formation of β-TCP at 800 ^C, whereas no such peak occurred in HA even up to

1000 ^C. In contrast, the substitution like Eu and F resulted in an increased thermal

stability. Similarly, Si substitution and K and Si co-substitution resulted in an enhanced

thermal stability in comparison with stoichiometric HA (Table 23.7). Zn and F

co-substitution in HA resulted in a decreased thermal stability with a total weight loss

of 25.09%. Co-substitution of strontium and fluorine in HA enhanced its thermal

stability. Only 7.56% total weight loss occurred in SrFHA nanopowders. MgSrHA

nanopowder exhibited only 1.8% weight loss after 700 ^C indicating its higher thermal

stability than MgHA. Among all the as-synthesized nanopowders, MgSrFHA

nanopowder exhibited highest thermal stability with a total weight loss of 5.28%.

23.5.4 Surface Area of Novel Hydroxyapatites

The novel hydroxyapatites have potential application as implants, and therefore their

mechanical properties are important, and these depend on the porosity of powders.

The BET surface area of nanopowders was evaluated by N2 adsorption using

Quantachrome Instruments NOVA 2200e Surface Area Analyzer employing

Brunauer-Emmett-Teller (BET) method (Joseph and Tanner 2005). The BET equa-

tion in linearized form is expressed as:

p

v p0
p

ð

Þ ^{¼ 1}

vmz ^{þ z
1}

vmz

p

p0

ð23:6Þ

where z is a constant related to energy of adsorption, p/p0 is the relative vapor

pressure of adsorbate, v is the volume of gas adsorbed, and vm is the volume of gas

adsorbed in a monolayer. The minimum resolution for p/p0 was 2  10
⁵. A linear

regression of the left side of the BET equation and p/p0 yielded a slope and intercept

from which z and vm were obtained. Table 23.8 presents the BET surface area of

nanopowders as calculated from vm (Currey 2005). Interestingly, ionic substitution

lowered the surface area of HA nanopowders, indicating that substituted

hydroxyapatites are denser than stoichiometric HA. The order of the BET surface

area of nanopowders is as follows:

HA > ZnHA > FHA > SrFHA > MgHA > KSiHA > SiHA > MgSrFHA

> MgSrHA

23.5.5 Bioactivity of Novel Hydroxyapatites

In vitro bioactivity estimation of nanopowders was performed in a simulated body

fluid (SBF). The various reagents used in the preparation of SBF are given in

Table

23.9.

SBF

has

ionic

composition

matching

human

blood

plasma

(Table 23.10).

23

Unleashing Potential of Bone Mimicking Nanodimensional Hydroxyapatites and. . .

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