Patent Description:
As is known from scientific publications, among others <NPL>), boron carbide occurs in the form of numerous crystalline phases. The general formula of boron carbide is presented as BnCm, where n comprises the ranges: <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> and <NUM>-<NUM>, m takes the values <NUM>-<NUM>, and the most often occurring phases are B<NUM>C, B<NUM>C<NUM>, BC, B<NUM>C, B<NUM>C, B<NUM>C, B<NUM>C<NUM>, B<NUM>C<NUM> and B<NUM>C<NUM>. Boron carbide powders are used, among others, to produce dense, polycrystalline sinters, which have found a number of industrial applications due to their specific properties, such as extreme hardness (<NUM>-<NUM> GPa), relatively low density (~<NUM>/cm<NUM>), high Young's modulus (<NUM>-<NUM> GPA) and high chemical resistance.

Boron carbide powders are currently synthesized by a number of methods, among others, by carbothermic reduction, magnesothermic reduction, synthesis from elements, gas phase synthesis, synthesis from polymer precursors, liquid phase reaction, ion beam synthesis, VLS method (Vapour-Liquid-Solid Growth). A commonly used method for synthesis of boron carbide is synthesis by carbothermic reduction, which uses boric acid H<NUM>BO<NUM> as a boron precursor and petroleum coke as a carbon precursor. The optimum ratio of boric acid to petroleum coke is <NUM>:<NUM>. At higher boron to carbon ratios, achieving pure powder without the addition of free carbon is possible, however due to the excessive boron content and its melting point, recovery of the achieved boron carbide powder from the furnace is impossible, due to fusion of the powders into the graphite crucibles. The addition of a small amount of sodium chloride is then usually used: <NUM>-<NUM>% by weight, which significantly improves the process yield by lowering the content of residual free carbon and at the same time reducing the energy requirement by <NUM>%. Unfortunately, simultaneously during the direct synthesis of boron-enriched boron carbide, there is a loss of boron in the furnace, which causes the contamination of the reactor and the losses of a raw material. For this reason, this process has not found wider application in the commercial synthesis of boron carbide. The boron carbide powders obtained by this method are characterized by a particle size above <NUM> and very often contain unreacted boron.

A method for producing submicron sized boron carbide powder from boric acid H<NUM>BO<NUM> or boron oxide B<NUM>O<NUM> is known from patent specification <CIT>, and the carbon source useful for the B<NUM>C synthesis is any carbon-containing material that will form carbon when heated. The carbon source used during the synthesis must be characterized by high purity and does not contain heavy metals (Fe, Cr, Ni), their content is maximum of <NUM> ppm, and the most preferable concentration is below <NUM> ppm. Increasing the chemical purity of boron carbide requires the removal of residual carbon remaining after synthesis. To eliminate carbon in the synthesis products, an excess of B<NUM>O<NUM> is used in the reaction to minimize the carbon content to <NUM>% in the achieved powder. After synthesis, B<NUM>O<NUM> is recycled or reused to the synthesis of of boron carbide.

Another known method, is a synthesis of boron carbide from boron and carbon powders, which are heated and next comminuted. For example, patent description <CIT> discloses a method for producing of boron carbide with different stoichiometric configurations. The synthesis method is characterized by mixing <NUM>% by weight of boron particles and <NUM>% by weight of carbon, in the temperature range from <NUM> to <NUM>, with heating rate from <NUM> to <NUM>, over a period of <NUM> hour. The boron carbide particles obtained in this manner can be used to produce dense sinters by hot pressing at the temperature in the range from <NUM> to <NUM>.

Document <CIT> discloses a method for preparing boron carbide nanoparticles. The process involves heating and reducing boron powder, which serves as the raw material, and a carbon material, which acts as a reducing agent, together with a transition metal inorganic salt serving as a catalyst in a furnace to generate B4C nanoparticles. The molar ratio of boron powder to carbon material ranges from <NUM>: <NUM> to <NUM>: <NUM>. Additionally, a transition metal inorganic salt catalyst with a mass fraction of <NUM>-<NUM>% is added. The components are then ultrasonically mixed uniformly, dried, and heated in a high-temperature furnace to obtain the final product.

Document <CIT> discloses a process for the preparation of boron carbide (B<NUM>C) comprising the step of carbidizing boron oxide, whereby: a. said carbidizing comprises heating boron oxide and carbon particles in an inert atmosphere, at a temperature not to exceed <NUM>; and b. said carbon particles have a diameter which does not exceed <NUM>.

Document <CIT> discloses a method for preparing nano twinned boron carbide powder by using boric acid and carbon source, adding to deionized water, stirring, heating to dryness, grinding, placing in graphite crucible in furnace, heating and finally cooling the resulting powder.

Document <CIT> discloses a method for preparing a fine-grained boron carbide ceramic, comprising the steps of:.

Although many methods of boron carbide synthesis are known and used, most of them do not allow to achieve a high purity powder. A tremendous problem of the currently used synthesis methods is, in particular, high agglomeration and aggregation of the obtained products after synthesis with particle size above <NUM>-<NUM>. For this reason, intensive mechanical processing of the achieved powders is necessary, which in turn causes their significant contamination with iron molecules, associated with the use of steel grinding mediums for milling hard agglomerates. Commercially achieved boron carbide powders are characterized by a purity of <NUM>-<NUM>% and a particle size above <NUM>. Long-term purification methods, generate increase of production costs and make it impossible to achieve boron carbide particles with a size below <NUM>, despite increased milling time. Products achieved from such powders have a low density and are porous, which in turn leads to their low strength and poor resistance to brittle cracking.

The aim of the invention is to achieve boron carbide particles of submicron size (less than <NUM>) and high purity, not achievable previously by methods commonly used, what will improve the properties of products obtained from these powders and increase the possibilities of its application.

The gist of the method of obtaining boron carbide nanoparticles by direct synthesis process from boron and carbon powders, which are heated at the temperature of <NUM>-<NUM> and next comminuted, is characterized by the fact that in a graphite crucible, on a graphite pad, the following are placed successively: a layer of amorphous boron powder, and next a layer of carbon in the form of amorphous carbon black, wherein the ratio of amorphous boron to amorphous carbon black is <NUM>:<NUM> by weight, and all of it is heated for <NUM> minutes to <NUM> hours. From the obtained, according to the kinetics of the reaction, bed of the product being in the form of three clearly distinguishable layers, the top two layers are poured down and recycled and/or returned as a raw material for the re-synthesis of boron carbide, and the layer located at the lowest, near the bottom of the crucible, distinguished by a light gray colour and showing the greatest compactness and hardness, is subjected to intensive milling using steel elements in an isopropanol environment for a period of <NUM> to <NUM> hours, and next is subjected to chemical etching in order to wash out impurities in the form of iron ions derived from milling , using successively: concentrated HCl, next concentrated HNO<NUM> and again concentrated HCl. The obtained powder is subjected to repeated washing out using distilled water until achieving pH of the suspension from <NUM> to <NUM>, and in the final step the suspension is subjected to intensive centrifugation at speed from <NUM> to <NUM> rpm, with centrifugation time from <NUM> minutes to <NUM> hours, achieving rounded off grains of boron carbide from <NUM> to <NUM> in size.

The method according to the invention allows to achieve boron-rich boron carbide powder having high purity and small grain size (below <NUM>), with rounded off shapes. The advantage of the proposed solution is the possibility to precisely control the morphology of the obtained particles through the parallel use of milling and chemical etching. The obtained powder, due to size and shape of the grains, can significantly improve the sinterability of boron carbide-based composites intended for producing of wear-resistant machine parts and devices. These features are also beneficial for medical applications, such as drug carriers. Furthermore, the configuration of the substrates bed allows for complete utilization of boron during synthesis and does not cause furnace contamination resulting from increased boron vapor pressure, since all boron reacts with carbon monoxide during synthesis, which is assured by a layer of carbon black over a layer of boron and it is visible as synthesis effect.

The method of obtaining boron carbide nanoparticles according to the invention, is explained below in practical embodiments and in the figure, in which <FIG> shows the XRD phase analysis of boron carbide obtained by the method described in example <NUM>, <FIG> - a transmission electron microscope (TEM) image of boron carbide powder obtained by the method described in example <NUM>, <FIG> - the XRD phase analysis of boron carbide obtained by the method described in example <NUM>, and <FIG> - the transmission electron microscope (TEM) image of boron carbide powder obtained by the method described in example <NUM>.

The following powders were prepared: amorphous boron (Sigma Aldrich p. ) and technical carbon black P-<NUM> (Thujmazy). A phase composition analysis performed showed that the commercial boron powder consisted of <NUM> phases: beta boron (card number <NUM>-<NUM>-<NUM> according to ISCD database), boric acid (card number <NUM>-<NUM>-<NUM> according to the ICSD database) and boron (card number <NUM>-<NUM>-<NUM> according to ICSD database), while the carbon black was fully amorphous. Direct synthesis from the above-mentioned powders was used to produce boron carbide. A graphite pad was placed at the bottom of the graphite crucible, a layer of amorphous boron powder in the amount of <NUM> was placed on it, and next, after levelling the surface, a layer of amorphous carbon black in the amount of <NUM>. This procedure simultaneously reduces the diffusion of boron in the bed and its loss during synthesis in the furnace. It also has a significant effect on the morphology of the achieved powder because the use of such method of synthesis reduces the agglomeration and aggregation of the resulting boron carbide grains and allows to achieve powder of a high purity. The crucible was placed in the graphite furnace and heated at the temperature of <NUM> under argon atmosphere for <NUM> hour. The bed containing three layers was achieved. The layer I, located at the lowest, near the bottom of the crucible, was distinguished by its light gray colour and showed clearly greater compactness and hardness than the other layers. The top two layers (layer II and layer III) showed dark gray to black in colour and were characterized by a looser consistency compared to the layer I. The layer III was composed of a loose powder entirely consisting of carbon black as the studies showed. The layer II was recycled and the layer III was returned as a raw material for the re-synthesis of boron carbide. The layer I, in the form of powder with an average particle size of <NUM>, was subjected to intensive milling using steel elements in a rotary-vibrating mill, in an isopropanol environment, for a period of <NUM> hours. Next the powder was subjected to chemical etching in order to wash out impurities in the form of iron ions derived from milling , using successively: concentrated HCl (<NUM>%) for <NUM> hours, next concentrated HNO<NUM> (<NUM>%) for <NUM> hours and again concentrated HCl (<NUM>%) for <NUM> hours. The achieved product, as confirmed by the XRD phase analysis (<FIG>), consisted exclusively of the B<NUM>C<NUM> phase, considered the most stable and with a high carbon solubility in the structure, which is particularly important for potential medical applications, and does not possess any of impurities detectable by X-ray diffraction and X-ray fluorescence. Next the product was washed out <NUM> times using distilled water until achieving pH = <NUM>, after which the obtained suspension was subjected to intensive centrifugation for <NUM> minutes at <NUM> rpm (centrifuge MPW-<NUM>, MPW Med. Instruments). A boron carbide powder of almost <NUM>% purity, with rounded off grains, average particle size less than <NUM> was achieved, the TEM image of which is shown in <FIG>.

Claim 1:
A method of obtaining boron carbide nanoparticles by direct synthesis process from boron and carbon powders, which are heated at the temperature of <NUM>-<NUM> and next comminuted characterized in that in a graphite crucible, on a graphite pad, the following are placed successively: a layer of amorphous boron powder, and next a layer of carbon in the form of amorphous carbon black, wherein a ratio of amorphous boron to amorphous carbon black is <NUM>:<NUM> by weight, and all of it is heated for <NUM> minutes to <NUM> hours, after which from the obtained bed of the product in the form of three clearly distinguishable layers, the top two layers are poured down and undergo recycling and/or are returned as a raw material to the re-synthesis of boron carbide, and the layer at the lowest, near the bottom of the crucible, distinguished by its light gray colour and showing the greatest compactness and hardness, is subjected to intensive milling using steel elements in an isopropanol environment for a period of <NUM> to <NUM> hours, and next is subjected to chemical etching to wash out impurities in the form of iron ions derived from milling , using successively: concentrated HCl, next concentrated HNO<NUM> and again concentrated HCl, after which the obtained powder is subjected to repeated washing out using distilled water until achieving pH of the suspension from <NUM> to <NUM>, and in final step, the suspension is subjected to intensive centrifugation at a speed of <NUM> to <NUM> rpm, with a centrifugation time from <NUM> minutes to <NUM> hours, achieving rounded off grains of boron carbide from <NUM> to <NUM> in size.