Results

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 A. Results obtained in 2017 (scientific report):

I. Preparation by the “acetate” route of the sol-gel method of Ce3+– doped BaTiO3 (BCT) nanopowders

I.1. Preparation by the “acetate” route of the sol-gel method of Ce3+– doped BaTiO3 (BCT) nanopowders

Fig

Fig. I.1. The flowchart for the preparation of BCT powders by the “acetate” route of the sol-gel method.

I.2. Complex characterization (phase purity, structure and morphology) of Ce3+ doped-BT (BCT) nanopowders synthesized by the “acetate” route of the sol-gel method

Fig Fig

(a) (b)

Fig

(c)

Fig. I.2. (a) XRD patterns of the stoichiometric powders described by the formula Ba1-xCexTiO3, prepared by the sol-gel method; (b) detail (the green, dashed rectangle of Fig. I.2(a)) of the region corresponding to the diffraction angles 2 = 25 – 30.5o, showing the presence of small amounts of secondary phases at the detection limit and (c) detail (the purple, dashed rectangle of Fig. I.2(a)) of the region corresponding to the diffraction angles 2 = 44,5 – 46,5o, proving the A-site incorporation of Ce3+ in the perovskite lattice.

Fig Fig

(a) (b)

Fig

(c)

Fig. II.3. (a) XRD patterns of the nonstoichiometric powders described by the formula Ba1-xCexTi1-x/4O3, prepared by the sol-gel method and (b) detail (the green, dashed rectangle of Fig. I.3(a)) of the region corresponding to the diffraction angles 2 = 25 – 30.5o, showing the absence of secondary phases at the detection limit and (c) detail (the purple, dashed rectangle of Fig. I.3(a)) of the region corresponding to the diffraction angles 2 = 44,5 – 46,5o, proving the A-site incorporation of Ce3+ in the perovskite lattice.

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(a) (b) (c)

Fig. I.4. Morphology of Ba0.9975Ce0.0025TiO3 powder: (a) TEM image; (b) HRTEM image and

(c) SAED pattern.

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(a) (b) (c)

Fig. I.5. Morphology of Ba0.995Ce0.005TiO3 powder: (a) TEM image; (b) HRTEM image and

(c) SAED pattern.

 

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(a) (b) (c)

Fig. I.6. Morphology of Ba0.995Ce0.005Ti0.99875O3 powder: (a) TEM image; (b) HRTEM image and

(c) SAED pattern.

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(a) (b) (c)

Fig. II.7. Morphology of Ba0.95Ce0.05TiO3 powder: (a) TEM image; (b) HRTEM image and

(c) SAED pattern.

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(a) (b) (c)

Fig. II.8. Morphology of Ba0.95Ce0.05Ti0.9875O3 powder: (a) TEM image; (b) HRTEM image and

(c) SAED pattern.

I.3. Preparation by the modified sol-gel method (Pechini procedure) of Ce3+ doped-BT (BCT) nanopowders

Schema de preparare Pechini_romana.bmp

Fig. I.9. Flowchart for the preparation of BCT powders by the modified sol-gel (Pechini) method.

I.4. Complex ccharacterization (phase purity, structure and morphology) of Ce3+-doped BaTiO3 (BCT) nanopowders synthesized by the modified sol-gel method

Pulberi BCT_Pechini_900_2h_FV_indexat.bmp Pulberi BCT_Pechini_900_2h_CV_indexat.bmp

(a) (b)

Fig. I.10. XRD patterns of the powders prepared by the Pechini method: (a) stoichiometric powder described by the formula Ba1-xCexTiO3 and (b) nonstoichiometric powders described by the formula

Ba1-xCexTi1-x/4O3.

BCT0025_FV BCT005_FV_bis BCT005_CV_bis

(a) (b) (c)

BTCe0 BaCeTiVO3_22 copy

(d) (e)

Fig. I.11. TEM images of Ce-BaTiO3 powders prepared by the Pechini method and thermally treated at 900oC for 2 hours, described by the formulae: (a) Ba0.9975Ce0.0025TiO3; (b) Ba0.995Ce0.005TiO3;

(c) Ba0.995Ce0.005Ti0.99875O3; (d) Ba0.95Ce0.05TiO3 şi (e) Ba0.95Ce0.05Ti0.9875O3.

B. Results obtained in 2018 I (scientific report):

II. A-site (Ce3+) doped-BaTiO3 (BCT) multiscale-structured ceramics, 2D and 1D nanostructures

II.1. Elaboration of microstructured BCT ceramics by conventional sintering (CS)

The as-synthesized oxide powders with compositions described by the nominal formulae Ba1-xCexTiO3 and Ba1-xCexTi1-x/4O3 (x = 0.005 and 0.05) were milled and shaped by uniaxial die-pressing at 174 MPa into pellets with a diameter of ~ 13 mm and a thickness of ~ 1.2 – 1.6 mm, using a small amount of organic binder (5% PVA aqueous solution). The green bodies were sintered in a muffle furnace, in static air at 1300oC for 4 hours plateau, using a heating rate of 5oC·min-1 and then they were slowly cooled (with the normal cooling rate of the furnace) at room temperature, in order to obtain dense Ce3+-doped BaTiO3 ceramic samples. After sintering, the colour of the ceramic samples varied from light orange to reddish, when the Ce3+ content varied from 0.5 at.% to 5 at.%.

II.2. Complex characterization (phase composition, microstructure, electrical properties) of BCT microstructured ceramics obtained by conventional sintering (CS)

 

Fig. II.1. (a) XRD patterns recorded at room temperature for the ceramic samples Ba0.9975Ce0.0025TiO3, Ba0..995Ce0.005TiO3 and Ba0.995Ce0.005Ti0.99875O3 derived from the Pechini powders and conventionally sintered at 1300oC / 4 hours; (b) detail (blue marked rectangle of Fig. II.1(a)) of the region corresponding to diffraction angles 2 = 25 – 30.5o and (c) detail (red marked rectangle of Fig. II.1(a)) of the region corresponding to diffraction angles 2 = 44.5 – 46.5o.

 

Fig. II.2. (a) XRD patterns recorded at room temperature for the ceramic samples Ba0..95Ce0.05TiO3 and Ba0.95Ce0.05Ti0.9875O3 derived from the Pechini powders and conventionally sintered at 1300oC / 4 hours; (b) detail (violet marked rectangle of Fig. II.2(a)) of the region corresponding to diffraction angles 2 = 26 – 30.5o and (c) detail (brown marked rectangle of Fig. II.2(a)) of the region corresponding to diffraction angles 2 = 44.5 – 46.5o.

Table II.1. Structural parameters of Ce3+-doped BaTiO3 ceramics derived from Pechini powders and consolidated by conventional sintering at 1300oC / 4 hours.

Structural parameters

Composition

Ba0,9975Ce0,0025TiO3

Ba0,995Ce0.005TiO3

Ba0.995Ce0.005Ti0.99875O3

Ba0.95Ce0.05TiO3

Ba0.95Ce0.05Ti0.9875O3

Symmetry

Tetragonal

Tetragonal

Tetragonal

Tetragonal

Cubic

Tetragonal

a (Å)

3.9959(14)

3.9956(12)

3.9952(15)

4.0239(11)

3.9721(24)

3.9878(7)

c (Å)

4.0243(25)

4.0233(24)

4.0164(22)

4.0465(26)

3.9721(24)

4.0006(9)

c/a

1.0071(17)

1.0069(14)

1.0053(21)

1.0056(9)

1.0000

1.0032(26)

V3)

64.26(8)

64.23(5)

64.11(7)

65.52(8)

62.67(11)

63.62(22)

SEM_BCT0025_Pechini_1300.jpg SEM_BCT005_Pechini_1300.jpg SEM_BCT-V-005_Pechini_1300.jpg

SEM_BCT05_Pechini_1300.jpg SEM_BCT05_Pechini_1300_EDX.jpg SEM_BCT-V-05_Pechini_1300_EDX.jpg

Fig. II.3. Microstructures of the BCT ceramics derived from the Pechini powders and conventionally sintered at 1300oC / 4 hours: (a) FE-SEM image (SE mode) of the sample Ba0.9975Ce0.0025TiO3;

(b) FE-SEM image (SE mode) of the sample Ba0.995Ce0.005TiO3; (c) FE-SEM image (SE mode) of the sample Ba0.995Ce0.005Ti0.99875O3; (d) FE-SEM image (SE mode) of the sample Ba0.95Ce0.05TiO3; (e) FE-SEM image (BSE mode) of the sample Ba0.95Ce0.05TiO3 revealing equiaxial grains belonging to the major perovskite phase (1) and non-equiaxial, elongated grains belonging to the secondary poly-titanate phases (2) and (f) FE-SEM image (SE mode) of the sample Ba0.95Ce0.05Ti0.9875O3.

 

Fig. II.4. Temperature dependence of the dielectric properties for the BCT ceramics described by the nominal formulae Ba0.9975Ce0.0025TiO3, Ba0.995Ce0.005TiO3 and Ba0.995Ce0.005Ti0.99875O3 derived from Pechini powders and conventionally sintered at 1300oC / 4 hours: (a) dielectric permittivity and

(b) dielectric losses.

 

Fig. II.5. Temperature dependence of the dielectric properties for the BCT ceramics described by the nominal formulae Ba0.95Ce0.05TiO3 and Ba0.95Ce0.05Ti0.9875O3 derived from Pechini powders and conventionally sintered at 1300oC / 4 hours: (a) dielectric permittivity and (b) dielectric losses.

Fig. 5.jpg

Fig. II. 6. (a) XRD patterns recorded at room temperature for the 0.5% Ce3+-doped BaTiO3 powders thermally treated at 900oC / 2 hours; (b) detail (light yellow rectangle of Fig. 1(a)) of the region corresponding to diffraction angles 2 = 25 – 30.5o and (c) detail (light blue rectangle of Fig. 1(a)) of the region corresponding to diffraction angles 2 = 44.5 – 46.5o.

Fig. 6.jpg

Fig. II.7. (a) XRD patterns recorded at room temperature for the 5% Ce3+-doped BaTiO3 powders thermally treated at 900oC / 2 hours; (b) detail (light yellow rectangle of Fig. 1(a)) of the region corresponding to diffraction angles 2 = 25 – 30.5o and (c) detail (light blue rectangle of Fig. 1(a)) of the region corresponding to diffraction angles 2 = 44.5 – 46.5o.

Table II.2. Structural parameters of Ce3+-doped BaTiO3 ceramics derived from sol-gel powders and consolidated by conventional sintering at 1300oC / 4 hours.

.

Formula

Ba0.995Ce0.005TiO3

Ba0.995Ce0.005Ti0.99875O3

Ba0.95Ce0.05TiO3

Ba0.95Ce0.05Ti0.9875O3

Symbol

BCT-005_CS

BCT-V-005_CS

BCT-05_CS

BCT-V-05_CS

Phase composition

BCTss – 100%

BCTss – 100%

BCTss – 96.5%

B6T17 – 3.5%

BCTss – 100%

Structure

Tetragonal, P4mm

Tetragonal, P4mm

Tetragonal, P4mm

Cubic, Pm-3m

Unit cell parameters

a (Å)

3.995339 ± 0.000115

3.994169 ± 0.000108

3.999700 ± 0.003248

3.999053 ± 0.000054

b (Å)

3.995339 ± 0.000115

3.994169 ± 0.000108

3.999700 ± 0.003248

3.999053 ± 0.000054

c (Å)

4.025707 ± 0.000135

4.024993 ± 0.000127

3.999820 ± 0.006488

3.999053 ± 0.000054

Tetragonality, c/a

1.0076

1.0077

1.00003

1.0000

Unit cell volume, V3)

64.26129

64.21228

63.98753

63.95455

BCTss = Ce3+-BaTiO3 solid solution – Tetragonal, P4mm (ICDD card no. 01-081-8524); Cubic – Pm-3m (ICDD card no. 01-083-3859)

B6T17 = Ba6Ti17O40 – Monoclinic C2/c (ICDD card no. 04-009-3291);

Fig. 8a.jpg Fig. 8b.jpg

Fig. 8c.jpg

Fig. 8d.jpg Fig. 8e.jpg

Fig. II.8. (a), (b) FE-SEM (SE mode) images of the Ba1-xCexTiO3 ceramics obtained by conventional sintering at 1300oC / 4 hours: (a) BCT-005_CS and (b) BCT-05_CS; (c) FE-SEM image in BSE mode of Ba0.95Ce0.05TiO3 ceramic sample, showing segregation of the Ba6Ti17O40 secondary phase and the related EDX spectra corresponding of the two regions marked by blue arrows; (d), (e) FE-SEM (SE mode) images of the Ba1-xCexTi1-x/4O3ceramics obtained by conventional sintering at 1300oC / 4 hours:

(d) BCT-V-005_CS and (e) BCT-V-05_CS.

Fig. 12a.jpgFig. 12b.jpg

Fig. II.9. (a), (b) Temperature dependence of the dielectric properties at 1 kHz frequency for all the Ce3+-doped BaTiO3 ceramics obtained by conventional sintering at 1300oC / 4 hours:

(a) relative permittivity; (b) dielectric losses and (c) hysteresis loops for single phase ceramics.

II.3. Elaboration of ultra-dense submicronic BCT ceramics consolidated by spark plasma sintering (SPS)

Amounts of powders synthesized by the modified Pechini method and by the “acetate” variant of the sol-gel route. with composition described by the nominal formula Ba1-xCexTi1-x/4O3 (x = 0.005 and 0.05). were also used to prepare dense ceramics by spark plasma sintering (SPS). The powders were poured into a graphite die and then sintered under vacuum to dense ceramic pellets of 10 mm diameter and ~ 1 mm thickness. using a commercial SPS equipment (FCT Systeme GmbH. Germany – Spark Plasma Sintering Furnace type HP D 1.25). The temperature was raised at a fixed heating rate 100°C·min-1 under a constant applied pressure of 50 MPa and then held at a constant value of temperature of 1050°C for 2 min. Rapid heating was provided by a pulsed DC current. After polishing. all the ceramic samples were annealed in air at 1000°C for 16 hours. with a heating rate of 10oC · min-1. The aims of this post-sintering thermal treatment are: (i) to reduce the concentration the oxygen vacancies originated by the reducing conditions of the SPS process and to ensure the reoxidation of the Ti3+ species to Ti4+. (ii) to remove possible surface carbon contamination and (iii) to relieve the residual stresses arising either from the SPS process or from polishing. After SPS the ceramics exhibited a dark-grey colour. while after the post-sintering thermal treatment they were yellowish.

   

Schematic representation of the

Spark Plasma Sintering process

     

II.4. Complex characterization (phase composition. microstructure. electrical properties) of BCT ceramics obtained by spark plasma sintering (SPS)

XRD_Ceramici SPS_colaj_final copy.bmp

Fig. II.10. (a) XRD patterns recorded at room temperature for Ba1-xCexTi1-x/4O3 ceramics obtained by spark plasma at 1050oC / 2 min, followed by post-sintering re-oxidation thermal treatment at 1000oC / 16 hours; (b) detail (light yellow rectangle of Fig. 7(a)) of the region corresponding to diffraction angles 2 = 25 – 30.5o and (c) detail (light blue rectangle of Fig. 7(a)) of the region corresponding to diffraction angles 2 = 44.5 – 46.5o.

Table II.3. Structural parameters of Ce3+-doped BaTiO3 ceramics derived from sol-gel powders and consolidated by spark plasma sintering at 1050oC / 2 min.

Formula

Ba0.995Ce0.005Ti0.99875O3

Ba0.95Ce0.05Ti0.9875O3

Symbol

BCT-V-005_SPS

BCT-V-05_SPS

Phase composition

BCT – 100%

BCT – 98.2% ; BT2 – 1.1% ; C – 0.7%

Structure

Tetragonal, P4mm

Cubic, Pm-3m

Unit cell parameters

a (Å)

4.000073 ± 0.000273

3.999968 ± 0.000167

b (Å)

4.000073 ± 0.000273

3.999968 ± 0.000167

c (Å)

4.021890 ± 0.000344

3.999968 ± 0.000167

Tetragonality, c/a

1.0054

1.0000

Unit cell volume, V3)

64.35261

63.99847

BCTss = Ce3+-BaTiO3 solid solution – Tetragonal, P4mm (ICDD card no. 01-081-8524); Cubic – Pm-3m (ICDD card no. 01-083-3859)

BT2= BaTi2O5 – Monoclinic C2 (ICDD card no. 04-012-4418);

C = CeO2 – Cubic Fm-3m (ICDD card no. 00-067-0121).

SEM_BCT-V-005_SPS.jpg SEM_BCT-V-05_SPS.jpg

Fig. II.11. FE-SEM (SE mode) images of the Ba1-xCexTi1-x/4O3 ceramics obtained by spark plasma sintering at 1050oC / 2 min followed by post-sintering re-oxidation thermal treatment at 1000oC / 16 hours: (a) BCT-V-005_SPS and (b) BCT-V-05_SPS.

Fig. 11c.jpg Fig. 11d.jpg

Fig. II.12. (a), (c) Relative permittivity and (b), (b) dielectric losses vs. temperature for the spark plasma sintered ceramics described by the nominal formula Ba1-xCexTi1-x/4O3: (a), (b) BCT-V-005_SPS and

(c), (d) BCT-V-05_SPS.

II.5. Preparation of multilayered Ce3+-doped BaTiO3 (BCT) thin films by the sol-gel method

Ba0.95Ce0.05Ti0.9875O3 thin films were prepared by the “acetate” route of the sol-gel method. As starting reagents were used barium acetate (Ba(CH3COO)2, 99%, Sigma Aldrich), titanium (IV) isopropoxide 97% solution in 2-propanol (Ti{OCH(CH3)2}4, Sigma Aldrich) and cerium acetate (Ce(CH3CO2)3, 99.9%, Sigma Aldrich).

Two different solutions were prepared by dissolving 5.2122 g barium acetate in 60 mL acetic acid and 0.3407 g cerium acetate in 20 mL acetic acid, at 70 C, under continuous stirring. As stabilizers for the sol, 2-methoxyethanol and acetylacetone in 2:1 volume ratio were used. Another solution resulted by mixing 6.51 mL titanium isopropoxide in 25 mL 2-propanol was added to barium acetate solution, under continuous stirring. Then, the cerium acetate solution was added to barium and titanium mixture solution. In a subsequent step, acetylacetone (CH3COCH2COCH3, 97%, Aldrich) was added to the as-obtained solution. A sol concentration of 0.3 M and pH = 5 were found to be optimal to produce thin films.

From the synthesized sol thin films were deposited on Pt-Si substrate. The structure of the substrate is Si/SiO2/TiO2/Pt, with the following thicknesses: 200 μm Si/450 nm SiO2/15 nm TiO2/100 nm Pt. The sol was deposited on the substrate by centrifugation with a rate of 3000 rot/min, for 20 s. After centrifugation, drying and solvent evaporation were carried out by heating the substrate – first deposition heterostructure on a plate at 200oC for 2 min. A subsequent thermal treatment at 400oC for 4 min was carried out in order to ensure the pyrolysis of the organic groups from the gel film. Multi-layered films were obtained by repeating for each deposit the as-described procedure in several deposition-heating cycles. The final thermal treatment was carried out al 700oC for 1 hour, with a heating rate of 5oC/min. The best properties were presented by the Ce3+-BaTiO3 thin films with 10 deposits (BCT10).

Configuratie.jpg

 

II. 13. Configuration of the substrate – BCT10 thin film heterostructure used for determining the film thickness by spectroellipsometric measurements

II.6. Complex characterization of multilayered BCT thin films (phase purity. topography. structure. microstructure. functional properties)

XRD_Film_engl

Fig. II. 14. XRD pattern at room temperature corresponding to the 10-deposit Ce3+-BaTiO3 thin film (BCT10).

SEM_3.jpg SEM_2.jpg SEM_1.jpg

(a) (b) (c)

 

Fig. II.15. FE-SEM images for the BCT10 thin film: (a) cross-section general view; cross-section detail and (c) surface general view.

AFM_3D_2x2.jpg

(a) (b)

(c)

(d) (e)

Fig. II.16. (a), (b) 2D and 3D AFM images on a surface area of (2 2) m for the BCT10 thin film; (c) the surface profile (red line of Fig. II.16(a); (d), (e) 2D and 3D AFM images on a surface area of

(5 5) m for the BCT10 thin film.

(a) (b)

(c)

Fig. II.17. Histograms corresponding to: (a) average grain length; (b) average grain width and

(c) average grain diameter approximated by the “Watershed” method.

 

(a) (b)

Fig. II.18. Temperature dependence of the dielectric properties for the BCT10 thin film at various frequencies: (a) dielectric permittivity and (b) dielectric losses.

 

(a) (b)

Fig. II.19. Hysteretic behaviour of the BCT10 thin film: (a) Total polarization – voltage hysteresis loop; and (b) Intrinsic polarization – voltage hysteresis loop.

II.7. Elaboration of 1D Ce3+-doped BaTiO3 (BCT) nanostructures by a template-mediated sol-gel route

Fig. II.20. Preparation flowchart of 1-D Ba0.95Ce0.05Ti0.9875O3 nanostructures.

II.8. Complex characterization of 1D BCT nanostructures (phase purity, structure, morphology, topography, functional properties) – first part

SEM_BCT fire+tuburi amorfe.jpg .

Fig. II.21. FE-SEEM images of different magnifications of amorphous 1D nanostructures: (a), (b) nanowires and (c) – (e) nanoshell tubes.

SEM_BCT fire+tuburi cristaline.jpg

Fig. II.22. FE-SEEM images of different magnifications of polycrystalline Ce3+-doped BaTiO3 1D nanostructures resulted after calcination at 800oC for 1 hour: (a), (b), (c) nanowires and (d), (e), (f) nanoshell tubes.

XRD_1D_engl

Fig. II.23. XRD patterns at room temperature for: (a) BCT nanowires; (b) BCT nanoshell tubes.

C. Results obtained in 2019 (intermediary scientific report 2017-2019):

III.1. Complex characterization of 1D BCT nanostructures (phase purity, structure, morphology, topography, functional properties) – second part

BCT_yuburi_Raman.jpg

Fig. III.1. Raman spectrum recorded at room temperature for the powder consisted of nanostructured Ce3+-BaTiO3 tubes resulted after calcination at 800oC, for 1 hour.

TEM_fire.jpg

Fig. III.2. TEM / HRTEM images and SAED patterns for the 5 mol% Ce3+-doped BaTiO3 nanowires calcined at 800oC for 1 hour: (a) low magnification TEM image on a bundle of nanowires; (b) high magnification TEM image on an individual wire; (c) HRTEM image inside a polycrystalline grain and

(d) SAED pattern of a polycrystalline zone.

TEM_tuburi.jpg

Fig. III.3. TEM / HRTEM images and SAED patterns for the 5 mol% Ce3+-doped BaTiO3 nanoshell tubes calcined at 800oC for 1 hour: (a) low magnification TEM image on a bundle of nanoshell tubes; (b) high magnification TEM image on an individual tube, indicating the fine-grained microstructure of the tube wall; (c) HRTEM image inside a polycrystalline grain consisting of randomly oriented crystallites and

(d) SAED pattern of a polycrystalline zone.

Fig. III.4. Local PFM hysteresis loops measured as a function of the applied DC field on 5 mol% Ce3+-doped barium titanate nanowires: (a) amplitude signal; inset – amplitude image and (b) phase signal; inset – phase image.

Fig. III.5. Local PFM hysteresis loops measured as a function of the applied DC field on 5 mol% Ce3+-doped barium titanate nanoshell tubes: (a) amplitude signal; inset – amplitude image and

(b) phase signal; inset – phase image.

III. B-site (Hf4+) doped-BaTiO3 (BTH) nanopowders and related multiscale-structured ceramics

III.2. Preparation by the sol-gel method of Hf4+ doped-BaTiO3 (BTH) nanopowders

Fig. III.6. The place of the selected compositions in the pseudo-binary system belonging to the ternary BaOTiO2HfO2 system.

Fig. III.7. Schematic representation of the preparation by the “acetate” variant of the sol-gel method of the BTH nanopowders and related ceramics.

III.3. Complex characterization (phase purity, structure and particle size / morphology) of BTH nanopowders

(a) Characterization of the amorphous precursor

ATD

Fig. III.8. Thermal analysis curves of the (Ba, Ti) amorphous precursor.

(b) Characterization of the BaTi1-xHfxO3 (BTH) oxide powders

XRD_BTH_sol-gel powders

Fig. III.9. (a) Room-temperature XRD patterns of BTH powders obtained after calcination at 900oC for 2 hours and (b) detail (yellow rectangle of Fig. III.9(a)) of the region corresponding to diffraction angles

2 = 44 – 46.5o.

TEM_pulberi sol-gel.jpg

Fig. III.11 (a), (d), (g), (j), (m), (p) TEM images; (b), (e), (h), (k), (n), (q) HRTEM images and (c), (f), (i), (l), (o), (r) SAED patterns for BTH powders prepared by “acetate” variant of the sol-gel method and calcined at 900oC, for 2 hours: (a)-(c) x = 0; (d)-(f) x = 0.03; (g)-(i) x = 0.05; (j)-(l) x = 0.1;

(m)-(o) x = 0.2 and (p)-(r) x = 0.3.

Average particle size

Fig. III.12. Evolution of the average particle size <d> as a function of Hf content (x) in the BTH powders obtained after calcination at 900oC for 2 hours.

EDX BTH30_powder

Fig. III.13. EDS spectrum of BaTi0,7Hf0,3O3 powder.

III.4. Elaboration of microstructured BTH ceramics by conventional sintering (CS)

From the BaTi1-xHfxO3 (BTH) powders, pellets with diameter of 13 mm and thickness of 1 2 mm were shaped by uniaxial pressing at a pressure P of 160 MPa. The green pellets were conventionally sintered in air at 1400oC for 2 hours, with a heating rate of 5oC/min. The obtained BTH ceramics were slowly cooled at the normal cooling rate of the furnace.

III.5. Complex characterization (phase purity, structure, microstructure, electrical properties) of BTH ceramics consolidated by conventional sintering (CS)

XRD_BTH_sol-gel ceramics_CS_colaj

Fig. III.14. (a) Room-temperature XRD patterns of BTH ceramics conventionally sintered at 1400oC for 4 hours and (b) detail (light-blue rectangle of Fig. III.14(a)) of the region corresponding to diffraction angles

2 = 44 – 46.5o.

SEM_ceramici sol-gel BTH_SC.jpg

Fig. III.15. FE-SEM images showing the microstructure of the sol-gel BTH ceramics conventionally sintered at 1400oC for 4 hours: (a) x = 0; (b) x = 0.03; (c) x = 0.05; (d) x = 0.10; (e) x = 0.20; (f) x = 0.30.

Average frain size_ Sol-gel BHT ceramics_CS

Fig. III.16. Evolution of the average grain size <GS> as a function of Hf content (x) in the BTH ceramics conventionally sintered at 1400oC for 4 hours.

Permittivity ve Losses vs temp

Curei temperature

Fig. III.17. Temperature dependence of the dielectric properties at 1 kHz frequency, for BHT ceramics conventionally sintered at 1400oC for 4 hours: (a) relative permittivity and (b) dielectric losses;

(c) evolution of Curie temperature as a function of Hf content.

III.6. Elaboration of BTH ceramics by spark plasma sintering (SPS)

Dense, fine-grained (in the submicronic range) ceramics with diameter of 8 mm and thickness of ~ 1 mm were obtained from the sol-gel BTH powders by spark plasma sintering (SPS). For shaping a graphite die was used. In order to prevent the reaction between the BTH powders and the surfaces of the graphite die and punch, graphite foils were used. After inserting the powders into the die, spark plasma sintering at a pressure of 50 MPa was carried out, the shaping and the consolidation processes occurring simultaneously. The samples were quenched at room temperature and then they were subjected to a post-sintering annealing in oxygen flow, at 1000oC for 6 hours., in order to reoxidize the grain boundary regions and to completely convert the Ti3+ ions (formed as a result of the partial reduction determined by a certain carbon contamination degree during the SPS process) into Ti4+ ions.

III.7. Complex characterization (phase purity, structure, microstructure, electrical properties) of BTH ceramics consolidated by spark plasma sintering (SPS)

XRD_BTH_sol-gel ceramics_SPS_colaj

Fig. III.19. (a) Room-temperature XRD patterns of BTH ceramics spark plasma sintered at 1200oC for

2 min and (b) detail (light-blue rectangle of Fig. III.19(a)) of the region corresponding to diffraction angles

2 = 44 – 46.5o.

SEM_ceramici sol-gel BTH_SPS.jpg

Fig. III.20. FE-SEM images showing the microstructure of the sol-gel BTH ceramics spark plasma sintered at 1200oC for 2 min: (a) x = 0; (b) x = 0.03; (c) x = 0.05; (d) x = 0.10; (e) x = 0.20; (f) x = 0.30.

Permittivity ve Losses vs temp

Fig. III.21. Temperature dependence of the dielectric properties at 1 kHz frequency, for BHT ceramics spark plasma sintered at 1200oC for 2 min: (a) relative permittivity and (b) dielectric losses.

III.8. Comparative study of phase transitions and ferroelectric-relaxor crossover in the as-prepared BTH ceramics

BTH3_Sol-gel_Comparing permittivity_100 KHz BTH10_Sol-gel_Comparing permittivity_100 KHz

BTH20_Sol-gel_Comparing permittivity_100 KHz BTH30_Sol-gel_Comparing permittivity_100 KHz

Fig. III.22. Comparative analysis regarding the ferroelectric / relaxor character of BaTi1-xHfxO3 (BTH) ceramics obtained by conventional sintering and spark plasma sintering, respectively: (a) x = 0.03;

(b) x = 0.1; (c) x = 0.2 and (d) x = 0.3.