Synthesis of iron borides

Abstract

In this report, the synthesis of iron borides has been studied. Iron and boron powders have been mixed in amounts corresponding to 50 at-% of each element. Differential scanning calorimetry (DSC) has been used to observe any reactions taking place up to a temperature of 1550 °C at heating rates of 5, 10 and 10 K/min, both for pre-compacted Fe-B powder mixtures as well as loose powders. The results indicate a reaction taking place at around 1150 °C, which was mostly seen when the highest heating rate was used. Additionally, Fe-Al-B mixtures have been studied in the DSC. Reactions could be observed at around 600 °C, most likely belonging to the Fe-Al system.
X-ray diffraction experiments have been performed on all samples that had gone through the DSC experiments, and the results indicated that for the Fe-B mixtures the two known stable iron borides had formed, FeB and FeB2. For the Fe-Al-B mixtures the x-ray diffraction data proved too noisy to be able to clearly determine what had formed.
Since the reaction mostly was visible in the DSC at higher heating rates, it has been assumed that the actual formation of the iron borides take place at a wider temperature range, possibly due to diffusion. With higher heating rates there is less time for this process to occur, and a larger amount of the iron boride will be formed at 1150 °C.

Popular Abstract

Iron Borides - Synthesis, powders and analysis

An important factor in the production industry is, quite obviously, the actual method of production. Using metal powders as the raw ingredients will require lower furnace temperature leading to lower costs while also reducing the safety risks of the operation, compared to methods where the materials would need to be melted.
However, before any production of this kind can take place some ground work needs to be done. One must find out what happens when the metal powders in question are combined, and how to obtain the desired product. In short, studying the synthesis.
There is a variety of compounds, iron borides, that can be formed from reactions between iron and boron, with a rather wide range of properties and possible applications. The two most common of these iron borides are FeB and Fe2B. These two are strong, hard and resistant to corrosion, and as such they are primarily used to coat and strengthen other materials. Furthermore, a number of metastable, more boron-rich iron borides have previously been formed under special synthesis conditions such as laser coating, as well as during very high pressure experiments. These compounds display a different set of properties compared to the stable iron borides. FeB4, for example, is a compound with high hardness that is presumed to have potential as a superconductor and may very well be the future of studies in the iron boron system, but as mentioned above the formation requires very high pressure, and equipment suitable for these kinds of operations.
In this case, the method of studying the iron boron system was as follows. Iron and boron powder were mixed and poured into a die, after which the powder was compacted into a small plate. This plate was analyzed using differential scanning calorimetry (DSC), where the sample is heated up in an oxygen-free environment at a steady heating rate following a predetermined temperature program. The DSC analysis produces a graph from which one can estimate if a reaction or transition has taken place, as well as showing if said reaction was endothermic or exothermic. The formation of FeB and Fe2B are exothermic reactions, and any endothermic transitions in this analysis would likely come from a melting transition.
DSC analyses of samples containing 50-50 mol-% iron and boron that were heated to 1550 °C only showed a rather small exothermic reaction peak at around 1150 °C, and for the lower heating rates it appeared that no transition at all took place during heating. The same general trend applied for the following DSC analyses as well, both when iron powder with larger particle size was used as well as when non-compacted powder samples were analyzed.


To be able to determine what, if anything, had actually been formed during the DSC analyses, powder that was grinded down from the DSC samples were analysed using X-ray diffraction spectroscopy where the X-ray spectra obtained can be compared to a database of known compounds to determine which compounds and elements that are present. These analyses indicated that both FeB as well as Fe2B had been formed, also for samples where no reaction could be seen in the DSC graph. Therefore, the question was: Why was the iron boride formation not detectable in the DSC at lower heating rates? Why was there no reaction peak?

One reason might be that the entire formation of iron boride does not take place at a single temperature. Instead, the formation may initially occur through diffusion of boron into the iron matrix. Then, at 1150 °C there is enough thermal energy to initiate the reaction between iron and boron. A higher heating rate would leave less time for the diffusion process, meaning that a larger part of the iron boride will be formed at this temperature, and the reaction peak will thereby be larger in the DSC spectrum.

• author: Almskoug, Jerker
• supervisor: Kumar Babu Surreddi; Hossein Sina
• organization: Materials Engineering
• course: FKM820 20161
• year: 2016
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• language: English
• id: 8873565
• date added to LUP: 2016-05-23 07:36:48
• date last changed: 2016-05-23 07:36:48