Fabrication and performance of Li4SiO4 pebbles by the melt

Report
Fabrication and performance of Li4SiO4 pebbles by
the melt spraying method
Yongjin Feng
Southwestern Institute of Physics (SWIP),
Chengdu, Sichuan, China
CBBI-16, Portland, 8-10, September
Outlines
•
Background
•
Fabrication Process and Results of Li4SiO4 pebbles
•
Deuterium Retention and Desorption Behavior of Li4SiO4
•
R&D Plans on Breeder Materials at SWIP
•
Summary
CBBI-16, Portland, 8-10, September
1. Background
CN Helium Cooled Ceramic Breeder (HCCB) TBM designs based on the SB/He/FM concept.
Component
Material
Structure
RAFM
Coolant
He
Purge gas
He+0.1%H2
Neutron
multiplier
Be
Breeder
Li4SiO4 , Li2TiO3
1-ton Ingot of CLF-1
Explosive view of CN HCCB TBM Sub-module design
Be pebbles by REP method
CBBI-16, Portland, 8-10, September
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;
CBBI-16, Portland, 8-10, September
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.
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.
CBBI-16, Portland, 8-10, September
2. Fabrication process and Results
Melting pot
Raw materials: Li2CO3 (Purity:99.99%)
SiO2 (Purity:99.99 )
Li/Si Molar ratio: 4
Melting Pot: Corundum Crucible
The raw materials are melted at temperature of
about 1400℃ .
Gas pressure: 1.5 bar,
Gas: Nitrogen,
Falling distance: 3.5 m.
Heat treatment condition:
Heating and insulation
Gas jet sprayer
Bottom feeder
Schematic drawing of fabrication setup
1000℃, 2h
Production: 100Kg/year pebbles with 1.0 mm diameter
Fabrication facility
CBBI-16, Portland, 8-10, September
 Shape and surface structure
 Broad size distribution.
 Most of the pebbles are
well spherically shaped,
smooth surface.
Optical micrographs and SEM
Optical micrographs and SEM of
the pebbles with 1mm diameter
SEM of pebble’s surface
CBBI-16, Portland, 8-10, September
 Phase analysis
Heat treatment atmosphere: Vacuum, air
temperature: 1000℃
time: 2h
5000
3000
Li2CO3
Li2SiO3
2500
Internsity (Counts)
Internsity (Counts)
4000
Li4SiO4
3000
2000
Li2SiO3
Li4SiO4
2000
1500
1000
500
1000
0
15
20
25
30
0
15
20
25
30
35
40
45
50
55
60
65
2
XRD pattern of pebbles annealed at air
70
35
40
45
50
55
60
65
70
2
XRD pattern of pebbles annealed at vacuum
TG curve of Li4SiO4 at CO2 atmosphere
Li4SiO4 as the major phase,
The diffraction peaks of Li2CO3, Li2SiO3,Li4SiO4
Li2SiOabsorb
second
are observed. Carbon dioxide are easily
3 as a rate
Temp. <500℃
veryphase
slow;
500℃ < Temp.< 720℃ absorption obviously;
absorbed by Li4SiO4
720℃ < Temp.< 900℃ CO2desorption
CBBI-16, Portland, 8-10, September
 Thermal analysis
 Physical properties
Measurement of Density and porosity by Hgporosimetry and He-pycnometry.
Specific surface area measurement by a
multipoint BET method.
TG
Mass change:-41.67%
DSC
716.7℃
Thermoanalysis of mixed raw materials
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.
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
CBBI-16, Portland, 8-10, September
 Chemical Composition of pebbles
 Behavior in air
Pebbles 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.
Elements analysis by ICP-OES
0.6
Weight increase (wt%)
0.5
0.4
0.3
Initial state
After annealing
0.2
0.1
0.0
0
10
20
30
40
50
Days
Weight increase of initial state pebbles
and after annealing pebbles.
The amount of impurities are 0.116186%
Li/Si molar ratio ≠ 4
CBBI-16, Portland, 8-10, September
 Mechanical properties
Mechanical 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
16
Min. load (N)
4.3
5.2
Average load (N)
6.5
7.0
press
pebble
After heat treatment , the crush load increased. The value is
scattered. The mechanical stability must be improved.
CBBI-16, Portland, 8-10, September
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
Experimental procedures of D2+ implantation
Sintering
Heating
treatment
XPS
Temperature: 1173 K
Heating time: 3 h
Heating temperature: 1000 K
Heating time: 10 min
X-ray source: K α of Al
Ion energy: 3.0 keV D2+
D2+ Imp.
Ion fluence : (0.4, 0.6, 0.8, 1.0)× 1022 D+ m-2
Ion flux: 2.0×1018 D+ m-2 s-1
XPS
TDS
Implantation temperature: R.T.
Heating rate: 5 K min-1
Heating region: R.T. - 1000 K
CBBI-16, Portland, 8-10, September
13
XPS results
 Comparision of before implanation and after implanation
22
+
22
-2
+
22
+
-2
22
+
-2
22
+
-2
22
+
-2
+
-2
22
+
-2
22
+
-2
22
+
-2
0.6*10 D m
0.8*10 D m
0.6*10 D m
-2
22
0.4*10 D m
0.4*10 D m
1.0*10 D m
0.6*10 D m
Before implanation
Before implanation
Before implantation
22 + -2
22 + -2
0.8*10 D m
0.4*10 D m
1.0*10 D m
0.8*10 D m
1.0*10 D m
64
62
60
58
56
54
52
Binding Energy (eV)
Li-1s XPS spectra
50
48 112
110
108
106
104
102
100
540
Binding Energy (eV)
Si-2p XPS spectra
538
536
534
532
530
Binding Energy (eV)
O-1s XPS spectra
Atom Li: 55.6 eV
Si-O- : 107.1 eV
O-Si: 536.1eV
Li-O- : 53.3 eV
Si-O-D : 105.2 eV
D-O-D: 533.8eV
-O-D: 531.3eV
CBBI-16, Portland, 8-10, September
528
 Comparision of before implanation and after implanation
Before Dimplan
After TDS
After Dimplan
64
62
60
58
Before Dimplan
After TDS
After Dimplan
56
54
52
Binding Energy (eV)
Li-1s XPS spectra
50
48
46
112
110
108
106
104
Binding Energy (eV)
102
Before Dimplan
After TDS
After Dimplan
100
542
Si-2p XPS spectra
540
538
536
534
532
Binding Energy (eV)
O-1s XPS spectra
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.
CBBI-16, Portland, 8-10, September
530
52
0.4
0.2
0.0
300
400
500
600
700
800
-2
14.0
Dm
1.0*10 D m
22 + -2
0.8*10 D m
22 + -2
0.6*10 D m
22 + -2
0.4*10 D m
0.8
0.6
19
-2
D retention / 10
-2 -1
+
18
0.6
22
Desorption rate / 10 m s
-2 -1
0.8
18
Desorption Rate (10 D2m s )
TDS results
0.4
0.2
0.0
Temperature (K)
TDS spectra of D2 for Li4SiO4 at different fluence
10.0
8.0
6.0
4.0
2.0
0.0
300
400
500 600 700
Temperture / K
800
Peak analysis for TDS spectrum at
the 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.
Peak 1
Peak 2
Peak 3
Total
12.0
0.4
0.6
0.8
22
1.0
-2
fluence / 10 D m
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
 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.
CBBI-16, Portland, 8-10, September
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-spheronizationsintering 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) ).
CBBI-16, Portland, 8-10, September
 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℃ )
CBBI-16, Portland, 8-10, September
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.
CBBI-16, Portland, 8-10, September
CBBI-16, Portland, 8-10, September

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