Nb4AlC3 block ceramic synthesized by hot pressing at in-situ reaction and preparation method thereof

Technical Field

The invention relates to single-phase block ceramic and a preparation method thereof, in particular to an in-situ reaction hot pressing synthesis Nb₄ AlC₃ block ceramic and a method for preparing the same.

Background Art

M_n+1 AX_n (M is a transition metal, a series of elements A A, X is or C N, n=1-3) has a layered hexagonal structure. Such self-M.W.Barsoum (the American ceramic Society, J.Am.Ceram.Soc. 79 (1996) 1953) for the first time by the reactive hot pressing technique synthetic block Ti₃ SiC₂ since, M_n+1 AX_n to its unique characteristic attract the world's growing number of scientific research workers. SUMMARY said, M_n+1 AX_n phase has the characteristics of ceramic and metal: low-density, low-hardness, high-modulus, high fracture toughness, good thermal shock resistance, good characteristics of the conductive and heat conductivity, a potential high-temperature application structure material. Such C.F.Hu (material, urgent, Scripta   Mater. 57 (2007) 893) through the high-temperature heat treatment Nb₂ AlC prepare single-phase Nb₄ AlC₃, and determines the crystal structure. By this kind of method, however, preparation of compact block Nb₄ AlC₃ complex process, no utility.

Content of the invention

The purpose of this invention is to provide an in-situ reaction hot pressing synthesis of high purity, high density Nb₄ AlC₃ block ceramic and a method for preparing the same.

The technical scheme of the invention is:

An in-situ reaction hot pressing synthesis Nb₄ AlC₃ bulk ceramic, is composed of a single phase Nb₄ AlC₃ composition. Nb₄ AlC₃ bulk ceramic refers to the hexagonal crystal system, space group is P6₃/mmc, in the crystal structure with Al Nb1-C-Nb2-C-Nb2-C-Nb1 chain in order to weak covalent bond, the Nb₄ AlC₃ along the (0001) basal plane to produce shearing deformation, so that the crystalline grain avoids the kink and easy to produce, macro show microscopic plastic. The hot pressing technique by in-situ reaction of the single-phase Nb₄ AlC₃ the relative density 98-99%, the length of the average grain size of the 45-55  m, wide for the 15-19  m.

The above-mentioned in-situ reaction hot pressing preparation Nb₄ AlC₃ bulk ceramic method, through the raw material powder in the high-temperature furnace and subjected to pressure reaction in the densification.

The preparation Nb₄ AlC₃ bulk ceramic method, to niobium powder, aluminum powder, the graphite powder as the raw material, raw material powder in a molar ratio of 4 the [...] (1 +x) : (3-y), wherein the 0   x≤ 0.3, 0 ≤ y≤ 0.3. The drying condition in the resin pot mill 12-24 hours, siebtechnik the cold press molding in the graphite die (10-20MPa), in the vacuum or through a sintering the hot pressing furnace of the argon, for the rate of rise of the 10-15 [...] /minutes, in 1600-1700 the sintering [...] , the holding time is 60-120 minutes, applied pressure is 20-30MPa. Therefore, preparing a high purity, high density Nb₄ AlC₃ single-phase ceramic.

Said adding the niobium powder, aluminum powder, graphite powder the grain size range of the 200-600 mesh; the sintering mode is hot-pressing sintering; the sintering atmosphere is vacuum (vacuum degree is 10^-4 -10^-2 MPa) or argon; said compound under dry conditions for the ball in the resin tank.

In this invention, the molar ratio of the raw material used for the niobium powder: aluminum powder : = 4 the graphite powder [...] 1 +x:3-y, wherein the 0   x≤ 0.3, 0 ≤ y≤ 0.3. According to the chemical dose ratio is because the in the course of synthesizing different sintering temperature and heating rate different, and Nb Al in the sintering process also have different a small amount of loss, but the inner composition of this range, the more pure preparation Nb₄ AlC₃.

The advantage of this invention is to:

1. Process is simple, low cost. The invention relates to niobium powder, aluminum powder, the graphite powder as the raw material, according to the appropriate proportion and simple process, which can be in-situ synthesizing Nb₄ AlC₃ bulk ceramic.

2. High purity, high density. The invention by in-situ reaction of the hot-pressing the prepared Nb₄ AlC₃ block ceramic having a high-density, the characteristics of high-purity, its relative density can reach 98-99%, the purity can reach to 96-98%.

3. The invention the stated Nb₄ AlC₃ bulk ceramic at the macro show microscopic plastic, can be used ordinary tool steel to carry out mechanical processing. Furthermore, it is fine thermoelectric conductor, has good tolerance, insensitive to thermal shock, a potential high-temperature structure and functional material.

4. The prepared Nb₄ AlC₃ bulk ceramic also has low hardness and good high temperature rigidity.

Description of drawings

Figure 1 for Nb₄ AlC₃ block schematic diagram of the ceramic crystal structure.

Figure 2 for Nb₄ AlC₃ bulk ceramic of the X-ray diffraction spectrum (Nb, Al, C powder in a molar ratio of 4 the [...] the 1.1 [...] 2.7).

Figure 3 (a) for Nb₄ AlC₃ bulk ceramic corrosion surface topography, Figure 3 (b) as the fracture morphology (secondary electron Image).

Figure 4 as Nb₄ AlC₃ the Vickers hardness of the ceramic block is the trend of changes in load value. Inserted into the graph (a)-(c) display indentation lead to surface damage.

Figure 5 as Nb₄ AlC₃ bulk ceramic the elastic modulus of the curve with the change of the temperature.

Mode of execution

Through the embodiment of the present invention detailed below.

Embodiment 1

Niobium powder to 24.78 g, aluminum powder 1.98 g, graphite powder 2.16 g is the raw material (molar ratio of 4 the [...] 1.1 the [...] 2.7, raw material powder having a particle size of 200 mesh), drying conditions in the ball in the resin tank 12 hours, in the cold press molding into the graphite mold (10 MPa), in the through a sintering the hot pressing furnace of the argon, for the rate of rise of the 15 [...] /minutes, in the 1650 [...] sintering, the holding time is 60 minutes, applied pressure is 25 MPa. For the density of the measured arab League base Mi �fa 6.97g/cm³, of the theoretical density is 99%. The X-ray diffraction analysis for the basic full Nb₄ AlC₃. Measuring Nb₄ AlC₃ block ceramic Vickers hardness 2.6GPa, compression strength is 515 MPa, shear strength is 116 MPa.

Embodiment 2

With the embodiment 1 is characterized in that the different: than powder mole different raw materials, the sintering temperature, different pressure and applying the heat preservation time.

Niobium powder to 24.78 g, aluminum powder 2.16 g, graphite powder 2.24 g is the raw material (molar ratio of 4 the [...] the 1.2 [...] 2.8), drying the resin under the conditions of milling in the tank 24 hours, in the cold press molding into the graphite mold (10 MPa), in the through a sintering the hot pressing furnace of the argon, for the rate of rise of the 15 [...] /minutes, in the 1700 [...] sintering, the holding time is 120 minutes, applied pressure is 30 MPa. For the density of the measured arab League base Mi �fa 6.98g/cm³, of the theoretical density is 99%. The X-ray diffraction analysis for the basic full Nb₄ AlC₃. Measuring Nb₄ AlC₃ the elasticity modulus of the bulk ceramic 306GPa, for modulus of shearing 127GPa.

Embodiment 3

With the embodiment 1 is characterized in that the different: than powder mole different raw materials, the sintering temperature, the rate of rise of temperature, pressure and applying the heat preservation time are different.

Niobium powder to 24.78 g, aluminum powder 2.34 g, graphite powder 2.32 g is the raw material (molar ratio of 4 the [...] 1.3 the [...] 2.9, raw material powder having a particle size of 200 mesh), drying conditions in the ball in the resin tank 20 hours, siebtechnik the cold press molding in the graphite die (20 MPa), in the through a sintering the hot pressing furnace of the argon, for the rate of rise of the 10 [...] /minutes, in 1600 the sintering [...] , the holding time is 120 minutes, applied pressure is 20 MPa. For the density of the measured arab League base Mi �fa 6.94g/cm³, of the theoretical density is 98%. The X-ray diffraction analysis for the basic full Nb₄ AlC₃. Measuring Nb₄ AlC₃ to the bending strength of the ceramic block 346 MPa, fracture toughness is 7.1 MPa. m^1/2.

Comparative example

The preparation of this Nb₄ AlC₃ C.F.Hu such as ceramic and of the block (the American ceramic Society, J.Am.Ceram.Soc. 90 (2007) 2542) of preparing Ta₄ AlC₃ and the like as the preparation A.T.Procopio Ti₄ AlN₃ (metallurgy material Journal, Metal.Mater.Trans.A   31 (2000) 333) compared with. Ta₄ AlC₃ density into 13.18g/cm³, Vickers hardness 5.1GPa, elastic modulus is 324GPa. Ti₄ AlN₃ density into 4.6g/cm³, Vickers hardness 2.5GPa, elastic modulus is 310GPa. Nb₄ AlC₃ bulk ceramic in Vickers hardness and elastic modulus with Ti₄ AlN₃ close.

Details below Nb₄ AlC₃ the crystal structure of the bulk ceramic, X-ray diffraction spectrum, micro-structure, the change of the hardness and the indentation topography load value , and elastic modulus at high temperature.

The stated Nb₄ AlC₃ bulk ceramic refers to the hexagonal crystal system, space group is P6₃/mmc, unit cell lattice constant as a 3.15 , For c 24.22 , The theoretical density is 7.06g/cm³. As shown in Figure 1 the crystal structure, in the crystal structure with Al Nb1-C-Nb2-C-Nb2-C-Nb1 chain in order to weak covalent bond, the Nb₄ AlC₃when Yi Yan of the crystal lattice in the deformation of the (0001) basal plane to produce shearing deformation, so that the crystalline grain avoids the kink and easy to produce, macro show microscopic plastic. It is fine thermoelectric conductor, can be easily processed and insensitive to thermal shock, a potential high-temperature structure and functional material. Figure 2 for Nb₄ AlC₃ block ceramic X-ray spreads out the spectrum , the molar ratio of the raw materials used for Nb:Al: the C=4 [...] the 1.1 [...] 2.7. In the Figure are all of the peaks Nb₄ AlC₃ phase. Figure 3 (a) for Nb₄ AlC₃ bulk ceramic corrosion surface. Strip Nb₄ AlC₃ the orientation of the grain without rules, the generated-to-diameter ratio of the crystal grain to a change within the range of the very large, the length of the average grain size of the 50  m, the width is 17 the  m. Figure 3 (b) for Nb₄ AlC₃   topography of the fracture of the ceramic block (secondary electron Image). Can be observed from graph Nb₄ AlC₃ shown as injury mainly of crystalline grain avoids, kink, and intergranular fracture transgranulation , with ternary processable ceramic is the typical damage. Figure 4 as Nb₄ AlC₃ the Vickers hardness of the ceramic block is the trend of changes in load value. With the increase of the load is imposed, Nb₄ AlC₃ from the Vickers hardness of the ceramic block 6.2GPa (3N) gradually reduces to 2.6GPa (200N), this is in accordance with the Vickers hardness of the ceramic material with the general rules of variations in the load (indentation size effect). Illustrations for Nb₄ AlC₃   bulk ceramic impression topography (secondary electron Image). Chart (a) indicates no crack is generated on the indentation tip, shear stress induced indentation surrounding grain extrusion; Figure (b) display in a single grain internal kink and layered; Figure (c) display in a single grain internal crack deflection. Figure 5 as Nb₄ AlC₃ bulk ceramic the elastic modulus of the curve with the change of the temperature. In the picture display Nb₄ AlC₃ can be maintained until the rigidity of the 1580 [...] , in elastic modulus is only 25%, affordances Nb₄ AlC₃ bulk ceramic the application potential under the high temperature.

It can be seen by the embodiment, the method for preparing Nb₄ AlC₃ bulk ceramic has high purity, high density, low hardness and good high temperature rigidity.