Fabrication and performance of Li 4 SiO 4 pebbles by the melt spraying method

CBBI-16, Portland, 8-10, September

Fabrication and performance of Li4SiO4 pebbles by

the melt spraying method

Yongjin Feng

Southwestern Institute of Physics (SWIP), Chengdu, Sichuan, China


• Background• Fabrication Process and Results of Li4SiO4 pebbles• Deuterium Retention and Desorption Behavior of

Li4SiO4 • R&D Plans on Breeder Materials at SWIP• Summary

Outlines


CN Helium Cooled Ceramic Breeder (HCCB) TBM designs based on the SB/He/FM concept.

1. Background

Explosive view of CN HCCB TBM Sub-module design

Be pebbles by REP method1-ton Ingot of CLF-1

Component MaterialStructure RAFMCoolant He

Purge gas He+0.1%H2

Neutron multiplier

Be

Breeder Li4SiO4 , Li2TiO3


The ceramic breeder material must satisfy the following requirements:

High tritium breeding capability;

Adequate mechanical properties;

Limited pebble fragmentation ;

Adequate pebble bed thermal conductivity;

Compatibility with ferritic steel and the purge gas;

Chemical stability to avoid mass transport and material restructuring;

Radiation resistance;

Low tritium residence time;

Low activation;


The fabrication trials have been investigated, such as, Melt spraying method,

Freezing-Sintering method, Extrusion-spheronization-sintering, Sol-gel.

The pebbles produced by the melt-spraying method have several advantages: Higher density; Smooth surface; Higher sphericity; Less contamination source; Simpler reprocessing.

The selection of fabrication process for the pebbles based on the following criteria: Capability to meet the pebbles goal specifications adequate for the HCCB TBM; Simplicity and economics; Scalability to industrial range; Sufficient production yield; Conveniently recycling the unburned 6Li from the pebbles.


Schematic drawing of fabrication setup

Heating and insulation

Melting pot

Bottom feeder

Gas jet sprayer

2. Fabrication process and Results

Fabrication facility

Raw materials: Li2CO3 (Purity:99.99%)

SiO2 (Purity:99.99 )Li/Si Molar ratio: 4Melting Pot: Corundum CrucibleThe raw materials are melted at temperature of about 1400 .℃Gas pressure: 1.5 bar, Gas: Nitrogen,Falling distance: 3.5 m.

Heat treatment condition: 1000 , 2h℃Production: 100Kg/year pebbles with 1.0 mm diameter


Optical micrographs and SEM

Most of the pebbles are well spherically shaped, smooth surface.

Shape and surface structure

Optical micrographs and SEM of the pebbles with 1mm diameter SEM of pebble’s surface

Broad size distribution.


Phase analysisHeat treatment atmosphere: Vacuum, air temperature: 1000℃ time: 2h

XRD pattern of pebbles annealed at air

The diffraction peaks of Li2CO3, Li2SiO3,Li4SiO4 are observed. Carbon dioxide are easily absorbed by Li4SiO4

15 20 25 30 35 40 45 50 55 60 65 700

500

1000

1500

2000

2500

3000

Inte

rnsi

ty (C

ount

s)

2

Li4SiO4

Li2SiO3

XRD pattern of pebbles annealed at vacuum

Li4SiO4 as the major phase,Li2SiO3 as a second phase

15 20 25 30 35 40 45 50 55 60 65 700

1000

2000

3000

4000

5000

Inte

rnsi

ty (C

ount

s)

2

Li4SiO4

Li2SiO3

Li2CO3

TG curve of Li4SiO4 at CO2 atmosphere

Temp. <500 absorb rate very slow;℃ 500 < Temp.< 720 absorption obviously;℃ ℃720 < Temp.< 900 CO℃ ℃ 2desorption


Thermal analysis

The weight loss of about 40% occurred between 550 and 800 . the significant ℃ ℃weight lost taking place at 720 . ℃The reaction is a endothermic reaction.

Thermoanalysis of mixed raw materials

DSC

TG

Mass change:-41.67%716.7℃

Physical properties

Measurement of Density and porosity by Hg-porosimetry and He-pycnometry.Specific surface area measurement by a multipoint BET method.

Initial state After Heat treatment

Density (% TD) ~ 93.5 ~ 94

Open porosity (%) ~ 5.7 ~ 5.2

Closed porosity(%)

~ 0.8 ~ 0.75

Specific surface area (m2/g)

2.796 1.095

Total pore volume for pores (cc/g)

3.403e-03 2.012e-03


Behavior in airPebbles were exposed to air for 50 days at room temperature. The influence of the exposed surface area on the rate of uptake was measured. The uptake of moisture was determined by the weight increase.

Weight increase of initial state pebblesand after annealing pebbles.

0 10 20 30 40 500.0

0.1

0.2

0.3

0.4

0.5

0.6

Wei

ght i

ncre

ase

(wt%

)

Days

Initial state After annealing

Chemical Composition of pebbles

The amount of impurities are 0.116186%

Li/Si molar ratio ≠ 4

Elements analysis by ICP-OES


Mechanical propertiesMechanical stability analysis by crush load tests. Single sphere was placed between two parallel plates. A continuously increasing load is imposed by a piston to a single pebble until it breaks. 40 pebbles with a diameter ~1.0 mm were tested, respectively.

Initial state After Heat treatment

Max. load (N) 12 16Min. load (N) 4.3 5.2

Average load (N) 6.5 7.0

After heat treatment , the crush load increased. The value is scattered. The mechanical stability must be improved.

pebble

press


3. Deuterium Retention and Desorption Behavior The elucidation of tritium recovery from Li4SiO4 is one of key issues of

TBM design. The study of hydrogen isotopes behavior in solid breeder materials is a important subject in the design for D-T fusion blanket module.

D2 irradiation has been applied as a technique of hydrogen isotopes implantation. Deuterium ion implantation was used to induce hydrogen isotopes and other irradiation defects into the surface of irradiated breeder material.

Desorption of hydrogen isotopes as water forms and hydrogen molecular forms might be due to the existence states of hydrogen isotopes on the surface of irradiated breeder material.

In Shizuoka University of Japan, the X-ray Photoelectron Spectroscopy (XPS) and Thermal Desorption Spectroscopy (TDS) apparatuses can be utilized for the elucidation of D2 desorption behavior in solid breeding materials.

CBBI-16, Portland, 8-10, September 13

Experimental procedures of D2+ implantation

TDS Heating rate: 5 K min-1

Heating region: R.T. - 1000 K

Heating treatment

Heating temperature: 1000 K

Heating time: 10 min

Ion energy: 3.0 keV D2+

Ion fluence : (0.4, 0.6, 0.8, 1.0)× 1022 D+ m-2

Ion flux: 2.0×1018 D+ m-2 s-1

Implantation temperature: R.T.

XPS

D2+ Imp.

X-ray source: K α of Al

XPS

SinteringTemperature: 1173 K

Heating time: 3 h


64 62 60 58 56 54 52 50 48

Binding Energy (eV)

Before implantation 0.4*1022D+m-2

0.6*1022D+m-2

0.8*1022D+m-2

1.0*1022D+m-2

Atom Li: 55.6 eV

Li-O- : 53.3 eV

Li-1s XPS spectra

112 110 108 106 104 102 100

Binding Energy (eV)

Before implanation 0.4*1022D+m-2

0.6*1022D+m-2

0.8*1022D+m-2

1.0*1022D+m-2

Si-O- : 107.1 eV

Si-O-D : 105.2 eV

Si-2p XPS spectra

540 538 536 534 532 530 528

Binding Energy (eV)

Before implanation 0.4*1022D+m-2 0.6*1022D+m-2

0.8*1022D+m-2 1.0*1022D+m-2

O-Si: 536.1eV

D-O-D: 533.8eV

-O-D: 531.3eV

O-1s XPS spectra

XPS results

Comparision of before implanation and after implanation


64 62 60 58 56 54 52 50 48 46

Binding Energy (eV)

Before Dimplan After TDS

After Dimplan

542 540 538 536 534 532 530 528

Binding Energy (eV)


After Dimplan

112 110 108 106 104 102 100

Binding Energy (eV)


After Dimplan

After TDS, the BE of electron for Li-1s,O-1s and Si-2p shift back to before

implantation. The irradiated influence for the chemical state of Li-1s,O-1s and

Si-2p in Li4SiO4 will be recovered after TDS.

Li-1s XPS spectra Si-2p XPS spectra O-1s XPS spectra

Comparision of before implanation and after implanation


TDS spectra of D2 for Li4SiO4 at different fluence

TDS results

D2 desorption rate and the total D2 retention increase with the increasing of implantation fluence. All of D+ are trapped by oxygen vacancy to form –OD bond.

Peak analysis for TDS spectrum atthe fluence 1.0×1022 D m-2

The D2 TDS spectrum of Li4SiO4 can be divided into 3 peaks. The first is due to the material surface adsorption, the second could be from the defects caused by D2

+ implantation, and the third would be from O-D bond.

0.4 0.6 0.8 1.00.02.04.06.08.0

10.012.014.0

D re

tent

ion

/ 10

19 D

m-2 Peak 1

Peak 2 Peak 3 Total

fluence / 1022 D m-2

D2 retention of Li4SiO4 at different fluence

Peak 1 (400 K) → Surface adsorption

Peak 2 (500 K) → Defect

Peak 3 (650 K) → -O-D- bond

300 400 500 600 700 8000.0

0.2

0.4

0.6

0.8

Deso

rptio

n ra

te /

1018 m

-2s-1

Temperture / K

300 400 500 600 700 8000.0

0.2

0.4

0.6

0.8

1.0*1022D+m-2

0.8*1022D+m-2

0.6*1022D+m-2

0.4*1022D+m-2

Des

orpt

ion

Rat

e (1

018D

2m-2s-1

)

Temperature (K)


4. R&D Plans on Breeder Materials For Fabrication:

• LiOH and SiO2 will be used as raw materials, and compared with the

current raw materials, the heat treatment will be optimized;• The reprocessing of Li4SiO4 pebbles will be considered by remelting;

• Li2TiO3 pebbles shall be produced using Extrusion-spheronization-

sintering method. For the properties of pebbles:

• Long-term annealing experiments under ITER TBM (DEMO blanket) relevant temperature and atmosphere; (Li content of the pebbles, Phase composition, microstructure, density, etc)

• Mechanical stability analysis will be tested as heat cycle test. After the tests, the amount of broken particles are determined.

(Temperature : 200-600℃, number of cycles: ~100 cycle (~1cycle/h) ).


Irradiation properties of pebbles:• Tritium behavior in thermal neutron irradiated Li4SiO4 will be considered to

carry out in this year; (Temp. : < 353 K, T. N. flux: 5.5×1012 cm2 s-1, T. N. fluence: 3.3×1015 cm2)

• Effect of implantation temperature on retention behavior of deuterium in Li4SiO4 will plan to investigate.

Thermo-mechanical of pebble bed• Uniaxial compression tests at temperatures up to 900 to determine the ℃

mechanical characteristics of pebble beds will be performed. (Stress-strain dependence during stress increase and decrease, thermal creep strain at constant stress levels. )

• Thermal conductivity measurements of pebbles bed and the effect of thermal creep on the thermal conductivity will be performed.

(Tests in helium and air atmosphere and temperatures up to 900 ) ℃


5. Summary A melt-spraying fabrication process for Li4SiO4 pebbles has been developed.

Li4SiO4 pebbles produced by spray of liquid droplets have almost spherical shape, a smooth surface and high density, but the produced pebbles exhibit a broad size distribution that limits the yield.

The mechanical stability of different batches are scattered. This would endanger the safety of TBM, and also does not satisfy the requirements of TBM.

A series of tests with pebbles of different composition treated in an optimized heat treatment conditions will be performed in our following work.

Optimized process is undergoing at SWIP. It was confirmed that the new chemical states of lithium, oxygen and silicon on the surface of D2

+-irradiated Li4SiO4 was formed due to typical irradiation defects induced by D2

+-irradiation. Thermo-mechanical behavior, long-term stability, the behavior under neutron irradiation and the tritium release properties will be performed.


Documents

Fabrication and performance of Li 4 SiO 4 pebbles by the melt spraying method