3rd review meeting 14.04.2008

New Generation Thermal Barrier Coating

A. Bhattacharya V. Shklover W. Steurer

Step 1: uniaxial pressing, 85 MPaStep 2: sintering: 1100C for 4 hr

Step 1: Hot pressing: 1150C, 1hr, 30MPa

Sample preparation

7.6mol% Ti doped 7YSZ

step 1: reverse coprecipitationproblems: alkoxides are very hygroscopic and handling of huge volume of chemicalssolution: preparation of small amounts, use of nitrogen atmosphere and dry ethanolstep 2: calcination in air at 900C for 2 hrsstep 3: uniaxial pressing, 50 MPastep 4: sintering,1200C for 4 hrs

7YSZ

density 4.09 g/cc, the=5.64 g/cc, rel=72%

density 3.88 g/cc, the= 6.0 g/cc, rel= 64%

density 2.99 g/cc, the= 6.0 g/cc, rel= 50%

SrZrO3step 1: uniaxial pressing, 25 MPaProblems: need binderSolution: powder was water sprayedstep 2: sintering in steps, RT to 150 C and then to 1100C, 1hr at each temperature

step 3: hot pressing: 30MPa, 1200C, 1hr

density 2.65 g/cc, the= 5.48 g/cc, rel= 48%

density 4.6 g/cc, the= 5.48 g/cc, rel= 84%

Sample preparation

ConclusionOptimisation of pressure, temperature and dwelling time is required to obtain the desired density

YSZ can be prepared to high density without hot pressing

SrZrO3 and TiYSZ may require hot pressing

SrZrO3 - state of the artSample: solid state reaction, calcination at 1050C, pellets by uniaxial pressing and sintered in atmosphere at 1500C

Experiments: XRD, DSC for heat capacity, Dilatometer for thermal expansion

Results:

XRD: orthorhombic at RT a= 5.816, b=8.225, c=5.813 the= 5.48 g/cc, rel= 95%

Thermal properties: 1st phase transition was obserbed at 1035K (DSC), 1041(CTE)inflection point increases with increase in heating ratetransition at 1035K is assumed to be from orthorhombic(Pnma) to orthorhombic(Cmcm) andthe thermal expansion of Cmcm is higher than Pnmathermal expansion: 9.69x10-6 K-1, after transition 1.10x10-5 K-1 for 95% dense sample ---- Matsuda et. al.

Thermal expansion: 10x10-6 to 11x10-6 (till 1000K) for 99.45% dense sample --Shakin et. al.

Sample density: 48%Thermal expansion:5.70x10-6 K-1(400K to 1000K)

5.1x10-6 K-1 (1000K to 1300K)

Sintering Temperature: 1370K

SrZrO3 -our work

Increase in density increases CTE

DTA analyses doesn’t show any phase transition--contradictory to carlsson et.al.

CT

E

m

m

m

m

m

m

m

m

m

ttt t t t t

t: t-ZrO2

m: m-ZrO2

In-situ XRD of SrZrO3

orthorhombic, Pbnm

orthorhombic, Cmcm

tetragonal, I4/mcm

cubic, Pm3m

Phase analyses

Orthorhombic (Pbnm) Orthorhombic (Cmcm) tetragonal (I4/mcm) Cubic(Pm3m) 750C 850C 1200C

Orthorhombic (Pbnm) Orthorhombic (Cmcm) Tetragonal (I4/mcm) Cubic (Pm3m) 970K 1100K 1440K ….Ahtee et.al. and kennedy et.al.Orthorhombic (Pbnm) tetragonal (I4/mcm) Tetragonal (I4/mcm) Cubic (Pm3m) 970K 1100K 1440K

….Carlsson

“SrZrO3 available with us contain a 2nd phase of ZrO2 which is monoclinic at room temperature till 1200C and then transforms to tetragonal”

: our sample

7YSZ

14YSZ

7Ti7YSZ

RT XRD of YSZ and TiYSZ

Ti YSZ obtained is single phase and have t’ structure

EDX analysis shows the atomic ration as Zr:Ti:Y = 0.8: 0.08: 0.12Composition has to be confirmed yet !!

Targeted: Zr0.84Y0.07Ti0.07O1.96

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Phase analyses

-- Levi et.al.

7 YSZ is t’

14YSZ is cubic

New compositions?

Yb doped SrZrO3---Osaka et.al

Replacing Zr4+ ion with Yb3+ will increase the oxygen ion vacanciesYbSRZ loose oxygen at high temperature(1200C)SRZ undergoes a weight loss of 0.8% from 65 to 1200C due to emission of carbonates produced by reaction with H2O and CO2 in airNo peaks were observed by DTA of SRZ

CaZrO3

COF: 7.7 x 10-6 K-1 (20-800C) [9.9] 9.7 x 10-6 K-1 (20-1200C) [9.0]Undergoes same phase transitions as SRZ ---Poluboyarinov et.al.

SrTiO3Tetragonal below room temperature but cubic after RT till its melting temperature--- ligny et.al.=3.23(2)x10-5 K-1 from RT to 1500K, below RT it decreases due to C to t transition.=2.98(2)x10-5 K-1 for SrZrO3 till 970 K

Values in red color corresponds to that of SrZrO3(SRZ)

Perovskites (ABO3)It has a cubic symmetry, however, rhombohedral, orthorhombic and tetragonal symmetries arealso observed. This is due to tilting and distortion of BO6 octahedra.

Distortion of simple perovskite depends on tilting angle or the tolerance factor t=<A-O>/ √2<B-O>

In ideal cubic perovskite would have a tolerance factor 1 and tilting angle 0.

For SrZrO3 this tilting angle is 6 deg and it tends to 0 with temperature.

SrTiO3 is cubic at RT, when Ti is replaced by Zr, the BO distance increases which results in too big SrO8 dodecahedra in a cubic unit cell and this polyhedra mismatch leads to an orthorhombic symmetry through tilting of BO6 octahedra by an angle of 6°----Ligny et.al.

m.p. of SrTiO3 is 2150C and of SrZrO3 is 2650C

Coming up

Dilatometer study of dense SrZrO3, 7YSZ, 14 YSZ, Ca doped YSZ and Ti doped YSZ

Chemical compatability of these oxides through in-situ and ex-situ XRD, complementedby SEM, EDX and EPMA analyses

Chemical compatability can also be characterised by phase diagram calculations (CALPHAD) or by chemical potential diagrams (CHD)??

Exploring raman spectroscopy for futher analyses

Problems1. Furnace availability: work is on to repair one old good one, as well as, it will be callibrated2. Availability of hot press in very limited and uniaxial press with small diameter (6-7mm) is not available3. Metallography lab access is also limited