This article was downloaded by: [Indian Institute of Science]On: 10 January 2012, At: 20:19Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registeredoffice: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

FerroelectricsPublication details, including instructions for authors andsubscription information:http://www.tandfonline.com/loi/gfer20

Dielectric and Pyroelectric Properties ofHAp-BaTiO3 CompositesA. K. Dubey a , B. Basu a , K. Balani a , R. Guo b & A. S. Bhalla ba Laboratory for Biomaterials, Department of Materials Science andEngineering, Indian Institute of Technology, Kanpur, 208016, UP,Indiab MeMDRL, Department of Electrical and Computer Engineering,University of Texas at San Antonio, San Antonio, TX, 78249, USA

Available online: 14 Dec 2011

To cite this article: A. K. Dubey, B. Basu, K. Balani, R. Guo & A. S. Bhalla (2011): Dielectric andPyroelectric Properties of HAp-BaTiO3 Composites, Ferroelectrics, 423:1, 63-76

To link to this article: http://dx.doi.org/10.1080/00150193.2011.618382

PLEASE SCROLL DOWN FOR ARTICLE

Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions

This article may be used for research, teaching, and private study purposes. Anysubstantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,systematic supply, or distribution in any form to anyone is expressly forbidden.

The publisher does not give any warranty express or implied or make any representationthat the contents will be complete or accurate or up to date. The accuracy of anyinstructions, formulae, and drug doses should be independently verified with primarysources. The publisher shall not be liable for any loss, actions, claims, proceedings,demand, or costs or damages whatsoever or howsoever caused arising directly orindirectly in connection with or arising out of the use of this material.

Ferroelectrics, 423:63–76, 2011Copyright © Taylor & Francis Group, LLCISSN: 0015-0193 print / 1563-5112 onlineDOI: 10.1080/00150193.2011.618382

Dielectric and Pyroelectric Propertiesof HAp-BaTiO3 Composites

A. K. DUBEY,1 B. BASU,1 K. BALANI,1 R. GUO,2

AND A. S. BHALLA2,∗

1Laboratory for Biomaterials, Department of Materials Science and Engineering,Indian Institute of Technology, Kanpur-208016, UP, India2MeMDRL, Department of Electrical and Computer Engineering, University ofTexas at San Antonio, San Antonio, TX-78249, USA

In order to mimic the electrical properties of natural bone, the present work investigatedthe dielectric, AC conductivity, pyroelectric and piezoelectric properties of HA-40 wt%BaTiO3 (HA-26 vol% BaTiO3) and HA-60 wt% BaTiO3 (HA-44 vol% BaTiO3) compos-ites. Multistage spark plasma sintering was used to achieve the desired combination ofproperties. The electrical parameters were measured as a function of temperature andfrequency. The values of dielectric constant and loss for both the developed compos-ites, measured at room temperature and at 1 KHz frequency was 21, 38 and 0.01 and0.02, respectively. The AC conductivity for both the composites is found to be of theorder of 10−10 and 10−9 (cm)−1, measured under similar conditions. Activation energycalculated from σ ac vs. temperature plot for HA-40 wt% BaTiO3 is 0.50 eV. The roomtemperature pyroelectric coefficients for both the compositions are 2.35 and 21 µC/m2K,respectively. The piezoelectric coefficient values (d33) for both the compositions are 0.9and 1 pC/N, respectively. The observed values of electrical parameters closely resemblewith that of the natural human bone.

1. Introduction

Hydroxyapatite (HA) is the most widely investigated calcium phosphate biomaterial fororthopaedic applications, as it interacts with host bone by exchanging Ca2+ and PO4

3−

ions. This helps in stimulating the bone growth and enhances the integration of the implantmaterial with the host bone [1, 2]. Therefore, HA is recognized as highly biocompatibleand bioactive material. However, the poor electrical and mechanical properties limit itsapplication as an implant material. On the other hand, natural bone possesses significantpiezoelectricity [3], streaming potential [4] and pyroelectricity [5]. The electric potentialgenerated due to piezoelectric effect has an important role in new bone formation [6]. Itis, therefore, possible that the addition of a biocompatible piezoelectric component to HAmay improve the host response to that implant material.

In this perspective, BaTiO3 (BT), a piezoelectric material, seems to be potential bio-material as it can enhance the bone formation in complex physiological environment [7, 8].BT containing composite (PVDF-TrFE/BT) has also been found as in vivo biocompatiblematerial after testing in rabbit tibiae [9]. Apart from improving the electrical properties with

Received in final form June 7, 2011.∗Corresponding author. E-mail: amar.bhalla@utsa.edu

63

Dow

nloa

ded

by [

Indi

an I

nstit

ute

of S

cien

ce]

at 2

0:19

10

Janu

ary

2012

64 A. K. Dubey et al.

the addition of ferroelectric phase, Chen et al. [10] suggested that a piezoelectric secondaryphase can improve the toughness of composite due to the energy dissipation and domainwall’s motion. It is, therefore, expected that the addition of BT in HA may improve themechanical and electrical response of developed biocomposite.

In order to exploit the known bioactivity of HA and electrical activity of BT, two com-posites HA-40 wt% BT (H6B4) (HA-26 vol% BaTiO3) and HA-60 wt% BT (H4B6) (HA-44vol% BaTiO3) have been optimally processed using spark plasma sintering technique. Thepurpose of this study is to examine the dielectric, piezoelectric and pyroelectric propertiesof the developed composites and compared these properties with the natural bone. Thetemperature and frequency variation of dielectric constant, loss and AC frequency has alsobeen discussed.

2. Experimental

2.1 Materials Synthesis

HA is synthesized by widely reported suspension-precipitation route [11, 12]. HA powderswere then calcined at 1073 K for 2 hrs and the XRD peaks confirmed the pure hydroxyapatitephase.

For the preparation of BaTiO3, stoichiometric amounts of BaCO3 and TiO2 wereweighed and mixed in a ball mill using agate balls as grinding media and acetone as amilling medium for 6 hrs in agate jar. The mixed powders were dried overnight in oven.The dried powders were calcined at 1273 K for 6 hrs in a platinum crucible. For composites,HA-40 wt% BT (HA-26 vol% BaTiO3) and HA-60 wt% BT (HA-44 vol% BaTiO3) wereball milled for 16 hrs in wet medium (acetone) using agate balls and jar as milling media.The purpose of the milling was the homogeneous mixing of particles and reduction ofinitial particle size.

The multistage spark plasma sintering route was adopted to consolidate the powdercompact. The spark plasma sintering process can densify a ceramic body in a very shortduration and at a lower temperature [13]. The sintering of HA at a lower temperature(∼1273 K) was required to inhibit a deficiency of OH− ions in HA structures and this willhave an implication on electrical and biological properties. The sintering cycle is shown inFig. 1. In this approach, initially the powder compact was held for 5 min. at a temperatureof 0.4 Tm (Tm is the average melting point of both the phases) to promote the initialsurface activation, then heated to a temperature 0.5 Tm and hold it there for 5 min, followedby the heating to 0.57 Tm without holding. The samples were then furnace cooled. Theheating rate was kept at 373 K/min. Such processing condition has been reported to resultin uniform and better mechanical properties of the ceramics [14, 15].

2.2 Materials Characterization

The capacitance and loss were measured using an automated measurement system having anLCR meter (HP-4284 A, Hewlett-Packard). The measurements were done in the temperaturerange 173 K-423 K at frequencies between 100 Hz and 1 MHz. The samples were heated atheating rate of 5 K/min. The pyroelectric measurements were made using the Byer–Roundymethod [16]. The samples were poled while slowly cooling from 403 K temperature to 173 Kand applying a field of 10 kV/cm. After poling, the samples were electrically shorted for5 minutes before collecting the pyroelectric data. A picoammeter (HP- 4140B, Hewlett

Dow

nloa

ded

by [

Indi

an I

nstit

ute

of S

cien

ce]

at 2

0:19

10

Janu

ary

2012

Properties of HAp-BaTiO3 Composites 65

0 5 10 15 20 250

200

400

600

800

1000

Hea

ting

rate

(100

C/m

in)

950 C (0 min)

850 C (5 min)

670 C (5 min) Furnace

cooling

Time (min.)

Tem

pera

ture

(° C

)

(~300C

/min)

°

°

°

°

°

Figure 1. Sintering cycle used during multistage spark plasma sintering of composites.

Packard Inc.) was used to monitor the pyroelectric current produced by the samples, whileheating at a constant rate (∼3 K/min). The piezoelectric measurements were conductedusing Berlin Court d33 meter.

3. Results and Discussion

3.1 Dielectric Behavior

Plots of dielectric constant (εr) and dielectric loss (D) with temperature in the temperaturerange of 173–423 K at a few selected frequencies for both the compositions are shown inFig. 2. Plots of εr and D with frequency for these compositions at a few temperatures areshown in Fig. 3. It is observed that the permittivity value increases with increase in thetemperature at a given frequency. However, an increase of εr and D with temperature ismuch more significant at lower frequencies than at higher frequencies. This may be dueto the development of interfacial polarization in the presence of two phases with differentelectrical conductivities. The permittivity value decreases with increase in frequency andthe high frequency plateau is weakly dependent on temperature.

The relative permittivity exhibits a frequency dispersion in which the magnitude de-creases with increasing frequency. This may be due to the presence of conductivity in thematerial, most probably the ionic conduction at high temperatures. A common phenomenonthat occurs during the sintering of this composite is the partial decomposition of HA due tothe dehyroxyaltion (removal of OH− ions). At high temperatures OH− ions are responsiblefor conduction.

In Figs. 2(a) and (b) D vs. T plots for both the compositions show the similar behavior.At higher frequencies (>100 KHz), a low temperature plateau (upto 300 K) appears whichis followed by a temperature dependent region. The plateaus extend to higher temperaturewith increasing frequencies. In the D vs. log f plots, no peak is obtained in any of thecurves. Thus, relaxation time τ cannot be calculated from these plots. However, a diffusemaxima for high temperatures (373 and 423 K) is seen, which is the signature of some kindof relaxation occurring in the composite at 100 KHz.

Dow

nloa

ded

by [

Indi

an I

nstit

ute

of S

cien

ce]

at 2

0:19

10

Janu

ary

2012

66 A. K. Dubey et al.

Figure 2. Variation of dielectric constant (εr) and loss (D) with temperature at a few selectedfrequencies for composition (a) HAp-40 wt%BaTiO3 and (b) HAp-60 wt%BaTiO3.

The permittivities of the developed composites were calculated using parallel andseries Wiener bounds [17] using eq. (1) and (2)

εcomposite = VHAεHA + VBT εBT (1)

1

εcomposite

= VHA

εHA

+ VBT

εBT

(2)

Where, ε is the permittivity of the material and V is the volume fraction (HA and BT referto hydroxyapatite and BaTiO3). The calculated permittivity values for both the developedcomposites using eq. (1) are 295 and 490 (at 1 KHz), respectively. However, the calculated

Dow

nloa

ded

by [

Indi

an I

nstit

ute

of S

cien

ce]

at 2

0:19

10

Janu

ary

2012

Properties of HAp-BaTiO3 Composites 67

Figure 3. Variation of εr and D with log frequency at a few steady temperatures for composition(a) HAp-40 wt%BaTiO3 and (b) HAp-60 wt%BaTiO3.

permittivities of these composites by considering the series Wiener bound are 16 and 21(at 1 KHz), respectively.

Logarithmic mixture rule [18] for the dielectric constant of a polyphase material isgiven as eq. (3)

log ετ = iVi log εi (3)

Dow

nloa

ded

by [

Indi

an I

nstit

ute

of S

cien

ce]

at 2

0:19

10

Janu

ary

2012

68 A. K. Dubey et al.

Figure 4. Schematic illustration showing the connectivity between HA and BT phases with 0–3 con-nectivity. The interconnected (gray) phase represents HA whereas non- connected (white) representsBT phase.

This gives a value between the series and parallel Wiener bounds. The calculated permit-tivity values for both the developed composites using eq. (3) are 38.8 and 87.6 (at 1 KHz)respectively.

The electrical and mechanical properties of composite are influenced by the connec-tivity of the constituent phases [19]. It can be seen that there is a considerable decrease inpermittivity of the composite in comparison to high dielectric constant phase BT (∼1100at 1 KHz). This may be due to the isolation of ferroelectric phase by HA particles. Here, weconsider 0–3 connectivity between the ferroelectric and HA phase of the developed com-posite. Figure 4 represents the schematic diagram showing the 0–3 connectivity betweenboth the phases. Landauer [20] expression for 0–3 connectivity in a binary system is validfor large volume fraction of second phase and hence, should be appropriate for H6B4 andH4B6 composites.

The Landauer equation is

V1(ε1 − εc)

(ε1 + 2εc)+ V2

(ε2 − εc)

(ε2 + 2εc)= 0 (4)

Where, V1 and V2 are the volume fractions of two different phases, ε1 is the dielectricconstant of piezoelectric phase and ε2 is that of continuous phase, where ε2>>ε1. εc is thedielectric constant of the composite. For our case, the value of above expression for H6B4

at 100 Hz is 0.13 and 0.12 for higher frequencies. For H4B6, it is 0.24 at 100 Hz and 0.23for higher frequencies.

Jayasundere and Smith [21] derived an expression for dielectric constant of the com-posite having 0–3 connectivity by taking into account the interaction between piezoelectricspheres.

ε = ε1V1 + ε2V2[ 3ε1

ε2 + 2ε1

][1 + 3V2(ε2 − ε1)

ε2 + 2ε1]

V1 + V2[ 3ε1

ε2 + 2ε1

][1 + 3V2(ε2 − ε1)

ε2 + 2ε1

(5)

Dow

nloa

ded

by [

Indi

an I

nstit

ute

of S

cien

ce]

at 2

0:19

10

Janu

ary

2012

Properties of HAp-BaTiO3 Composites 69

As the interaction between the piezoelectric phases increases with increase in the concen-tration of second phase. The above expression is found to valid for higher volume fractionof the second phase.

The Maxwell- Garnet [22] expression for the effective permittivity (εeff) of a polyphasematerial is given as,

εeff =εmVm

[23 + εi

3εm

]+ εiVi

Vm

[23 + εi

3εm

]+ Vi

(6)

Where, Vm and εm are the volume fraction and dielectric constant of the matrix phaseand Vi and ε1 are the volume fraction and dielectric constant of the inclusion phase. Forthe present case, measured values of dielectric constant are not in full agreement with thevalues predicted by this theory.

Figure 5 represents the comparison of dielectric constant obtained from eq. (5) and (6)with the present study at a few selected frequencies at room temperature. It is observed thatfor H6B4, the values of dielectric constants are very close (<4% deviation at 1 KHz andhigher frequencies) to that obtained from eq. (5) for all the measuring frequency range.

The dielectric constant of the human cortical bone has been found to a very sen-sitive function of water content [23]. The dielectric constant of dry human corticalbone is around 10. However, the room temperature value of dielectric constant andloss for both the developed composites, H6B4 and H4B6 are 21, 38 and 0.01 and 0.02respectively.

3.2 AC Conductivity Behavior

AC conductivity was measured as a function of frequency and temperature. The plots oflog σ ac vs. log f at a few selected temperatures and log σ ac vs. 1000/T at a few selectedfrequencies are shown in Figs. 6 & 7. At low temperature, the conductivity is almostindependent of temperature and depends strongly on frequency. However, in the hightemperature region, the logσ ac varies linearly with temperature, obeying the Arrheniusequation, σ = σ 0 exp (−Ea/kT), where Ea is the activation energy for conduction. ForH6B4, a diffuse maxima is seen in log σ ac vs.1000/T plot, before the high temperaturelinear region for the frequencies up to 1 KHz. This may be due to the dielectric relaxation.The plot of log σ ac vs.1000/T at the frequency of diffuse maxima for H6B4 is shown inFig. 8. The plot is found to be linear. The value of activation energy from this plot is0.50 eV.

It is observed that the conductivity increases with increasing frequencies [Fig. 6(a)].The temperature up to which conductivity remains constant decreases with decrease infrequency [Fig. 6(b)]. Thus, it is inferred that at high frequency, conductivity is almostconstant at low temperature and increases only at high temperature. However, at lowfrequencies, conductivity increases with increasing temperature uniformly.

The experimentally recorded behavior of log σ ac vs. log f (at different steady temper-atures) for H6B4 and H4B6 is essentially similar. From the slope of the dispersion region, itis inferred that the temperature dependence of AC conductivity increases, but its frequencydependence decreases with increasing temperature. At high temperature, conductivity isalmost frequency independent, but linearly dependent on temperature.

AC conductivity with frequency varies linearly with frequency at all measuring tem-peratures. However, there is a little signature of plateau in the curve at higher frequencies,

Dow

nloa

ded

by [

Indi

an I

nstit

ute

of S

cien

ce]

at 2

0:19

10

Janu

ary

2012

70 A. K. Dubey et al.

Figure 5. Comparison of dielectric constant (εr) calculated from Jayasundare and Smith theory(eq. 5) and Maxwell-Garnet theory (eq. 6) with the present study.

which is beyond our measuring range [Fig. 6(a)]. It is therefore, evident that the linear(dispersion) region represents some relaxation process. In case of H4B6 [Fig. 6(b)], ACconductivity increases linearly with frequency in the measuring frequency range.

Although the composites were sintered at low temperature, but because of momentarygeneration of spark of plasma, local temperature is high [24, 25]. Due to this, a small lossof oxygen from BT or OH− ion from HA is possible. Such defects potentially rise to thelocalized energy levels between valence and conduction band.

The dependence of AC conductivity on frequency can be explained by the followingmechanisms [26]:

Dow

nloa

ded

by [

Indi

an I

nstit

ute

of S

cien

ce]

at 2

0:19

10

Janu

ary

2012

Properties of HAp-BaTiO3 Composites 71

Figure 6. Variation of log σ ac with (a) log frequency and (b) inverse of temperature for the compo-sition HAp-40 wt%BaTiO3.

(a) Transport by carriers excited to localized states near band edge and hopping atenergy close to it. Here, the dependence of AC conductivity with temperature isgiven by

σac α exp[−(EF − EA)/KT] (7)

(b) Hopping of charge carriers in the localized states near the Fermi level, the corre-sponding expression is

σac α T (8)

Shamos and Lavine [27] have reported that the electrical properties of the dry bone liesbetween the good insulator and poor semiconductor. The dielectric constant of dry bonesample at 100 KHz varies from 8 to 9.2 [28].

Dow

nloa

ded

by [

Indi

an I

nstit

ute

of S

cien

ce]

at 2

0:19

10

Janu

ary

2012

72 A. K. Dubey et al.

Figure 7. Variation of log σ ac with (a) log frequency and (b) inverse of temperature for the compo-sition HAp-60 wt%BaTiO3.

In the present study, the AC conductivity for both the composites is found to be ofthe order of 10−10 and 10−9 (cm)−1 respectively. This is the same order as reported byReinish et al.[29] for dry bone. AC conductivity of dry human femer is 10−10 Ω−1 cm−1.

3.3 Pyroelectric and Piezoelectric Behavior

The pyroelectric coefficient was calculated from the pyroelectric current by:

p = i

A[

dTdt

] (9)

Dow

nloa

ded

by [

Indi

an I

nstit

ute

of S

cien

ce]

at 2

0:19

10

Janu

ary

2012

Properties of HAp-BaTiO3 Composites 73

3.15 3.20 3.25 3.30 3.35 3.40 3.45 3.50

-10.5

-10.4

-10.3

-10.2

-10.1

-10.0

-9.9

-9.8

-9.7

1000/T (K)-1

log

σ ac(o

hm c

m)-1

HAp-40BTLinear fit

Ea= 0.50eV

Figure 8. Variation of AC conductivity with temperature at crossover frequencies for HAp-40wt%BaTiO3.

where p is the pyroelectric coefficient, i is the pyroelectric current, A is the electrodearea, and (dT/dt) is the heating rate. The polarization was calculated by the pyroelectriccoefficient:

Ps =∫

pdT (10)

Figure 9 shows the pyroelectric coefficient and spontaneous polarization for both thecompositions as a function of temperature. In case of H6B4 (Fig. 9(a)), the pyroelctriccoefficient reaches its maximum value, 8.6 µC/m2 K at temperature of 393 K. This tem-perature corresponds to the maximum polarization change and phase transition. The otherphase transitions are clearly noticeable at 331.6 K and 278.6 K, where the peak values ofthe pyroelectric coefficients are 4.3 µC/m2 K and 2 µC/m2 K, respectively. The maximumpolarization, as determined from pyroelectric measurement, was 0.84 mC/m2 at 100 K. Thepolarization then slowly decays to 0.76 at 174 K. With further increase in temperature, thepolarization decreases rapidly and finally, attains a small value.

In case of H4B6 (Fig. 9(b)), the phase transitions at 380 K and 270 K are clearly evident,where the peak value of the pyroelectric coefficients are 85 µC/m2 K and 29 µC/m2 K,respectively. The maximum polarization, as determined from pyroelectric measurement,was 13.82 mC/m2 at 100 K. The polarization is then slowly decays to 13.5 mC/m2 at 235.64K. With further increase in temperature, the polarization decreases rapidly.

The room temperature pyroelecric coefficient for both the compositions is 2.35 and21 µC/m2 K, respectively. Lang [5] reported the pyroelectric coefficient for dry bovine femurto be 0.0036 ± 0.0021 µC/m2 0C. The measured room temperature piezoelectric coefficientvalues (d33) for both H6B4 and H4B6 composites are 0.9 and 1 pC/N respectively, whichcompare well with that observed in bone (0.7 pC/ N) [30].

Dow

nloa

ded

by [

Indi

an I

nstit

ute

of S

cien

ce]

at 2

0:19

10

Janu

ary

2012

74 A. K. Dubey et al.

100 150 200 250 300 350 400 4500

2

4

6

8

10

x10-6

p (C

/m2 K

)

Temperature (K)

0

2

4

6

8

10x(-10-4 )

Ps (C

/m2)

100 150 200 250 300 350 400 4500

2

4

6

8

10

12

14

16

18x10-5

Temperature (K)

p (C

/m2 K

)

0

2

4

6

8

10

12

14x(-10-3)

Ps (C

/m2)

(a)

(b)

Figure 9. Variation of pyroelectric coefficient (p) and polarization (Ps) with temperature for(a) HAp-40 wt% BaTiO3 and (b) HAp-60 wt% BaTiO3.

4. Conclusion

The permittivity value increases more rapidly with temperature at lower frequencies(<1 KHz) than at higher frequency. This may be the result of interfacial polarization,which arises due to the presence of two phases with different electrical conductivities.The relative permittivity exhibits a frequency dispersion in which the magnitude decreaseswith increasing frequency. The frequency dispersion may be attributed to the presenceof conductivity in the material, most likely the ionic conduction at high temperatures.The room temperature value of dielectric constant and loss for both the developed com-posites was [21, 38] and (0.01, 0.02), respectively, measured at 1 KHz of frequency.The significant decrease in the permittivity of the developed composite in comparisonto high dielectric constant phase (BaTiO3) may be due to the isolation of ferroelectricphase by hydroxyapatite. In the present study, the AC conductivity for HA-40 wt% BaTiO3

and HA-60 wt% BaTiO3, measured at 1 KHz, is found to be of the order of 10−10 and

Dow

nloa

ded

by [

Indi

an I

nstit

ute

of S

cien

ce]

at 2

0:19

10

Janu

ary

2012

Properties of HAp-BaTiO3 Composites 75

10−9 (cm)−1, respectively. From pyroelectric coefficient measurement, it is confirmedthat the phase transitions in the developed composite HA-40 wt% BaTiO3 occurred attemperatures 393, 331.6 and 278.6 K. These temperatures correspond to the maximumpolarization change. The maximum polarization, as determined from pyroelectric mea-surement, was 0.84 mC/m2 at 100 K. In case of HA-60 wt% BaTiO3, the phase transitionsoccurred at 380 and 270 K are evident. The maximum polarization, as determined frompyroelectric measurement, was 13.82 mC/m2 at 100 K. The room temperature pyroelecriccoefficient for both the compositions is 2.35 and 21 µC/m2 K, respectively. The roomtemperature measured piezoelectric coefficient values (d33) for both the compositions are0.9 and 1 pC/N. From the above mentioned properties, it can be clearly inferred that thedeveloped composite can be a promising material for orthopedic applications, as far as theelectrical compatibility of the composite with the bone is concerned.

Acknowledgments

Thanks are due to Indo-US Joint Center on Biomaterials for Health Care, Dept. of Scienceand Technology, India and National Science Foundation (grant/0844081), USA for financialsupport.

References

1. M. Jarcho, Calcium phosphate ceramics as hard tissue prosthetics. Clin. Orthop. Relat. Res. 157,259–278 (1981).

2. L. L. Hench, Bioceramics. J. Am. Ceram. Soc. 81, 1705–1728 (1998).3. E. Fukada and I. Yasuda, On the Piezoelectric effect of bone. J. Phys. Soc. Jpn. 12, 1158–1162

(1957).4. G. V. B. Cochran, R. J. Pawluk, and C. A. L. Bassett, Electro-mechanical characteristics of bone

under physiologic moisture conditions. Clin. Orthop. Relat. Res. 58, 249–270 (1968).5. S. B. Lang, Pyroelectric effect in bone and tendon. Nature 212, 704–705 (1966).6. C. A. L. Bassett and R. O. Becker, Generation of electric potentials by bone in response to

mechanical stress. Science 137, 1063–1064 (1962).7. Y. J. Park, K. S. Hwang, J. E. Song, J. L. Ong, and H. R. Rawls, Growth of calcium phosphate

on poling treated ferroelectric BaTiO3 ceramics. Biomaterials 23, 3859–3864 (2002).8. J. Q. Feng, H. P. Yuan, and X. D. Zhang, Promotion of osteogenesis by a piezoelectric biological

ceramic. Biomaterials 18, 1531–1534 (1997).9. R. Gimenes, M. A. Zaghete, M. Bertolini, J. A. Varela, L. O. Coelho, and N. F. Silva, Smart

struct. and mat. 2004: Electroactive polymer actuators and devices (EAPAD). In: Bar-Cohen Yeditor. SPIE; 2004. p. 539.

10. X. M. Chen and B. A. Yang, New approach for toughening of ceramics. Mater. Lett. 33, 237–240(1997).

11. W. Rathje, Zur Kentnis de Phosphate: I. Uber. Hydroxyapatite. Bodenk Pflernah. 12, 121–128(1939).

12. M. H. Santos, M. Oliveira, L. P. F. Souza, H. S. Mansur, and W. L. Vasconcelos, Synthesiscontrol and characterization of hydroxyapatite prepared by wet precipitation process. Mater. Res.7, 625–630 (2004).

13. T. Nishimura, M. Mitomo, H. Hirotsuru, and M. Kawahara, Fabrication of silicon nitride nano-ceramics by spark plasma sintering. J. Mater. Sci. Lett. 14, 1046–1047 (1995).

14. K. M. Reddy, N. Kumar, and B. Basu, Innovative multi-stage spark plasma sintering to obtainstrong and tough ultrafine-grained ceramics. Scripta. Mat. 62, 435–438 (2010).

15. K. M. Reddy, N. Kumar, and B. Basu, Inhibition of Grain growth during final stage of multistagespark plasma sintering of oxide ceramics. Scripta. Mat. (in Press, 2010).

Dow

nloa

ded

by [

Indi

an I

nstit

ute

of S

cien

ce]

at 2

0:19

10

Janu

ary

2012

76 A. K. Dubey et al.

16. R. L. Byer and C. B. Roundy, Pyroelectric coefficient directs measurement technique and appli-cation to a nsec response time detector. Ferroelectrics 3, 333–338 (1972).

17. K. K. Karkkainen, A. H. Sihvola, and K. I. Nikoskinen, Effective permittivity of mixtures:Numerical validation by the FDTD method. IEEE Trans. Geosci. Remote Sens. 38, 1303–1308(2000).

18. W. D. Kingery, H. K. Bowen, and D. R. Uhlmann, Introduction to Ceramics, Second Edition,John Wiley and Sons; 1976, p. 497.

19. R. E. Newnham, D. E. Skinner, and L. E. Cross, Connectivity and piezoelectric-pyroelectriccomposites. Mat. Res. Bull. 13, 525–536 (1978).

20. R. Landauer, The electrical resistance of binary metallic mixtures. J. Appl. Phys. 23, 779–784(1952).

21. N. Jayasundere and B. V. Smith, Dielectric constant for binary piezoelectric 0–3 composites. J.Appl. Phys. 73, 2462–2466 (1993).

22. W. D. Kingery, H. K. Bowen, and D. R. Uhlmann, Introduction to Ceramics, 2nd ed. New York:Wiley Interscience; 1976, pp. 948–949.

23. A. A. Marino, R. O. Becker, and C. H. Bachman, Dielectric determination of bound water ofbone. Phys. Med. Biol. 12, 367–378 (1967).

24. M. Tokita, Trends in advanced SPS spark plasma sintering systems and technology. Joztrital ofthe Society of Powder Techtrology Jayat. 30, 790–804 (1993).

25. Z. A. Munir and U. A. Tamburini, The effect of electric field and pressure on the synthesis andconsolidation of materials: A review of the spark plasma sintering method. J. Mater. Sci. 41,763–777 (2006).

26. N. F. Mott and E. A. Davis, Electronic Processes in Non-crystalline Materials, Chapter 6.Clarenden, London; 1979.

27. M. H. Shames and L. S. Lavine, Physical bases for bioelectrie effects in mineralized tissues.Clin. Othop. 355, 177–188 (1964).

28. V. D. Shtraus and G. O. Pfafrod, Electrical relaxation properties of dry human compact bonetissue in the infra low frequency range. Mechanics of Composite Mat. 17, 716–720 (1981).

29. G. B. Reinish and A. S. Nowick, Effect of moisture on the electrical properties of bone. J.Electrochem. Soc. 123, 1451–1455 (1976).

30. J. B. Park and R. S. Lakes, Biomaterials: An Introduction, Plenum, New York; 1992.

Dow

nloa

ded

by [

Indi

an I

nstit

ute

of S

cien

ce]

at 2

0:19

10

Janu

ary

2012