](2)σ=ε′ε0ωtanδ=ε′′ε0ω,
where “ ε0”  is  the  dielectric  constant  of  free space,  and  ω=2πf  is  the  angular  velocity. Figure 6 shows the variation of conductivity as a function of frequency for  β-CaSiO3  before  and after γ-irradiation.  Both unirradiated and irradiated samples exhibit the same behavior, that is, high frequency dispersion and flat frequency insensitive conductivities at low frequencies. The nature of conductivity behavior can be explained using Almond-West type power law using single exponent [17](3)σ(ω)=σ(0)+Aωs,
where σ(0) is the DC conductivity which is the frequency independent part of σ versus ω plot, A is a constant,  and “s” is the power law exponent. The data were computer-fitted to the above equation. The value of exponent lies in the range 0.7 to 0.9. Figure 6 shows the fit of the conductivity data to power law. The DC conductivity (frequency independent plateau in the low frequency region) evaluated from the fit for the unirradiated sample is of the order of 10−9 mho cm−1. The frequency dependent conductivity can be explained as follows: in a hopping model, it is possible to distinguish different characteristic regions of frequency. In the low frequency region where the conductivity almost remains constant, the transport would take place on infinite paths. In the region of frequencies where the conductivity increases with frequency, the transport is dominated by contributions from hopping of infinite clusters [18]. The DC conductivity increases by a factor of 3 after exposure to 1 KGy, while for higher doses of exposure, the conductivity decreases. Since the dielectric constant and dielectric conductivity are directly related, both exhibit similar variation after exposure to γ-radiation. Similarly, the decrease in dielectric constant and hence dielectric conductivity at higher dose may be attributed to the porosity of the CaSiO3 powder. However, the effect of porosity on the dielectric conductivity needs further investigation. In semiconducting SrTiO3 doped with Y2O3, it is reported that increase in porosity has resulted in decrease in conductivity [19]. However, in polycrystalline NaNbO3 ceramic, porous counterpart is known to exhibit higher bulk conductivity than dense ones [20]. In any case, exposure of ceramic powders  to γ-radiation has resulted in modifications in the dielectric and electrical properties of nano-β-CaSiO3. The investigation thus reveals the possibility of employing γ-radiation in controlling the properties of ceramic powders for specific technological applications.Figure 6
Variation of  conductivity with frequency forβ-CaSiO3. The solid lines  are fit to power law equation.


## 4. Conclusion

Crystalline  CaSiO3  ceramic  powder  synthesized  by solution  combustion  route exhibits a single  β-phase when calcined at  900°C in the nanoregime  with a particle size of the order of 20–30 nm.  The powder exhibits a quite high dielectric constant as well as dielectric conductivity at low frequency. Exposure of the nanopowder to γ-radiation results in substantial changes in the dielectric and conductivity perhaps due to increase in porosity of the material. However, detailed investigation would be required to ascertain the effect of porosity on the properties of the material. The investigation reveals the possibility of employing the ionizing radiation to tailor the dielectric and electrical properties of the Wollastonite.



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*Source: 808560-2011-07-13.xml*