Method for production of anorganic nanofibres and/or nanofibrous structures comprising TiN, anorganic nanofibres and/or nanofibrous structures comprising TiN

Technical field

The invention relates to the method for production of anorganic nanofibres and/or nanofibrous structures comprising TiN.

Next to this, the invention relates to anorganic nanofibres and/or nanofibrous structure comprising TiN.

Background art

TiN excels with high hardness of cca 85 of Rockwell scale, high wear resistance, chemical stability and further special physical properties. It is used for surface finish of alloys of titanium, steels, aluminium and further materials for the purpose to improve their utility properties. Mostly this is applied as coating on surfaces of drills, lathe tools, screw cutters and similar tools by which they increase their service life up to several times. They provide the same effect on pressing tools, they prolong the life of forms for pressing of plastics and thanks to parameters of surface they also increase the quality of pressings. TiN coatings are produced directly through the PVD or CVD methods. Another property of TiN is electrical conductivity, resistance against corrosion and IR reflectivity. TiN features a gold-like colour, therefore it is often used for decoration purposes in jewellery, as a coating for spectacle frames, etc. It is hardly recognizable from gold coatings, but it features many times higher abrasion resistance. Recently the TiN coatings appear also on surgical instruments, at which they increase, besides their life, also the smoothness of cuts, moreover the TiN coating features antibacterial effects.

TiN in micro or nano form is advantageous from the point of view of high specific surface, at the same time of a minimum size, thanks to which it can be

better incorporated into the structure of treated surfaces and composites. At present it is being produced only in a form of nano-powders or thin TiN layers.

Nanofibrous morphology of TiN, with thickness of fibres from tens to hundreds of nanometers, has a number of further important attributes at simultaneous keeping of all the above mentioned properties of TiN. Besides the high specific surface it features a distinct improvement in accessibility of surface.

For the time being, no other production methods of pure TiN nanofibres without further admixtures are known. For example patent JP1215718 discloses production of TiN nanofibres through annealing of Tiθ ₂ nanofibres in NH3 atmosphere, nevertheless the resultant nanofibres whose length is within the range from 300 to 2000 nm and the ratio of their length and diameter from 3 to 200, contain up to 40 % of residual O ₂ , or more precisely they contain the residual TiU2. There are also known production methods of nanoparticles of a pure TiN through annealing of TiO ₂ in NH ₃ , nevertheless in the resultant product remain the residual structures of TiO ₂ . Phase-pure TiN is usually created at high temperatures when it is difficult to keep the fibrous morphology of the product.

The goal of the invention is to develop a production method of anorganic nanofibres and/or nanofibrous structures comprising TiN in pure state without residual TiO ₂ . At the same time the goal of the invention is to develop anorganic nanofibres and/or nanofibrous structures comprising TiN in pure state without residual TiO ₂ .

The principle of invention

The goal of the invention has been achieved through the metod for production of anorganic nanofibres and/or nanofibrous structures comprising TiN, whose principle consists in that, from the polymer matrix containing titanium alkoxide dissolved in a solvent system on basis of alcohol containing a chelating agent, poly(vinylpyrrolidone) and addition of concentrated hydrochloric acid are through electrostatic spinning produced the organic/anorganic

nanofibres arranged in a nanofibrous structure, which are subject to calcination in the air atmosphere at the temperature from 350 to 700 ⁰ C and the produced ^" TϊO ₂ nanofibres arranged in a nanofibrous structure are annealed in stream of NH ₃ atmosphere at the temperature from 400 to 900 ⁰ C, by which TiN nanofibres are created without residual TiO ₂ .

The method according to the invention represents a low-temperature conversion of TiO ₂ polycrystallic nanofibres with a high accessibility of surface to TiN. The synthesis runs in ammonia atmosphere at low temperatures along full keeping of the nanofibrous structure and it is a complete one, so that the resultant product does not contain TiO ₂ . Anorganic nanofibres or anorganic TiN nanofibrous structures without any further additions and especially without TiO ₂ , represent a new form of TiN and provide new possibilities how to utilise properties of TiN in many branches.

To reach a high quality of anorganic nanofibres and/or anorganic nanofibrous structures it is advantageous, if the calcination temperature of organic/anorganic nanofibres lies within an interval from 500 to 700 ⁰ C and the temperature of annealing in NH ₃ atmosphere within an interval from 500 to 800

⁰ C.

At the same time it is preferred if the alcohol in the solvent system is selected from group of ethanol, 1-propanol, 2-propanol or of their mixtures.

Further it is preferred if the poly(vinylpyrrolidone) has the molecular weight of 1300000 g/mol and its weight concentration in the solution is within the range from 4 to 9 %.

At the same time it is preferred, if the chelating agent is beta - diketone, which is according to the claim 6 preferably acetylacetone.

At all above mentioned methods of production it is preferred, if the titanium alkoxide is selected from the group of titanium tetrabutoxide, titanium tetraisopropoxide.

Further, at all above mentioned methods it is preferred, if molecular ratio of alkoxide and chelating agent in the solution is within the range of 1 :0,8 to 1:2,2.

The goal of the invention has been achieved by anorganic nanofibres, whose principle consists in that, they are formed of TiN without residual TiO ₂ .

TiN nanofibres of such purity have not been produced yet, and their application will be based especially on their electrical conductivity, high specific surface and high accessibility of this surface.

Anorganic nanofibres are polycrystalline ones and at the same time they may be hollow or compact, according to the manner of their desired application.

The goal of the invention has also been achieved by anorganic nanofibrous structure comprising the TiN nanofibres, whose principle consists in that, it is totally formed of the TiN anorganic nanofibres without residual TiO ₂ . This anorganic nanofibrous structure may be formed, according to the requirements to application of the resultant product, of polycrystalline compact or hollow anorganic nanofibres.

One of advantages of such anorganic nanofibrous structure consists in that, its high sintering ability is kept. This ability may be e.g. used after grinding the fibres into individual particles and their further application for production of compact parts. At increasing the temperature above the temperature for preparation of nanofibres, the TiN nanocrystals are easy to sinter into greater aggregates and enable creation of dense and strong formations. The anorganic nanofibrous structure shown above is according to the claim 15 able of low-temperature reversible reaction of TiN to TiO ₂ and back to TiN with high cycling ability at simultaneous keeping the initial structure in the temperature interval of 400 - 900 ⁰ C, that means the nanofibrous structure with high specific surface or sintered nanofibrous structure. Utilisation of this reaction may be expected in physical and/or chemical processes, at which e.g. a change in colour and electrical conductivity may be utilised.

At the same time it is preferred if the temperature interval lies within 450 to 600 ⁰ C.

According to the claim 17 the anorganic nanofibrous structure is a part of a composite, which is preferably a bullet-proof material or material with high wear resistance.

Due to its high specific surface and accessibility of this surface, the anorganic nanofibrous structure presents supporting structure for catalysis.

Thanks to its electrical conductivity the anorganic nanofibrous structure is preferably a part of electrodes of electrochemical cells. According to the claim 23, the anorganic nanofibrous structure is preferably a part of solar cells.

Description of the drawings

To better understand the principle of the invention, the Fig. 1 schematically represents one of the applicable devices for production of precursor organic/anorganic nanofibres through electrostatic spinning. Fig. 2 represents the nanofibrous layer of precursor of TiO ₂ nanofibres, the Fig. 3 represents TiO ₂ nanofibres for TiN synthesis, the Fig. 4a shows keeping of morphology of nanofibres during the TiN synthesis, the Fig. 4b represents the size of nanofibres, the Figs. 4c, 4d indicate the size of TiN crystals and type of porosity. The Fig. 5a shows keeping of morphology of nanofibres during TiN synthesis, the Fig. 5b represents the size of nanofibres. Figs. 5c, 5d indicate the size of crystals, a dense sinter of TiN crystals and type of porosity of hollow fibres.

Examples of embodiment

Exemplary embodiment of a device for electrostatic spinning of polymer matrices is schematically represented in the Fig. 1 and it comprises a spinning chamber j[, in which there are against each other arranged a spinning electrode 2 and a collecting electrode 3. The spinning electrode 2 comprises the spinning mean 2A_ formed in the represented embodiment according to the CZ patent 294274 of rotatably mounted cylinder 211, extending by a section of its circumference into the polymer matrix 4 being situated in the reservoir 22 of polymer matrix. Rotating cylinder 211 carries out thanks to its rotation the polymer matrix into electrostatic field, which is induced between the spinning

electrode 2 and collecting electrode 3, while a portion of surface of the rotating cylinder 211 positioned against the collecting electrode 3 represents an active spinning zone of the spinning means 21.. During spinning thus the polymer matrix 4 is to be found in electrostatic field on surface of active spinning zone of the spinning mean 21 _, of the spinning electrode. Between the active spinning zone of the spinning mean 2J. of the spinning electrode and collecting electrode 3 a substrate material 5 led.

The substrate material 5 is mostly formed of non-woven fabric of suitable properties, nevertheless it may be formed of other suitable fabric, foil or paper. The spinning electrode 2, respective its spinning mean 21., may be formed also of different embodiment, for example according to CZ PV 2006-545 or CZ PV 2007-485 or in other suitable manner, while especially for discontinuous production is even not excluded usage of jet or needle spinning electrodes. Similarly, the collecting electrode 3 may be formed of a cylindrical rod or of a plate or according to CZ PV 2006-477 or according to CZ PV 2007-108 as a corona emitting, or also according to CZ PV 2007-727, eventually in other suitable manner.

Polymer matrix 4 containing titanium alkoxide dissolved in the solvent system on basis of alcohol comprising a chelating agent, poly(vinylpyrrolidone) and additive of concentrated hydrochloric acid is in a known, not represented manner, brought into the reservoir 22 of polymer matrix, out of which is, in example of embodiment according to the Fig. 1 , carried out by a circumference of the rotating cylinder 211 into the electrostatic field induced between the spinning electrode 2 and the collecting electrode 3, in which on the active spinning zone of the rotating cylinder 211 the organic/anorganic nanofibres 41. are produced, which are drifted towards the collecting electrode 3 and which are deposited on the substrate material 5, formed of non-woven fabric of organic fibres (microfibres), on which they create the nanofibrous structure 410 having the thickness of layer up to several hundreds of micrometers. This nanofibrous structure 410 is together with the substrate material 5 in a known manner drawn off from the spinning chamber 1. by means of take-up device 6 and behind it is deposited on a conveyor 61., and in a known manner, e.g. by means of a knife

and pad, is divided into fabric formations 51. of required length or shape. Organic/anorganic nanofibres 4J. are formed of organic polymer, in which a anorganic component formed of complex titanium compounds, is dispersed.

Fabric formations 5J _, comprising the nanofibrous structure 410 formed of organic/anorganic nanofibres 41 and the substrate material 5 are removed form the conveyor and consequently inserted into the calcining furnace 7, in which they are subjected to calcination in air atmosphere at the temperature from 350 to 800 ⁰ C, through which the substrate material 5 and organic components are removed from organic/anorganic nanofibres 41. so that the nanofibrous structure 410 contains only the anorganic nanofibres 40 of TϊO ₂ , which are polycrystalline and have a high accessibility of surface. Optimum temperature of calcination lies within interval from 500 to 700 ⁰ C.

Before inserting into the calcining furnace 7 at not represented embodiment the substrate material 5 may be removed from the fabric formations 51., so that only a part of fabric formations formed of nanofibrous structure 4_iO is subjected to calcination.

According to another not represented example of embodiment the fabric formations 51. or their parts formed of nanofibrous structures 410 are subjected to calcination continually. Once the calcination is finished, the nanofibrous structure 410 formed of

T1O ₂ nanofibres is inserted or is brought into annealing furnace 8 with streaming NH ₃ atmosphere and temperature from 400 to 900 ⁰ C, in which total conversion of TiO ₂ nanofibres 40 to TiN nanofibres 400 runs while fully keeping the nanofibrous structure and high accessibility of surface of nanofibres. The resultant nanofibrous structure does not contain TiO ₂ . Optimum temperature of annealing lies within interval from 500 to 800 ⁰ C. Streaming of NH ₃ atmosphere through the nanofibrous structure contributes especially to ideal conversion.

The obtained TiO ₂ nanofibres were e.g. after calcination warmed in streaming NH ₃ atmosphere at the temperature of 500 ⁰ C, for a period of two hours. Upon this process the TiO ₂ nanofibres were completely converted into pure TiN nanofibres without the residual TiO ₂ . Diameter of obtained nanofibres

was within the range from 30 to 1000 nanometers. The specific surface of this structure was 46 m ² /g, which corresponds to average size of individual TiN crystals about 25 nanometers.

The obtained nanofibres kept their dimensions with respect to the initial TiO ₂ nanofibres. Keeping of morphology of nanofibres during the TiN synthesis, the size of nanofibres, size of TiN crystals and type of porosity are witnessed by Figs. 4a - d.

Example 1 The polymer matrix for spinning was prepared by dissolution of 100 g of titanium tetrabutoxide in a mixture of 250 g of ethanol and 29,4 g of acetylacetone. After homogenising the obtained solution was carefully mixed with solution of 35,2 g of poly(vinylpyrrolidone) having molecular weight of 1300000 g/mol in 758,8 g of ethanol and after then acidified with concentrated hydrochloric acid. The polymer matrix for spinning thus in this example contained titanium tetrabutoxide dissolved in the solvent system containing ethanol, acetylacetone, poly(vinylpyrrolidone) and additive of concentrated hydrochloric acid.

From this polymer matrix through the electrostatic spinning the organic/anorganic nanofibres containing poly(vinylpyrrolidone) were produced, which comprised the complex titanium compounds. From these organic

/anorganic nanofibres on the substrate material there was created the nanofibrous structure, which was removed from the substrate material and consequently calcinated in furnace in the air atmosphere at the temperature of 500 ⁰ C upon production of pure polycrystalline TiO ₂ nanofibres with the size of surface being 50m ² /g. Such TiO ₂ nanofibres are shown in the Fig. 3. The nanofibrous structure of organic/anorganic nanofibres formed on the substrate material could be calcinated also together with substrate material, which would burn at the same time, so that the result would again be the nanofibrous structure from pure polycrystalline TiO ₂ nanofibres.

Alternatively the following polymer matrices were subjected to spinning through electrostatic method . a) for production of polymer matrix for production of TiO ₂ nanofibres the mixture of 250 g of 2-propanol and 29,4 g of acetylacetone was used, in which 100 g of titanium tetrabutoxide was dissolved. After homogenising the obtained solution was mixed with solution of 35,2 g of poly(vinylpyrrolidone) having molecular weight of 1300000 g/mol in 758,8 g of ethanol, and after then acidified with concentrated hydrochloric acid. Through electrostatic spinning from this polymer matrix a layer of organic/anorganic nanofibres was produced, which was consequently calcinated at the temperature of 600 ⁰ C. b) for production of polymer matrix for production of TiO ₂ nanofibres the mixture of 250 g of 1-propanol and 29,4 g of acetylacetone was used, in which 100 g of titanium tetrabutoxide was dissolved. After homogenising the obtained solution was mixed with solution of 35,2 g of poly(vinylpyrrolidone) having molecular weight of 1300000 g/mol in 758,8 g of ethanol, and after then acidified with concentrated hydrochloric acid. Through electrostatic spinning from this polymer matrix a layer of organic/anorganic nanofibres was produced, while the layer of organic/anorganic nanofibres was consequently calcinated at the temperature of 700 ⁰ C. c) for production of polymer matrix for production of TiO ₂ nanofibres the mixture of 250 g of ethanol and 58,8 g of acetylacetone was used, in which 100 g of titanium tetrabutoxide. was dissolved. After homogenising the obtained solution was mixed with solution of 35,2 g of poly(vinylpyrrolidone) having molecular weight of 1300000 g/mol in 729,4 g of ethanol, and after then acidified with concentrated hydrochloric acid. Through electrostatic spinning from this polymer matrix a layer of organic/anorganic nanofibres was produced, while the layer of organic/anorganic nanofibres was consequently calcinated at the temperature of 400 to 800 ⁰ C. d) for production of polymer matrix for production of TiO ₂ nanofibres the mixture of 150 g of ethanol and 29,4 g of acetylacetone was used, in which 100 g of titanium tetrabutoxide was dissolved. After homogenising the obtained solution was mixed with solution of 35,2 g of poly(vinylpyrrolidone) having

molecular weight of 1300000 g/mol in 272,1 g of ethanol and after then acidified with concentrated hydrochloric acid. Through electrostatic spinning from this polymer matrix a layer of organic/anorganic nanofibres was produced, while the layer of organic/anorganic nanofibres consequently calcinated at the temperature of 400 to 800 ⁰ C. e) for production of polymer matrix for production of TiO ₂ nanofibres the mixture of 250 g of ethanol and 35,2 g of acetylacetone was used, in which 100 g of titanium tetraisopropoxide was dissolved. After homogenising the obtained solution was mixed with solution of 42,2 g of poly(vinylpyrrolidone) having molecular weight of 1300000 g/mol in 977,7 g of ethanol and after then acidified with concentrated hydrochloric acid. Through electrostatic spinning from this polymer matrix a layer of organic/anorganic nanofibres was produced, while the layer of organic/anorganic nanofibres consequently calcinated at the temperature of 500 to 700 ⁰ C. In all cases a long-term continuous spinning process was achieved and thickness of produced nanofibres was within the range from 30 to 1000 nanometers.

Example 2 TiO ₂ nanofibres obtained in the same manner as in the example 1 were warmed in streaming NH ₃ atmosphere at the temperature of 500 ⁰ C. After two hours the temperature was increased to 800 ⁰ C at a rapid increase, and after thirty minutes the annealing furnace was switched off. At this process, there occurred a complete conversion of TiO ₂ nanofibres to pure TiN nanofibres without residual TiO ₂ . Nanofibrous morphology was kept, but fibres consisting of TiN nanocrystals were of more compact character. Specific surface of this anorganic nanofibrous structure was 20,3 m ² /g, which corresponds to an average size of individual TiN crystals being 53 nanometers.

The obtained hollow fibres were created of a compact TiN. Keeping of morphology of nanofibres during the TiN synthesis, the size of nanofibres, size

of TiN crystals, the dense sintering of TiN crystals and type of porosity of hollow nanofibres are witnessed by Figs. 5a - d.

Example 3 TiN nanofibres obtained in the same manner as in the example 2 were warmed upon presence of air at the temperature of 450 ⁰ C for a period of thirty minutes. At his process, there occurred a complete reverse conversion of TiN nanofibres to pure Tiθ ₂ nanofibres without residual TiN. Nanofibrous morphology was kept also at this reversible reaction. Tiθ ₂ nanofibres obtained in this manner were warmed in streaming NH3 at 55O ⁰ C upon simultaneous production of TiN nanofibres without residual TiO ₂ . The original structure of nanofibres was still kept.

Said procedure of reverse conversion to TiO ₂ and backwards to TiN was repeated in cycles at simultaneous keeping the original structure of nanofibres both Of TiN and Of TiO ₂ .

Example 4

TiO ₂ nanofibres obtained in the same manner as in the example 1 were warmed upon presence of air at the temperature of 600° C for a period of 6 hours. The initial structure of nanofibres was kept, only sintering of crystals occurred. The resultant product contained anatase with cca 5% of rutile (Figs. 6, 7). In parallel TiO ₂ nanofibres were cyclically subjected to calcination in the stream of NH ₃ at the same temperature and for the same period. Phase-pure TiN nanofibres with the same nanofibrous morphology were produced. These nanofibres were used for cycling of TiN-TiO ₂ and backwards. There were performed 6 cycles, while oxidation was performed at presence of air at 450° C for 30 minutes and reduction in stream of NH ₃ at 60O ⁰ C for 6 hours. This process was fully reversible.

Industrial applicability

Pure TiN in nanofibrous form, especially in the form of anorganic nanofibrous structure, that has not existed to date, may be used in many industrial branches. Electrical conductivity and a high specific surface enable a perfect distribution of electrons, which enables application at production of more efficient electrodes. TiN nanofibres substantially improve mechanical properties of composite materials and products made of them, and simultaneously improve their anticorrosion properties. Inertness, high specific surface and accessibility of this surface enable usage of nanofibrous structure of TiN as a support structure for catalysts. Further possibilities of application appear in devices for conversion and storage of energy, like solar cells and electrodes for electrochemical cells.