SK292013A3 - Composite material with homogenous distribution of carbon nano-tubes and a method for production thereof - Google Patents

Abstract

A composite material with a homogeneous distribution of carbon nanotubes consisting of alumina, corundum or zirconium oxide, or silicon nitride, or silicon carbide, or aluminum with a volume distribution of 90 to 99.5% and carbon nanotubes of 0.5 to 10% by volume . The resulting composite material has a min. 99% of theoretical density while ensuring fine-grained microstructure, in case of corundum matrix with average particle size less than 1 µm. The method of preparing the composite material is based on a combination of methods of stabilizing CNT suspensions with ceramic or metal particles, utilizing the "freezing" of the suspension after spraying with liquid nitrogen followed by freeze-drying and sintering under inert atmosphere using or without compressed composite. material.

Description

The technical solution concerns the area of carbon nanotubes (CNT) composite materials and technologies used in the production of these materials.

BACKGROUND OF THE INVENTION

Achieving a homogeneous distribution of carbon nanotubes in a ceramic or metal matrix is a problem because carbon nanotubes (CNTs) show a strong tendency to form agglomerates in bundles due to van der Waals forces due to the large specific surface area and diameter to length ratio of CNT. Thus, the high CNT content of the composite materials leads to their agglomeration, thereby limiting the desired physical and mechanical properties of the resulting composite materials.

Methods for achieving a homogeneous distribution of discrete CNTs in a ceramic or ceramic matrix, respectively. The metallic material has so far been intensively investigated. Various approaches to the dispersion of CNT in the matrix have been suggested, e.g. by conventional powder mixing by ultrasound or ball milling; a colloidal process with the addition of dispersants, surfactants or acid treatment; a sol-gel technique of attaching the CNT nets of the resulting gel; as well as in-situ synthesis (growth) of CNT using the Chemical vapor deposition (CVD) technique.

Conventional blending of powders, generally used to disperse the secondary phase in composites, does not have a high efficiency in the case of CNT homogenization due to possible mechanical damage to the CNT, which limits the use of high mixing energy. The application of conventional mixing has been observed on various types of materials such as e.g. aluminum oxide, silicon nitride, etc. as also mentioned in the following documents [1-5]:

[1] Zhan G.D., Kuntz J.D., Wan J., Mukherjee A.K. Nat. Mater. 2003, 2, 38-42 [2] Zhan G.D., Kuntz J.D., Garay J.E., Mukheijee A.K. Appl. Phys. Lett. 2003, 83, 1228-1230 [3] Wang J., Kou H. M., Liu X.J., Mr. Y.B., Guo J.K. Ceram. Int. 2007, 33, 719-722 [4] Balaszi, C., Shen, Z., Konia, Z., Kasztovski, Z., Weber, F., Vertesy, Z., et al. Compos. Sci. Technol. 2005, 65, 727-733 [5] Balaszi C., Sedlackova K., Czigany Z. Compos. Sci. Technol. 2008, 68, 1596-1599

The colloidal process uses change, respectively. treating the surface chemistry of the CNT, thereby stabilizing it in suspension with the ceramic or metal particles. Treatment of CNTs requires their functionalization, as the synthesized CNTs have a low surface charge. In addition, they contain impurities derived from the synthesis itself (amorphous carbon) as well as metal catalyst residues. The use of an acid treatment also purifies the CNT and at the same time functionalises by adsorption or by the formation of functional groups (mostly negatively charged), in particular of carboxylic acids and oxygen-containing groups. Functionalization also changes the hydrophobic nature of CNT to hydrophilic, which greatly facilitates further processing, especially when using polar liquids as suspension media, as indicated by the following sources [6-9]:

[6] Liu J, Rinzler AG, Dai H, Hafiier J H, Bradley RK, Boul PJ, et al. Fullerene pipes. Science 1998; 280: 1253-6.

[7] Esumi K, Ishigami M, Nakajima A, Awada KS, Hodna H. Chemical treatment of carbon nanotubes. Carbon 1996; 34: 279-81.

[8] Kim B, Park II, Sigmund W. Electrostatic Interactions of short-range multi-carbon nanotubes and polyelectrolytes. Langmuir 2003; 19: 2525-7.

[9] Hu H, Yu A, Kim E, Zhao B, Itkis ME, Bekyarova E, et al. Influence of zeta potential on disposability and purification of single-walled carbon nanotues. J Phys Chem B 2005; 109: 11520-4

The sol-gel process, as an alternative method of homogenizing CNT, utilizes mixing of CNT with the appropriate sol, while avoiding negative phenomena such as destruction of CNT, such as mixing with powders, mixing of solid phases due to different densities of ceramic or metal particles and CNT. The sol-CNT mixture is due to gelling agents and / or gels. by changing the conditions (pH, temperature, humidity) it transforms into a gel, thereby making such a system rigid (increasing viscosity) and thus preventing the formation of CNT aggregates. Subsequently, the gel-CNT system is converted into a mixture of powders and CNT by heat treatment. The disadvantage of such a process is the possibility to prepare composite materials with only ceramic matrix, negative factor also economical demands of production and high price of raw materials, especially ceramic precursors.

Of the prior art processes for the preparation of composite materials with a homogeneous CNT distribution in a ceramic or metal matrix, the in-situ method of growth of CNT in a gas-phase matrix (CVD technique) is currently most used. This technique is suitable for both ceramics and metals. However, the homogeneity of the resulting CNT distribution in the composite material is limited in this case by achieving a homogeneous distribution of the CNT catalysts. Another factor limiting the wider application of the process is its economic demands and the necessity of using a matrix with relatively high open porosity. However, in the preparation of dense composites by sintering, this entails an increase in the costs associated with this technological step (using a higher sintering temperature, prolonging the sintering time, applying pressure), and in addition to economic burden it is also ecological (higher energy consumption).

Alumina-based ceramics (i.e., corundum ceramics) is an important material used mainly in the construction of various machines, devices and components. The reason for its wide use is its properties such as low density, hardness, resistance to wear, corrosion and temperature, chemical stability even at high temperatures. However, corundum ceramics also have low fracture toughness and low resistance to thermal shocks, which prevent its wider use. By adding carbon nanotubes (CNTs) to the corundum matrix, it is possible to substantially improve its mechanical properties (fracture toughness) and to extend the application potential of corundum ceramics by improving its functional properties (especially thermal and electrical conductivity).

In order to achieve improved properties of corundum ceramics, it is necessary to ensure a homogeneous distribution of both phases throughout the bulk of the material in the CNT composite system.

From the point of view of alumina-based composite materials (corundum), the main problem with their preparation was precisely the achievement of a homogeneous CNT distribution in the corundum matrix and, in connection therewith, the compaction of such composite materials. The present solutions have focused mainly on the use of knowledge and methods of stabilizing CNT by means of dispersing additives, surfactants, functionalization of the CNT surface, using the sol-gel method as well as the in-situ approach of CNT growth in the matrix.

The brittleness of corundum ceramic is the biggest disadvantage in terms of its application possibilities. An important property that characterizes the formation and propagation of cracks is fracture toughness, and therefore, in order to improve the fracture toughness of AI2O3-based composites, most of the published works have focused on the toughening of these materials. The mechanical properties of CNT are used as a reinforcing element in a ceramic matrix to improve fracture toughness, hardness and flexural strength [1, 2]. Research findings in this field are inconsistent. Some authors report an improvement in the fracture toughness of Al ₂ O ₃ - CNT nanocomposite. The authors [3] achieved the fracture toughness of the A1 ₂ O ₃ - SWCNTs of the composite of 6.4 ± 0.3 MPa.m ^1/2 , which is twice as high as the single crystalline A1 ₂ O ₃ . Zhan et al. with an Al ₂ O ₃ composite with 10 vol. % SWCNTs even nearly three-fold improvement over monocrystalline Al ₂ O ₃ ceramics, Siegel et al. achieved an improvement of 24% for a composite containing 10 vol. % of polyhedronic carbon nanotubes (MWCNT) [4], the authors [5] achieved an Al2O3 - MWCNT composite containing 3 vol. % CNTs of fracture toughness higher by 79% compared to nanocrystalline ceramics. Other authors, Laurent et al. [6], Wang et al. and Sun et al. have not observed any or slight improvement in fracture toughness compared to monocrystalline corundum ceramics [4, 7]. As far as the hardness of these composites is concerned, they usually have a lower hardness compared to monocrystalline Al2O3. Possible reasons are weak links between CNTs and AI2O3 grains, the lubricating nature of CNTs, and also the presence of a relatively soft phase (hardness of the polystyrene nanotubes in the radial direction is 6-10 GPa) at grain boundaries [9].

The mechanical properties of the materials thus prepared are below the requirements. This is due to the problems encountered in the production of Al2O3-CNT nanocomposites, the key problem being to achieve a homogeneous distribution of CNTs in the ceramic matrix. Several approaches have been proposed to achieve uniform distribution, including molecular-level mixing, which consists of a reaction between functionalized CNTs and metal ions in solution ([10] CHA, SI et al.: In Scripta Materialia, 2005, vol. 53, p. 793-797), further in situ synthesis of CNT in a ceramic matrix, preparation of dispersed suspensions of Al ₂ O ₃ and CNTs using ultrasound or milling in a surfactant attritor ([11] ZHANG, SC et al .: In J Eur. Ceram. Soc., 2010, vol. 30, p. 1373-1380), preparation of AI2O3-MWCNT using precursors ([12] YAMAMOTO, G. et al .: In Diamond and Related Materials, 2008, vol. 17, p. 1554) -1557), Al2O3 coating of nanotubes ([13] ESTILI, M., KAWASAKI, A .: In Scripta Materialia, 2008, vol. 58, p. 906-909), and others.

The uniform distribution of CNTs within the matrix alone is not sufficient to improve the material properties. Another problem is the weak interfacial compatibility due to the chemical inhomogeneity of the two phases due to the different nature of the CNTs-matrix linkages and the strong tendency of the CNTs to form clusters that are bound by attractive Van der Waals forces [13], ([14] LIU, S. et al .: In Carbon, 2011, vol. 49, p. 3698-3704), ([15] Sarkar, S., DAS, PK: In Ceramics International, 2012, vol. 38, p. 423-432). It is also necessary to achieve a high density of these composites, i.e. to optimize the sintering process without degrading the CNTs. Exposing nanotubes to high temperatures (above 1250 ° C) can destroy the structural integrity of carbon nanotubes, leading to the failure of many of their toughening mechanisms [8]. Various sintering methods, such as non-pressure sintering, hot pressing, and compaction have been used to compact ceramic AI2O3-CNT composites. SPS method. The SPS method seems to be the most promising, making it possible to achieve high densities at lower temperatures and shorter holding times compared to conventional methods, reducing the likelihood of CNT damage and also preventing grain growth [4, 5, 10, 15]. it prevents grain growth, and in general, refining the structure of the material has a positive effect on improving its strength, hardness, wear resistance and resistance to thermal shocks. In addition, the refinement of the structure also improves the fracture toughness of many materials [9]. Tensile mechanisms in composites of this type include crack deflection, crack bridging, nanotube drawing [3, 14] and ([16] HE, CN et al .: In Journal of Alloys and Compounds, 2009, vol. 478, pp. 816-819).

Further research has focused on the influence of CNTs on the functional properties of corundum ceramics. Alumina ceramic insulator in the electrical conductivity of the order of 10 ^{"10 to 10" 12} S / m. The addition of CNTs as a high conductivity component can significantly increase the electrical conductivity. As with mechanical properties, the homogeneous distribution of CNTs in the matrix and the solid interfacial interface between nanotubes and Al ₂ O ₃ are very important here. The electrical properties of the nanocomposite then depend mainly on the content of CNTs, the density of the ceramic and the sintering conditions. Zhan et al. achieved an electrical conductivity of 3345 S / m at 15 vol.% for the nanocomposite SWCNT-AI2O3. % SWCNT, sintered using the SPS method ([17] KUMARI, L. et al .: In Ceramics International, 2009, vol. 35, p. 1775-1781). The authors ([18] INAM, F. et al .: In J. Eur. Ceram. Soc., 2010, vol. 30, p. 153-157) achieved an electrical conductivity of 576 S / m for the AI2O3-CNT nanocomposite with 5 vol. % MWCNT. The SPS method was used for sintering, which preserves the structural integrity of CNTs, which is very important for improving the electrical conductivity of the nanocomposite. Also, the electrical conductivity of these nanocomposites increased with increasing grain size, due to the increasing number of conductive paths at the grain boundaries where the nanotubes are located [18],

Homogeneous distribution of nanotubes in the matrix and avoiding agglomerate formation are also important to improve the thermal conductivity of the material, since the thermal conductivity of CNT clusters is 3 orders of magnitude lower than that of individual CNTs. Unordered CNT films have a conductivity of about 30 W / m.K and magnetically arranged CNT films have a thermal conductivity of about 200 W / m.K. Also defects in CNTs reduce their thermal conductivity ([19] BAKSHI, S.R. et al .: In Computational Materials Science, 2010, vol. 50, p. 419-428). The addition of CNTs to the matrix (metal or ceramic) shows great potential for improving thermal conductivity. Shi et al. showed that the addition of 10 vol. % CNT coated with copper in W-Cu powder increased the thermal conductivity of the hot pressed samples from 180.97 to 640.53 W / mK ([20] SHI, XL et al .: In Materials Science and Engineering A, 2007, vol. 457, p. 18-23). The authors ([21] KUMARI, L. et al .: In Composites Science and Technology, 2008, vol. 68, p. 2178-2183) achieved the highest thermal conductivity of 90.4 W / mK measured at 100 ° C for Al 2 O 3 - CNT nanocomposite containing 7.39 wt. % CNT, an 229% improvement over pure Al 2 O 3. This composite was sintered at 1550 ° C by SPS. The authors attribute this improvement to several factors: phonons scattered more frequently in agglomerates bound by van der Waals forces, thereby reducing thermal conductivity; which is probably due to higher density and hence lower porosity [21],

SUMMARY OF THE INVENTION

The disadvantages of the prior art are largely overcome by a composite material with a homogeneous distribution of carbon nanotubes prepared by the process of the present invention.

The present invention deals with a composite material containing carbon nanotubes and a method for its production, which can achieve a significant change in material properties by increasing its fracture toughness, increasing electrical conductivity and increasing thermal conductivity. The present invention solves the problem of achieving a homogeneous distribution of carbon nanotubes. The composite material with a homogeneous distribution of carbon nanotubes consists of alumina, corundum or zirconium dioxide or silicon nitride or silicon carbide or aluminum with a volume fraction of 90 to 99.5% and carbon nanotubes with a volume fraction of 0.5 to 10%. For example, carbon nanotubes are distributed in an alumina matrix (corundum), and the resulting composite material exhibits a high densities of min. 99% of the theoretical density while maintaining a homogeneous distribution of carbon nanotubes while providing a fine-grain microstructure of corundum matrix with an average particle size of less than 1 µm.

The method of preparing the composite material according to this solution consists in combining methods of stabilizing CNT suspensions with ceramic or metal particles and utilizing the "freezing" of the suspension after spraying with liquid nitrogen followed by freeze-drying (lyophilization).

In the first step, the preparation technology is based on the destruction of CNT clumps by the action of acids, in particular sulfuric and nitric acids, in a volume ratio of 3: 1. In this material preparation step, the van der Wals forces acting between the individual nanotubes are disrupted, and at the same time the catalyst residues (after CNT production) and free amorphous carbon are removed (undesirable due to degrading properties). To increase the efficiency of the treatment, it is advisable to use the simultaneous action of ultrasound, which helps to separate the individual nanotubes. Subsequently, the treated CNTs are filtered and washed with water to remove acids. In the second step, the treated CNTs are admixed with aqueous (other types of liquid medium - isopropanol, or other higher alcohols, hexane) are commonly used in the suspension, with appropriate material (e.g. ceramic or metal powders) with the addition of a suitable steric stabilizer (sodium dodecyl sulphate). , dimethylformamide). During this step, homogenization occurs between the powder particles and the nanotubes, and the suspension (mixture) as a whole stabilizes due to the organic stabilizer molecules. Homogenization of the suspension may be by stirring in the presence of grinding bodies (package milling, attrition milling). In this step it is also possible to use other additives which act as granulating additives (binder agent). In the third step, a stable suspension is sprayed (sprayed) into liquid nitrogen, where at -196 ° C the carrier medium (water) is immediately frozen and thus the mixture of ceramic powder or metal and CNT is frozen. To remove the presence of water, respectively. To prevent the phase from returning to the liquid state (this could lead to separation of the phases from each other due to different densities) it is necessary to remove the solid water present by sublimation (lyophilization at reduced pressure).

The result of the technological process is a mixture in the form of a powder or granulate, which is further treated for further purposes by removing large particles by sieving to the desired fraction. The mixtures prepared in this way are subsequently compacted (compacted) by sintering under the influence of high temperatures and optionally also by pressures (i.e. by free sintering, by heat pressing) in an inert atmosphere (to prevent oxidation - CNT burning out). The composite material thus formed may consist of alumina (corundum), or zirconium oxide, or silicon nitride, or silicon carbide, or aluminum having a volume fraction of 90 to 99.5% and carbon nanotubes (polystyrene) containing 0.5 up to 10 vol. %.

The composite materials prepared according to this solution exhibit the characteristics listed in the table:

matrix Hardness (GPa) Fracture toughness (MPa. M ¹⁷² ) Electrical conductivity (Sm ² ) Thermal conductivity (W.mhK ¹ ) Aluminum oxide 18.1 + 0.3 5.5 ± 0.4 1150 ± 80 115 ± 1.7 Zirconium dioxide 12.8 ± 0.4 11.5 ± 0.3 280 ± 15 4.9 ± 0.2 Silicon nitride 18 ± 0.3 10.2 ± 0.4 35 ± 1 90 ± 0.8 Silicon carbide 28.2 ± 0.3 6.2 ± 0.3 4530 ± 105 5.2 ± 0.3 aluminum 230 ± 0.5 36.7 + 0.6 0.041 + 0.002 261 ± 1.8

DETAILED DESCRIPTION OF THE INVENTION

Example 1:

Polygene carbon nanotubes (MWCNT) in an amount of 5 g were purified and functionalized by stirring in 400 ml of a 3: 1 sulfuric and nitric acid mixture for 5 hours. After each hour of stirring, the mixture was sonicated for 20 min. to ensure more intensive separation of individual nanotubes from each other. After the stirring process, the mixture was diluted with distilled water and the MWCNT was filtered through a 0.1 µm nylon filter membrane and the MWCNT was washed with distilled water until pH 7 of the filtrate was reached. Subsequently, the MWCNTs were dried at 100 ° C for 10 h. The treated MWCNTs were dispersed by ultrasound in sodium dodecyl sulfate solution (SDS), the SDS: MWCNT weight ratio being 1.5: 1. The thus prepared dispersion of deagglomerated carbon nanotubes was further used to prepare composite materials.

Ceramic alumina powder having an average particle size of 400 nm was dispersed by ultrasound and subsequently mixed with a MWCNT dispersion prepared according to the above procedure, the pH being adjusted to 10.9 with ammonium hydroxide solution. Stirring was intensified using corundum grinding bodies and stirring time was 24 hours. After homogenization, the grinding bodies were separated by pouring through a sieve, and the homogeneous mixture was sprayed into liquid nitrogen. The spray rate was controlled by the dispersion flow rate, which was maintained at a constant value of 20 ml / min and a propellant gas pressure of 10 kPa. Under these conditions, granules of a mixture of alumina, MWCNT and frozen water were formed by supercooling the spray dispersion at -196 ° C. Water was subsequently removed by lyophilization, at a reduced pressure of 100 Pa, a -56 ° C freezer temperature, and a treatment time of 24 h. The resulting alumina granulate with a homogeneously distributed MWCNT phase was subsequently cold pressed at 100 MPa, and the blanks thus pressed were sintered by a hot pressing method. The hot pressing was carried out at a temperature of 1550 ° C and a pressure of 30 MPa under an atmosphere of inert gas (argon) for 1 hour.

This preparation process results in a composite ceramic material with carbon nanotubes (CNTs) homogeneously distributed in an alumina matrix (corundum), which exhibits high densities (min. 99% of theoretical density), while maintaining not only homogeneous CNT distribution but also fine-grained corundum matrix microstructure (narrow particle size distribution) with an average particle size of less than 1 µm.

Example # 2

Polygene carbon nanotubes (MWCNT) in an amount of 5 g were purified and functionalized by stirring in 400 ml of a 3: 1 sulfuric and nitric acid mixture for 5 h.

After each hour of stirring, the mixture was sonicated for 20 min. to ensure more intensive separation of individual nanotubes from each other. After the stirring process, the mixture was diluted with distilled water and the MWCNT was filtered through a 0.1 µm nylon filter membrane and the MWCNT was washed with distilled water until pH 7 of the filtrate was reached. Subsequently, the MWCNTs were dried at 100 ° C for 10 h. The treated MWCNTs were dispersed by ultrasound in sodium dodecyl sulfate (SDS) solution, the SDS: MWCNT weight ratio being 1.5: 1. The dispersion of deagglomerated carbon nanotubes thus prepared was further used to prepare composite materials.

Aluminum powder with an average particle size of 150 nm was dispersed by sonication followed by mixing with a MWCNT dispersion prepared according to the above procedure, the pH being adjusted to 10.9 with ammonium hydroxide solution. Stirring was intensified using corundum grinding bodies and stirring time was 24 h. After homogenization, the grinding bodies were separated, passed through a sieve and the homogeneous mixture was sprayed into liquid nitrogen. The spray rate was controlled by the dispersion flow rate, which was maintained at a constant value of 20 ml / min and a propellant gas pressure of 10 kPa. Under these conditions, granules of a mixture of aluminum, MWCNT and frozen water were formed by supercooling the spray dispersion at -196 ° C. This was then removed by freeze drying at a reduced pressure of 100 Pa, a -56 ° C freezer temperature and a 24 hour period. The resulting aluminum granulate with a homogeneously distributed MWCNT phase was subsequently cold pressed at 100 MPa, and the thus pressed blank was sintered by means of a heat pressing method. The hot pressing was carried out at a temperature of 550 ° C and a pressure of 30 MPa at a reduced pressure of 100 Pa for 5 minutes.

Industrial usability

The present invention represents a preferred alternative to the prior art solutions in terms of the technological process used, since it ensures savings of the input costs of the production process as well as the elimination of the negative environmental impacts of the currently used technologies. The carbon nanotube (CNT) composite materials prepared on the basis of this solution can be used as structural elements due to their improved properties, and are particularly useful in two areas:

1. in the armament, engineering and automotive industries (improvement of mechanical properties, namely fracture toughness)

2. in electrical engineering - the use of improved electrical properties of such materials, in particular low resistivity, as well as the use of a multifunctional nature (in the case of ceramics), i. a combination of aggressive environment resistance, mechanical properties and electrical properties. In electrical engineering, it is also possible to utilize the ability of such materials to conduct heat, due to increased thermal conductivity, e.g. as substrates.

Claims (3)

1. A composite material with a homogeneous distribution of carbon nanotubes, characterized in that the material consists of alumina, corundum or zirconium oxide or silicon nitride or silicon carbide or aluminum with a volume ratio of 90 to 99.5% and carbon nanotubes with a volume proportion of 0.5 to 10%. A composite material with a homogeneous distribution of carbon nanotubes according to claim 1, characterized in that the carbon nanotubes are distributed in an alumina matrix, corundum, and the composite material exhibits a high density of min. 99% of the theoretical density while maintaining a homogeneous distribution of carbon nanotubes while providing a fine-grain microstructure of corundum matrix with an average particle size of less than 1 µm. Method for producing a composite material with a homogeneous distribution of carbon nanotubes according to claims 1 and 2, characterized in that the carbon nanotube aggregates are disrupted by the action of acids, preferably sulfuric and nitric acid, in a volume ratio of 3: 1, disrupting the van der Wals forces between the individual nanotubes and at the same time to remove catalyst residues and free amorphous carbon, ultrasonic treatment is used to increase the treatment efficiency, which helps to separate the individual nanotubes, then carbon nanotubes are filtered and washed with water to remove acids, then carbon nanotubes are mixed an aqueous suspension with powdered ceramics or metals with the addition of a suitable steric stabilizer, preferably sodium dodecyl sulphate, dimethylformamide, homogenizing between the powder particles and the nanotubes by the action of organic stabilization of the suspension as a whole, followed by homogenization of the suspension, which can take place by stirring in the presence of grinding bodies, and advantageously the use of additives which act as granulating additives is possible, the stable suspension is then sprayed or sprayed into liquid nitrogen where C, the carrier medium, water is immediately frozen, and the mixture of powder and carbon nanotubes is frozen, thereby eliminating the separation of the phases from each other, and the resulting powder or granulate mixture is treated for further purposes by removing large particles by sieving to the desired fraction and The mixtures prepared by the process are then compacted, compacted by sintering under the action of high temperatures, preferably also by application of pressures by free sintering or by heat pressing in an inert atmosphere. INDUSTRIAL PROPERTY OFFICE OF THE SLOVAK REPUBLIC Švermova 43,974 04 Banská Bystrica 4 REQUEST REPORT