Development of Li4SiO4-based Ceramics for Solid Breeder and CO2 Absorption Applications

Abstract

Lithium orthosilicate (Li4SiO4) have been studied as an attractive ceramic solid breeder
material due to its high lithium atom density, low neutron activation characteristics
and prominent tritium release rate at low temperatures. In addition, Li4SiO4 is
also considered as a promising solid sorbent for CO2 capture owing to its sorption
capacity at high temperatures (450-700°C), excellent cyclability, faster sorption rate,
cost-effective and lower regeneration temperatures (≤ 800°C). However, Li4SiO4 has
several drawbacks, namely high phase formation temperature (> 900°C), poor sintered
density (< 80%), grain growth during sintering (> 10 μm), poor crush load in pebble (<
15N) and high moisture sensitivity. The phase purity and particle size of Li4SiO4 are
the two critical factors which can directly influence the CO2 absorption capacity. The
preparation of nanoscale Li4SiO4 powders may improve the sinterability and CO2 capture
property. For design aspect, the pebble configuration has been recognized as preferred
option in most of the blanket designs for tritium breeding. For solid breeder application,
the pebbles having a higher bulk density (> 85% TD), open porosity (around 10-15%),
phase purity, uniform grain size (< 10μm), good sphericity (~1) and reasonable crush
load (> 30N) are required. Extrusion-spherodization is a simple and feasible technique
for the bulk preparation of Li4SiO4 pebbles. However, still there is no detailed study that
addresses the different factors affecting the formation of Li4SiO4 pebbles with desired
properties using this technique.
Keeping the above factors in mind, the major objectives of the present work
are to synthesize finer Li4SiO4 powder using the solution and solid state routes at
lower calcination temperatures (< 900°C) and study different properties. To fabricate
Li4SiO4-based pebbles with narrow size distribution (0.8–1.2 mm), good sphericity
(0.9-1.0) and reasonable crush load using the extrusion-spherodisation technique by
monitoring different process parameters e.g. binder type, amount of binder and moisture
using the solid state derived Li4SiO4 powder. The other objectives are to prepare Li4SiO4-based composite ceramics for further improvement of the density, mechanical
property and stability and also to study the secondary phase addition on the electrical,
thermal and mechanical properties of the composite ceramic.
The phase pure Li4SiO4 powder has been synthesized by the solution combustion
method at 700°C using the cost effective synthetic silica source like silicic acid. In this
method fuel-to-oxidizer ratio of the solution can effect the nature of the combustion
reaction and phase formation. Li4SiO4 is also prepared by the solid-state method at a
calcination temperature of 800°C. The average particle size of the solution route Li4SiO4
powder (for Φe =0.6) is found to be 72 nm with high surface area (26.7 m2/gm). The
densification behavior, phase stability, thermal, electrical and CO2 absorption properties
of the solution route prepared sintered ceramic are studied and the obtained results are
compared with the solid-state route samples. The combustion derived powder shows
higher sinterability (~84% RD) and CO2 absorption capacity of 30.7wt% (307mg/g)
at 700°C after 60min which is equivalent to 83.9% of its theoretical efficiency. The
absorption kinetic model and CO2 absorption cyclability are studied to understand the
CO2 absorption behavior of the solution route prepared powder.
Further, Li4SiO4 pebbles are fabricated by extrusion-spherodization technique using
three different binders namely PVA, PVP and guar gum. The effect of these binders
on the fabrication process is studied and analyzed. We standardize the amount of the
binder and moisture to get uniform sized Li4SiO4 green pebbles. The fabricated pebbles
are found to be sintered to an appreciable density (~85%) with an average crush load of
17N. The surface energy and the microstructure of the green aggregates are studied to get
comprehensive information for achieving better properties in PVP derived pebbles. For
comparison, the pebbles are also prepared using combustion route powders. The crush
load and density values obtained for combustion route pebbles are marginally higher
than the solid state route.
To further enhance the density, crush strength, and moisture stability of the
Li4SiO4 pebbles, Li4SiO4-Li2TiO3 (LS-LT) and Li4SiO4-Li2ZrO3 (LS-LZ) composite
are studied. Four different solid state techniques are explored to synthesize LS-LT
and LS-LZ composite powder. It is observed that the composite powder prepared
with the precursor oxide/carbonates (Li2CO3, SiO2, TiO2, ZrO2 ) is more reactive
and shows better densification. For different amount of Li2TiO3/Li2ZrO3 (0-15wt%)
addition the effects on the densification, microstructure, thermal, electrical properties
and mechanical properties of Li4SiO4-composite samples have been studied. The
phase pure Li4SiO4-Li2TiO3 and Li4SiO4-Li2ZrO3 composite powders are successfully
synthesized using the solid-state method at 800 and 900°C respectively. The composite
pebbles result in 89 and 87% of its theoretical density for 5 wt% addition of LT and LZ
respectively at 1000°C. The average size of the Li4SiO4 grain in this sintered pebbles is
found to be reduced by 28.4% and 19.7% in comparison with the pure Li4SiO4. The
average crush load value of the composite (5 wt%) pebbles becomes doubled [33N (for
LS-LT) / 30N (for LS-LZ)] to that of the pure sample (17N). The thermal expansion,
thermal conductivity and electrical conductivity of the composite pellets are found to
be slightly reduced with increase of Li2TiO3/Li2ZrO3 content. The composite samples
show an excellent stability to moisture when compared to pure Li4SiO4.