High-Resolution Transmission Electron Microscopy

Observation of Thermal Decomposition Process of

Ca-Deficient Hydroxyapatite

Koji NISHIO, Masato TAMAI, Mitsuhiro NAKAMURA and Toshiyuki ISSHIKI

Nano-Structural Science Laboratory,Kyoto Institute of Technology,

Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan

Hydroxyapatite

Hydroxyapatite (Ca10(PO4)6(OH)2: HAp)

• Crystal structure: Similar to a bone

Hexagonal (a = 0.943 nm, c = 0.688 nm)

⇓ widely noticed and expected

• As an alternative material for bone

• Application to a biosensor making

good use of biocompatibility

[001̄]

a

b

[100]b

c

Synthesis of Hydroxyapatite

Dry process⇒ Stoichiometric HAp ( s-HAp )

6CaHPO4 + 4CaCO3 → s-HAp + 2H2O + 4CO2

Wet process⇒ Non-stoichiometric HAp

• Sol-gel method

• Hydrolysis method

A. Nakahira et al., J. Am. Ceram. Soc. 82 (1999) 2029-2032

Hydrolysis Method

α-tricalcium phosphate (α-Ca3(PO4)2: α-TCP)

⇓ Hydrolysis in a mixture of water and alcohol

Calcium-deficient HAp(Ca10−Z(HPO4)Z(PO4)6−Z(OH)2−Z ⋅ nH2O (Z = 0-1):

Ca-def HAp )

Advantage of this method:• Easy to systhesis within several hours

under mild conditions• Easy to control of crystal morphology

plate-, blade-,whisker-like shape, etc.

⇓⇓⇓⇓⇓

⇓⇓⇓⇓⇓

⇓⇓⇓⇓⇓

0 h0 h0 h0 h0 h

1 h1 h1 h1 h1 h

2 h2 h2 h2 h2 h

3 h3 h3 h3 h3 h

α-TCPα-TCPα-TCPα-TCPα-TCP

α-TCPα-TCPα-TCPα-TCPα-TCP

Ca-def HApCa-def HApCa-def HApCa-def HApCa-def HAp

500 nm500 nm500 nm500 nm500 nm

Ca-Deficient Hydroxyapatite

Ca-def HAp → s-HAp + β-TCPImportant reaction to developnew biomaterials

Whisker-like shape is favorableas a source to produce porousbiomaterials

TEM observation of thewhisker-like shape, Ca-def HAp crystals

770 °C770 °C770 °C770 °C770 °C

900 °C900 °C900 °C900 °C900 °C

980 °C980 °C980 °C980 °C980 °C

1040 °C1040 °C1040 °C1040 °C1040 °C

1 µm1 µm1 µm1 µm1 µm

Experimental Procedure

Specimen

Source: Whisker-like shape, Ca-def HAp synthesized byhydrolysis method (Ca/P molar ratio = 1.58)

Heat Annealed at 200-1100 °C in airtreatment: (heating rate: 5 °C/min, keep time: 2-6 hours)

Equipment

TEM JEOL, JEM-2010/SP(EDS) Noran, VantageXRD Rigaku, RINT 2000FT-IR JEOL, IR-WINSPEC100

TEM images of samples before & after annealing

before annealingbefore annealingbefore annealingbefore annealingbefore annealing annealed at 600 °Cannealed at 600 °Cannealed at 600 °Cannealed at 600 °Cannealed at 600 °C

700 °C700 °C700 °C700 °C700 °C 800 °C800 °C800 °C800 °C800 °C 900 °C900 °C900 °C900 °C900 °C

500 nm500 nm500 nm500 nm500 nm

Ca-def HAp whiskersLength ( c axis): 2 ∼ 5 µmWidth: ∼ 0.1 µmCa/P molar ratio: 1.58Heating rate: 5 °C/minKeep time: 2 hours

Analysis of XRD patterns

20 30 40 50 60

Inte

nsi

ty [

a.u

.]

Diffraction angle 2θ [degree]

900 °C900 °C900 °C900 °C900 °C

800 °C800 °C800 °C800 °C800 °C

600 °C600 °C600 °C600 °C600 °C

before annealingbefore annealingbefore annealingbefore annealingbefore annealing

1100 °C

1000 °C

900 °C

800 °C

600 °C

400 °C

200 °C

before annealing

β-TCPHAp

500 nm500 nm500 nm500 nm500 nm

���*

-

-

-

Ca-def HAp whisker annealed at 800 °C

50 nm50 nm50 nm50 nm50 nm

HApHApHApHApHAp

HApHApHApHApHAp

HApHApHApHApHAp←←←←←→→→→→

→→→→→ [010][010][010][010][010]

100100100100100002002002002002

3 nm3 nm3 nm3 nm3 nm(100)(100)(100)(100)(100)

Planar defect /Layered precipitate

• traversing in the whisker

• parallel to (100) plane

Planar defect and layered phase

600 °C600 °C600 °C600 °C600 °C 700 °C700 °C700 °C700 °C700 °C 800 °C800 °C800 °C800 °C800 °C

[021][021][021][021][021] [010][010][010][010][010] [010][010][010][010][010]

20 nm20 nm20 nm20 nm20 nm

5 nm5 nm5 nm5 nm5 nm

↑↑↑↑↑planar defectplanar defectplanar defectplanar defectplanar defect

(100)(100)(100)(100)(100)

(100)(100)(100)(100)(100)

(001)(001)(001)(001)(001)

1.43 nm1.43 nm1.43 nm1.43 nm1.43 nm↓↓↓↓↓

↑↑↑↑↑

TEM-EDS microanalysisC

ount

s[a

.u.]

Energy [keV]

O

P

Ca

Ca

A (layered phase)

[021] 2 nm2 nm2 nm2 nm2 nm

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AAAAA

BBBBB

Cou

nts

[a.u

.]

Energy [keV]

O

P

Ca

Ca

B (Ca-def HAp)

atomic molar ratioarea Ca/P Ca P

[atom. %]A 1.86 65 35B 1.27 56 44

s-HAp 1.67 62.5 37.5

⇒ Ca-rich

Analysis of IR absorption spectra

Wave number [cm −1]

Tran

smitt

ance

[a.u

.]

3800 3300 2800

900 °C

800 °C

600 °C

400 °C

3570 cm−1

HHY 3538 cm−1

Wave number [cm −1]

Tran

smitt

ance

[a.u

.]

1400 900 400

900 °C

800 °C

600 °C

400 °C

6

744 cm−1

IR absorption spectrum of thesample annealed at 800 °C

⇓

• IR absorption peak at3570 cm−1 shifted to3538 cm−1

• IR absorption peak ap-peared at 744 cm −1

⇓

Ca-rich HApG. Bonel et al., Ann. NYAcad. Sci. 523 (1988) 115

Ca-rich metastable phase

The unidentified phase appeared in the annealed Ca-def HAp

TEM ⇓

600 °C: planar defect700-800 °C: layered phase

900 °C: not observed

⇓

Metastable phase

⇓

⇓ TEM-EDS, IR

EDS: higher Ca/P molarratio than of s-HAp

IR: shift & appearanceof absorption peaks

⇓

Ca-rich phase

⇓Ca-rich metastable phase

Relation between the Ca-def HAp and the phase

[010]HAp[010]HAp[010]HAp[010]HAp[010]HAp

[011]HAp[011]HAp[011]HAp[011]HAp[011]HAp

[001]HAp[001]HAp[001]HAp[001]HAp[001]HAp

(100)(100)(100)(100)(100)→→→→→ ←←←←←1.43 nm1.43 nm1.43 nm1.43 nm1.43 nm

2 nm2 nm2 nm2 nm2 nm

100100100100100

002002002002002

100100100100100

011̄011̄011̄011̄011̄

100100100100100010010010010010

Ca-def HAp

metastablephase

[010]

[011][001]

0.943 nm

0.688 nm

2.86 nm

Metastable phasea = 2.86, b = 0.943, c = 0.688 [nm]

(Orthorhombic)(100)HAp || (100)MSP[010]HAp || [010]MSP

Summary

• Thermal decomposition of the Ca-def HAp whiskers pre-

pared by the hydrolysis method begins at about 800 °C and

finishes at about 900 °C.

• Planar defects , often observed in the specimen annealed at

600-800 °C, are considered as a metastable phase .

• The results of EDS microanalysis and IR analysis suggest

the metastable phase is a Ca-rich phase .

• Lattice constants of the phase are analyzed into a = 2.86,

b = 0.943,c = 0.688 nmwith orthorhombic crystal system.