Toshiyuki ISSHIKI, Mitsuhiro NAKAMURA, Masato TAMAI and Koji NISHIO Kyoto Institute of Technology

1. Seminar on Nanotechnology for Fabrication of Hybrid Materials, 6-8, Nov., 2002, Toyama, Japan(4th Japanese-Polish Joint Seminar on Materials Analysis) Investigation of Thermal Decomposition Process of Hydroxyapatite Crystals by In-Situ Scanning Electron Microscopy and Cathodoluminescence Microscopy Toshiyuki ISSHIKI, Mitsuhiro NAKAMURA, Masato TAMAI and Koji NISHIO Kyoto Institute of Technology

2. Equipment for High Temperature In-Situ SEM Observation Heating stage using direct heating method. Problems and their solutions for the in-situ SEM observation. Thermal Decomposition Process of Hydroxyapatite Direct observation of morphology change in thermal treatment. Nano precipitates created in electron beam irradiation. Contents

3. Direct heating method for TEM (developed by Kamino and Saka). Specimen is mounted on a narrow tungsten filament (20mmf) and heated directly by current through the filament. Simple and Small heating unit →Small thermal capacity Reachable temperature is over 1500oC with small current. Temperature and specimen drift are settled in a short time. Difficult to measure precise temperature. (×thermocouple) ◎ Non-contact method with radiation thermometer. Heating Stage for In-situ Observation of High Temperature Reactions

4. Disturbance of image detection Saturation of secondary electron detector caused by Incident light to photon multiplier tube (PMT) Thermal electron emitted from the filament Problem of High Temperature In-Situ SEM Influence of the incident light • Secondary electrons are converted with scintillator to blue light, and then detected with PMT. (ET-detector) • Strong light from thermal filament saturates the PMT. • Arrange the filament not to face the detector. • Cut off the light emitted from the filament with optical filter.

5. Around 1,000oC, thermal electrons emitted from a filament increase about tenfold as temperature rises at each 100oC. The thermal electrons saturate an SE-detector and contrast of SEM images decrease. Energy of thermal electrons →less than 1 eV Energy of secondary electrons　→　around a few tens eV Electrostatic filter is effective to separate these electrons. Influence of Thermal Electrons Emission density of thermal electrons from tungsten filament. Energy distribution of thermal electrons.

6. Design of In-situ Heating System for SEM • Key points of the system • Dichroic filter  Cutting off the light from filament • Thermal electron filter  Suppression of the thermal electrons • Radiation thermometer  Precise measurement of temperature Schematic illustration of heating system for in-situ SEM observation.

7. Thermal electron filter 20mmf tungsten wire wounded on the frame with 70mm(W) x 10mm(H) at 4turns/mm. Placed between the filament and the detector Disposable heating stage Light bulb removed grass cover Specimens are mounted on and between tungsten filament Industrial mass product  Easy to get,good uniformity and low price Overview of the Heating Unit Overview of heating stage equipped with thermal electron filter. Micrographs of heating stage (Light bulb removed grass cover).

8. Effect of Thermal Electron Filter withthermal electron filter loaded -20V Specimen: SiC particles • Good contrast images can be obtainedover 1400oC by using thermal electron filter, while it becomes difficult to observe images without the filter above 1300oC. • There is no need to re-adjust brightness and contrast of imagesas temperature changes. This make possible to record images with short intervals. withoutthermal electron filter Accel. voltage: 15 kV Probe current: 1 nA Magnification: x10,000

9. Calcium deficient hydroxyapatite (Ca10-Z(HPO4)Z(PO4)6-Z(OH)2-Z·nH2O, (Z=0~1): Ca-def HAp) above 800oC Stoichiometric HAp ((Z=0): s-HAp) ＋ b-tricalcium phosphate (b-Ca3(PO4)2: b-TCP) Thermal reaction of Ca-deficient hydroxyapatite The nano-composites composed of s-HAp and b-TCP, especially having porous morphology, show high bioactivities. They are taken a great interest as important bio-ceramics.

10. Experimental • Synthesis of Ca-def HAp whisker • Prepared by hydrolysis of a-tricalcium phosphate (a-Ca3(PO4)2) in octanol/water binary emulsion.   • In-situ SEM observation • JEOL JSM-845 equipped with the heating stage for in-situ observation. • How to change their morphology in thermal treatment. TEM image of Ca-def HAp before thermal treatment. JEOL JSM-845 scanning electron microscope.

11. Morphology Change of HAp Whiskers • There is no morphology change of whiskers below 800oC. • The morphology of whiskers began to change around850oC. Thermal decomposition proceeds in this temperature range. • The whiskers united each otherabove 900oC. The whiskers deformed into gnarled shape. • Above 1000oC, shape of whisker was lost and gnarled whiskers changed into round shape particles. In-situ observation of sintering process of Ca-def HAp whiskers.

12. Sintering into Porous Body from HAp Whiskers • The morphology of the whisker began to change around850oC. • The whiskers coalesced each other to form porous nano-composite composed of s-HAp and b-TCP above 900oC. • Each grain of the composite became large above 1000oC. Porosity of the composites decreased rapidly. • Heat treatment below 1000oC is preferred to obtain high-porosity composite. In-situ SEM observation of sintering process from aggregates of whisker-shaped Ca-def HAp into porous body.

13. Nano Particles Precipitated on Ca-def HAp HAp • Deformation of the whiskers also began around 850oC even under dense electron beam irradiation. • A lot of fine particles a few tens nm in size precipitated on the whiskers near 900oC and the particles grew over a hundred nm in size with temperature increasing. • Above 1100oC, the precipitated particles disappeared simultaneously with the b-TCP particles. The particles were considered to be b-TCP. b-TCP Decomposition process of Ca-def HAp whiskers under dense electron beam irradiation. Round-shaped particles are b-TCP to check difference of reaction between HAp and b-TCP.

14. The similar fine precipitates were observed on s-HAp crystals, even though s-HAp particles are usually stable at this temperature range. It is considered that electron beam irradiation makes Ca vacancies inside the s-HAp crystals and they decompose as well as Ca-def HAp crystals. Nano Particles Precipitated on Stoichiometric HAp Decomposition process of s-HAp under dense electron beam irradiation.

15. The particles showed blue CL emission which color was the same as that obtained from pure b-TCP powder. The precipitates were confirmed to beb-TCP Cathodoluminescence observation of Precipitates Nano-particles precipitated froms-HAp. Cathodoluminescence image of the nano-precipitates.

16. Summary • Thermal decomposition process of HAp was investigated by in-situ scanning electron microscopy with the aid of cathodo-luminescence microscopy. • Direct heating method was applied to heating stage for in-situ SEM. • It was revealed that the formation process of porous nano-composites of s-HAp and b-TCP and the relationship between the annealing temperature and morphology of the composites. • Nano-size b-TCP particles precipitate above 850oC not only on Ca-def HAp whiskers but also on s-HAp particles under dense electron beam irradiation. It is considered that the irradiation induces Ca vacancies in the HAp crystal and they act as nucleation sites of b-TCP.