1. Molten Salt Synthesis of Fe/Cr/Co Silicide Nanoparticles
Craig Cross, David Otto and Dr. Steven N. Girard University of Wisconsin – Whitewater, Department of Chemistry
Demonstration of Phase Control
Introduction
Reactions performed followed the equations:
Fe3O4 + 3 SiO Mg  3 FeSi + 10 MgO
Fe3O SiO Mg  3 FeSi MgO + 6 Mg2Si
Chromium-Silicon
Phase Diagram
Reactions performed followed the equations:
Cr2O3 + 8 SiO Mg  2 CrSi MgO + 4 Mg2Si
3Cr2O3 + 2 SiO Mg  2 Cr3Si + 11 MgO
Chromium Silicides
CrSi2
Previous work in Girard Lab showed the ability to synthesize Cr3Si (as shown below) and Cr5Si3 phases.
We attempted to synthesize CrSi, but due to the non-congruent melting point, failed to do so.
Cr3Si
Cobalt-Silicon
Phase Diagram
The reaction performed followed the equation:
Co3O4 + 3 SiO Mg  3 CoSi + 10 MgO
Cobalt Silicides
CoSi
Other Cobalt Silicide phases have not been attempted. These reactions will be tried in the future. The results for phase-control of other metal silicides leads us to believe other phase-pure Cobalt Silicides can be synthesized.
CoSi2 has a non-congruent melting point, which may make it difficult to synthesize using current methods.
Silicon is the second most abundant element in earths crust, most commonly found as SiO2 (silica).
Silica is an inexpensive material to purchase.
Even with silicon’s high abundance, it is expensive to obtain in pure form, as the process of separating silicon from silica requires temperatures above 1700oC.
Silicon has many applications within the technology and energy fields, being used within computer processors and renewable energy technologies.
SiO2 (s) + C (s) → Si (l) + CO2 (g)
Electric arc furnace used to produce metallurgic grade silicon (98% pure)
Silicon
Solar Cells
FeSi2 has a non-congruent melting point
At temperatures below 962 °C FeSi2 is semiconducting and can be used for solar and thermoelectric applications.
At temperatures above 962 °C FeSi2 is metallic and has poor properties for solar and thermoelectric applications.
Iron-Silicon Phase Diagram
Iron Silicides
Silicides
Silicides are compounds where a metal is bonded to silicon, the general formula being MxSiy
Most transition metals can form silicides, each also being able to form multiple crystalline phases.
Because of these multiple crystalline phases, it is difficult to create phase-pure metal silicides in a high volume.
Silicides show properties that could be useful for new technologies including lithium ion batteries, solar cells, thermoelectrics, and other products.
FeSi
FeSi2
The main focus of silicide production was for CrSi2, FeSi2, and CoSi, as these have been shown to have thermoelectric properties. Other members of Girard lab have also synthesized MnSi1.73 which possess thermoelectric properties as well.
Thermoelectrics
Producing Phase-Pure Silicides
Prior work in Girard lab showed that by doubling molar SiO2 values, the silicon rich phases formed in higher purities.
Prior work in Girard lab showed the addition of Li2O increases the solubility of silicon within the reaction.
Our approach is to find ratios of Li2O: SiO2 that maximize the purity of each individual product in an attempt to synthesize a pure product.
FeSi2, CoSi, and CrSi2 have shown properties that could be useful for solar and thermoelectric applications.
Lithium Oxide Addition
Metal Oxide Purity
Conclusions
Silica has low solubility in solution causing slower reactivity, leading to reactions favoring the metal-rich silicide phases.
Addition of Lithium Oxide increases solubility, driving the reaction towards the desired product.
Adding Lithium Oxide increases magnesium oxidation, freeing up electrons for the metal/silicon reduction, and preventing unwanted Mg2Si formation.
The oxide ion acts as a Lewis base, with more O2- dissolved in solution leading to an increase in basicity. This principle is called oxo-basicity.
Li2O concentrations were calculated based on percentage of total moles in the reaction.
While attempting to replicate results of another member of Girard Lab, we inadvertently discovered that the crystalline quality of the metal oxide starting material directly influenced the crystalline quality of the metal silicide product.
Phase-pure metal silicides were able to be produced at temperatures as low as half of typical liquid state reactions.
The amount of Lithium Oxide needed to create phase-pure metal silicides was found to be directly related to the amount of silica within the reaction.
1:1 phase – 5% Li2O
1:2 phase – 10% Li2O
The quality of the metal oxide starting material was found to affect the quality of the metal silicide product.
Battery Electrodes
Li2O(s) → 2Li+(aq) + O2-(aq)
Mg(s) + O2-(aq) → MgO + 2e-
Experimental O Si Li - +
The ionic charges of silicates make them more soluble than silica in ionic solutions.
Cr2O3
CrSi2
Synthesis of Metal Oxide Nanoparticles
Combine Starting Materials
Place Precipitate in Autoclave
Laboratory Oven
170oC for 18 h – 4 days
Wash and Centrifuge to Collect Pellet
Anneal at 400oC for 4 h
0% Li2O
5% Li2O
10% Li2O
20% Li2O
CrSi2
20% Li2O
10% Li2O
5% Li2O
1% Li2O
0% Li2O
FeSi2
Future Work
The downward slope on the left side of the blue plot signifies amorphous material within the sample. This along with the lower intensity of peaks is what classifies it as lower quality.
Lower quality chromium oxide was autoclaved for 18 hours, high quality chromium oxide was autoclaved for 4 days.
CrSi2 and CoSi nanoparticles were synthesized with excellent phase-purity. Larger quantities of these nanoparticles will need to be produced so their thermoelectric properties can be measured.
FeSi2 was shown to be synthesized in the desired 1:2 phase, but further analysis using other methods will need to be conducted to show certainty that the product formed was the semi-conducting low temp phase.
We will attempt to synthesize nanoparticles using new transition metals as starting materials.
New methods of creating manganese oxide and iron oxide starting materials will be tried to see if purity of these oxides follow the same trend as for both cobalt and chromium oxides.
Co3O4
Fe3O4
Cr2O3
Transmission Electron Microscopy (TEM) of low quality CrSi2 nanoparticles
Transmission Electron Microscopy (TEM) of high quality CrSi2 nanoparticles
200 nm
100 nm
PXRD data is analyzed by comparing obtained peaks from samples with known catalogued patterns.
This data shows samples made to be the metal oxide precursors required for the following reactions.
Final samples are then analyzed using Powder X-Ray Diffraction (PXRD)
Magnesiothermal Reduction
Magnesium
Nano SiO2
Nano Metal Oxide
KCl/LiCl
Eutectic
Materials combined in inert gas glove box, ground with mortar and pestle for 15 m
Ground powder is placed within sealed reactor before removing from glove box
Reactor placed in furnace at 700oC for 3 h
Product is washed and dried
These reactions use Magnesium as a reducing agent to oxidize both the metal and silicon present.
These reactions are performed at high temperatures, typically at 700°C.
The KCl:LiCl eutectic is used as a molten salt solvent to facilitate the reaction.
Low quality nanoparticles show amorphous formations surrounding the chromium silicide. This presence of amorphous material will negatively impact their thermoelectric properties.
High quality nanoparticles show clean, faceted edges on the chromium silicide particles formed.
Research shows that for double silica reactions and corresponding 1:2 phase products, a 10% Lithium Oxide concentration produces the highest purity products.
References
10% Li2O
5% Li2O
0% Li2O
CoSi
FeSi
Co3O4
CoSi
Liu, X.; Fechler, N.; Antonietti, M. Chemical Society Reviews 2013, 42,
Song, G.; Shi, X.; Sun, K.; Li, C.; Uher, C.; Baker, J.; Banaszak, M.; Orr, B.; Journal of Physical Chemistry 2009, 113,
Yang, J.; Liu, H.; Martens, W.; Frost, R.; Journal of Physical Chemistry 2010, 114,
Yang, J.; Baker, A. G.; Liu, H.; Martens, W. N.; Frost, R. L, Journal of Materials Science 2010, 50,
5% Li2O
Acknowledgements
0% Li2O
Special thank to the UW-Whitewater Provost funds and the College of Letters and Sciences
The lower quality cobalt oxide was annealed at 250°C, the higher quality cobalt oxide as annealed at 400°C.
As can be seen by the peak intensities, CoSi nanoparticle quality is greatly influenced by the cobalt oxide starting material.
Research shows that for single silica reactions and corresponding 1:1 phase products, a 5% Lithium Oxide concentration produces the highest purity products.