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TEM-based techniques as a powerful tool in development of ceramics materials

Yingda Yu, Inger-Lise Tangen, Tor Grande, Ragnvald Høier and Mari-Ann Einarsrud

IMT & Fysikk, NTNU

SUP Seminar on High Performance Ceramics and Heterogeneous Materials

13.06.2003 Trondheim

2 TEM only checked a small part of samples, i.e. a poor sampling

instrument. (►interdiscipline cooperation)

Which type information can be obtained from a TEM

TEM can provide both of materials morphology and microstructure information at the same time.

TEM is a powerful tool to reveal materials phenomena at atomic level with extremely higher resolution ability.

TEM is a powerful tool to reveal materials phenomena at atomic level with extremely higher resolution ability, together with chemical composition information.

0.27 nmX 6 000 000

3Interface dislocations

Microstructure Characterization of Epitaxial Grown Thin Film

The epitaxial growth relationship can be identified according to HRTEM and SAED results, and the misfit parameter can be calculated as 3.47% between the epitaxial grown LaFeO3 thin film and LaAlO3 substrate.

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Microstructure characterization related with AlN thermal conductivity

Properties of AlN

High thermal conductivity

Electrical isolator

High Hardness and strength

Excellent corrosion resistance

Good oxidation resistance

High thermal stability

High thermal conductivity

In this SUP subproject, searching for possible application AlN as sidelining materials in aluminium electrolysis, it is important to understand the relationbetween the sintered microstructure and its thermal conductivity.

Experimental details and selected properties of AlN samples

Sample code

Wt % Y2O3

Sintering time (h)

Sintering condition

Thermal Con. (W/mK)

Lattice oxygen (wt %)

80F 0.8 2 A 91 0.56 82F 2.3 2 A 130 0.43 84F 3.9 2 A 144 0.63 86F 7.7 2 A 148 0.84 81F 0.8 2 B 101 0.55 83F 2.3 2 B 142 0.43 85F 3.9 2 B 148 0.43 87F 7.7 2 B 144 0.97 90 0.8 2 C 105 0.53 91 0.8 6 C 119 0.28 92 2.3 2 C 145 0.39 93 2.3 6 C 163 0.19 94 3.9 2 C 154 0.52 95 3.9 6 C 180 0.20 96 7.7 2 C 160 0.49 97 7.7 6 C 192 0.15

Aase Marie Hundere PhD Thesis NTH 1995

SEM and TEM micrographs show the different distributions of secondary phases in the samples.

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The presence of secondary phases along AlN grain boundaries disrupts the connections between high thermal conductivity AlN grains. In addition, the thin amorphous layer between the secondary phase and the AlN grin will further reduce the thermal conductivity .

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Microstructural changes were found to disrupt the connectivity of the AlN grains, resulting in a decrease in the thermal conductivity of the materials.

Microstructure characterization related with AlN thermal conductivity

From GB

Al K Al K

N K Y K

O K Inside Grain

EDS spectra obtained using 0.6 nm probe from AlN grain boundary (GB) and inside grain in FEG-TEM.

Representative HRTEM images of vertical AlN GBs, with AlN grains close to low index planes.

No substantial differences were seen between grain boundaries (GBs) in the two representative samples

SEM and TEM micrographs show the different distributions of secondary phases in the samples.

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TEM Characterization of Pressureless Sintered AlN(CaO) Ceramics

To search new and improved materials to apply in Hall-Héroult aluminium electrolysis cells as alternative side linings materials, which demands more cheap AlN materials.

In the investigation, the low cost additive (Calcium aluminates C12A7, 12CaO-7Al2O3) and low-temperature sinter route to form dense material are explored. Eirik Haugen PhD Thesis NTNU 2000

TEM results confirmed that the high densification (>95%) AlN material was obtained by using 8 wt% C12A7 sintering additive with AlN fine staring powders at 1650C.

CA C12A7 C3ACA2CA6

TEM results confirmed that the high densification (>95%) AlN material was obtained by using 8 wt% C12A7 sintering additive with AlN fine staring powders at 1650C.

Crystallized secondary phases are located at the triple junctions and are determined as CA (CaAl2O3) phase through serise SAED patterns from large angle tilt.

TEM results confirmed that the high densification (>95%) AlN material was obtained by using 8 wt% C12A7 sintering additive with AlN fine staring powders at 1650C.

Crystallized secondary phases are located at the triple junctions and are determined as CA (CaAl2O3) phase through serise SAED patterns from large angle tilt.

HRTEM image reveal that the AlN grains are connected with a relative clean grain boundaries (GBs), which is important to have a higher corrosion resistance towards cryolite (anti-intergranualar corrosion IGC).

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However, only small amounts of crystalline CA secondary phases are found by XRD analysis.

TEM and EDX composition results reveal that secondary phases consist with the crystallized CA phase (CaAl2O4) and high Ca-containing amorphous C12A7 phase.

TEM Characterization of Pressureless Sintered AlN(CaO) Ceramics

CA C12A7 C3ACA2CA6

However, only small amounts of crystalline CA secondary phases are found by XRD analysis.

For further study the chemistry of the secondary phases in CaO-Al2O3 system, the sample was prepared in an inner Mo crucible with a Mo lid sintered for 24 h in a graphite furnace under a slight N2 overpressure.

14 wt% C12A7 (Calcium aluminates C12A7, 12CaO-7Al2O3) used as sintering additive, together with AlN coarse starting powder.

Considering the mass balance due to the Al2O3 on AlN powder surfaces, an Al2O3 contents of ~55 wt% is expected.

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20vol%TiN/AlN composite

4 m

Nano-pores (arrowed) appeared as increasing TiN to 20vol%

Inger-Lise Tangen PhD Thesis NTNU 2002

10vol%TiN/AlN

TEM Characterization of AlN-TiN Composite Ceramics For understanding the relationships between microstructures and sintering conditions to lead to improving AlN Composite mechanical properties. AlN-TiN samples are sintered by means of the eutectic Y-Al-O reaction for lowing the sintering temperature.

The smaller (<1μm) TiN particles are located at intragranular positions while large TiN particles are at intergranular positions of AlN matrix. 10vol%TiN/AlN

5 nm

AlN matrix

TiN particle

HRTEM reveals the amorphous grain boundary between AlN and TiN,

The local residual strain contrasts cause by the thermal expansion mismatch between AlN and TiN.

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AlN-SiC Solid Solution and AlN-SiC Composite Ceramics

For further improving AlN flexural strength and fracture toughness, the in-situ formed SiC reinforced AlN materials are investigated.

AlN-SiC Solid Solution formed as SiC content below 10vol%.

As further increasing SiC powders contents, the average grain size decreased from 5-8 down to 3-5 m.

The high density com-posite obtained under a flowing nitrogen atmosphere in a graphic furnace at 1880 C

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Elongated SiC grains associated with SiC polytype are formed along AlN matrix GBs during sintering.

EELS chemical mapping (from above white square area) used to understand the chemistry of secondary phase region.

HRTEM and Moiré fringe tech-niques can be used to investigate SiC polytype details and SiC re-nucleation in SiC/AlN composite.

Moiré fringe

SiC Polytype is formed by different stacking sequences

AlN Polytype is modified by different AlO6 octahedral layers.

AlN-SiC Solid Solution and AlN-SiC Composite Ceramics

For further improving AlN flexural strength and fracture toughness, the in-situ formed SiC reinforced AlN materials are investigated.

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HRTEM reveals the detailed AlN polytype phase information (such as here as intergrowth with 24H, 33R and 39R).

Two new AlN polytype phases 39R and 24H were identified for the first time in the AlN-Al2O3 system.

Fine phase identification of the elongated AlN polytype ceramic

The elongated grown AlN polytype grains are prepared for improving AlN fracture toughness by pressureless sintering with different Al2O3 additive contents 1950°C

The AlN polytype phase development in a series of sample with increasing Al2O3 contents in the system AlN-Al2O3-Y2O3 has been inverstigated.

nR polytype, consists of three rhombohedra related sub-blocks.

nH polytype, consists of two hexagonal related sub-blocks.

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Fine phase identification of the elongated AlN polytype ceramic

The elongated grown AlN polytype grains are prepared for improving AlN fracture toughness by pressureless sintering with different Al2O3 additive contents 1950°C

The new 24H AlN polytype phase was identified for the first time in the AlN-Al2O3 system.

The polytype structures can be decribed as flat inversion domain boundary (IDB) complexes consisting of an aluminium surrounding by six oxygen atoms (octahedron) to form along AlN basal (001) planes.

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Microstructure Characterization of AlN Fiber

Aluminum nitride whisker was prepared by nitridation of aluminium foil at 1500 C under 250 mbar N2 gas pressure for 5 hrs.

Both of SAED pattern (streak lines) and HRTEM provide the microstructure information of the whisker-like growth relationship, with AlN (001) perpendicular to the growth direction.

Amorphous covered surface

Configuration Fiber Tip

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The possible growth mechanisms

L

S

Growth direction: [001] Growth direction: perpendicular to [001]

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Conclusion Remarks

TEMs used in this SUP project

Philips CM30, operated at an accelerating voltage 300 kV

JEOL FEG TEM, operated at an accelerating voltage 200kV and 300 kV

TEM-based techniques is an powerful tool in development of ceramics materials, i.e. to understand the relationships between structures and sintering conditions and lead to improve the material mechanical property (material design).

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Acknowledgement

The sub-Sup project was fully supported by the Norwegian Research Council.

FunMat

NanoMat

Future work

TEM related techniques would be expected one of the most effective tools to provide both microstructual infomation and chemical composition in the new-funeded nanotechnology projects,