MAX PHASES BY REACTIVE FLASH SINTERING AND A METHOD FOR ULTRAFAST SYNTHESIS THEREOF

Abstract

The present invention discloses a novel flash sintering process for synthesis of MAX phases, namely, but not limited to Ti.sub.2SnC and Ti.sub.3SiC.sub.2 in an extremely short time. The process is a combustion synthesis where a relatively low voltage in a range of 20-60 V/cm is applied using a DC/AC power source across the compact precursor material prior to ignition. The flash event is observed with a quick rise in current flow in a range of 100-300 mA/mm.sup.2 followed by measured temperature range of 1200-1400? C. in a green compact body of different MAX phase compositions. The process of the present invention enables the synthesis of MAX phases in air by suppressing oxidation of the material because of the very short residence time of the flashing event. In addition, the present invention focuses on synthesis of MAX phase in bulk to serve as the starting material for the development of two-dimensional MXenes.

Claims

1. A process for synthesis of a dense and pure MAX phase, comprising the following steps: a) forming a mixture of a transition metal (M), a post-transition metal (A) and a non-metal (X) in a certain stoichiometric ratio, wherein the transition metal is titanium, wherein the post-transition metal is selected from tin and silicon, wherein the non-metal is carbon; b) wet-milling the mixture in a solvent followed by vacuum drying to form a compact disc; c) igniting exothermally the compact disc sandwiched between two flat graphite electrodes, to initiate combustion on an application of current having an electric field is in a range of 20-60 V/cm, thereby promoting propagation of the combustion in an extremely short time; d) terminating the application of current to obtain the MAX phase; wherein the MAX phase is a three-dimensional (3D) layered transition metal carbides or nitrides or carbonitrides.

2. The process as claimed in claim 1, wherein the mixture has a M:A:X molar ratio of about 2:0.2-1.0:1 for synthesis of Ti.sub.2SnC as MAX phase.

3. The process as claimed in claim 1, wherein the mixture has a M:A:X molar ratio of about 3:1.0-1.8:2 for synthesis of Ti.sub.3SiC.sub.2 as MAX phase.

4. The process as claimed in claim 1, wherein a pressure of about 5 MPa to 30 MPa is employed onto the mixture to form a compact disc.

5. The process as claimed in claim 1, wherein pre-heating of the compacted disc is done in a range of 200-400? C.

6. The process as claimed in claim 1, wherein the application of current is in a continuous mode and is in an optimized range of 100-300 mA/mm.sup.2.

7. The process as claimed in claim 1, wherein temperature of the compact disc during flash event is about 1200-1400? C. and duration of the flash sintering process is less than 10 seconds.

8. The process as claimed in claim 1, wherein the process is conducted in vacuum, or in an atmosphere of an inert gas.

9. The process as claimed in claim 1, wherein the process is conducted in air without oxidation of the transition metal, a post-transition metal and a non-metal.

10. The process as claimed in claim 1, wherein the MAX phase formed is grey in colour and is easily pulverizable.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] FIG. 1 is an illustration of an experimental setup of flash sintering technique

[0032] FIG. 2 depicts photographic images of flashing event of the sample in (a) air and (b) vacuum/inert.

[0033] FIG. 3 depicts photographic images of Ti/Sn/C compact mixture (a) before flash sintering and (b) after flash sintering

[0034] FIG. 4 depicts X-ray diffraction pattern of Ti.sub.2SnC MAX phase

[0035] FIG. 5 depicts FESEM image of compact layered Ti.sub.2SnC MAX phase with EDAX analysis (Inset).

[0036] FIG. 6 depicts X-ray diffraction pattern of Ti.sub.3SiC.sub.2 MAX phase.

[0037] FIG. 7 depicts FESEM image of compact layered Ti.sub.3SiC.sub.2 MAX phase with EDAX analysis (Inset).

DETAILED DESCRIPTION OF THE INVENTION

[0038] It will be apparent that the foregoing description of the specific embodiments will be appreciated by those skilled in the art without departing from the spirit of the invention and therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the terminology employed herein is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.

[0039] The present invention relates to synthesis of ternary carbides, nitrides and carbonitrides called MAX phases in an extremely short time by exposing pellets pressed from a mixture of precursor powders to high temperature in air or vacuum or in gas atmosphere via a novel approach known as Flash sintering. MAX phases are three-dimensional (3D) nano-layered, hexagonal, machinable ternary carbides/nitrides with a combination of both metallic and ceramic properties. Generally, these are synthesized from an early transition metal M (Sc, Ti, V, Mo, Zr etc.), a post-transition metal or metalloid A (Group-13 & 14 elements) and a non-metal X (C or N) having a general chemical formula of M.sub.n+1AX.sub.n. Depending on their n value, MAX phases are mainly classified into three categories: M.sub.2AX type, M.sub.3AX.sub.2 type and M.sub.4AX.sub.3 type. The recent discovery of two-dimensional nano-material called MXenes which is fast evolving with tremendous potential for application in the field of energy storage, electromagnetic interference, shielding, water purification, electrolysis, medicine etc. can be fabricated from MAX phase only, for which synthesis of latter is even more valuable. The synthesis of variety of compositions of MAX phases with more than 155 members have been reported so far. Preferred MAX phases include Ti.sub.2SnC, Ti.sub.3AlC.sub.2, Ti.sub.3SiC.sub.2, Zr.sub.2AlC, V.sub.4AlC.sub.3 etc.

[0040] Pursuant to the present invention, the first step in this process can be employed in preparing the mixture of elemental powders of a transition metal species, a co-metal species and a non-metal species as the starting material, followed by wet-milling at a particular molar ratio. A mixing time of about 12 hours in a roll-mill or ball-mill will typically provide a homogeneous mixture of powders suitable for use in the inventive process. Individual powders in the mixture typically possess average particle size of about 1 ?m to 250 ?m with greater than 98% purity.

[0041] In accordance with one particularly preferred embodiment of the present invention, the mixture of powder material is compacted or compressed uniaxially to form a cylindrical green pellet having a desired relative density for combustion synthesis. A uniaxial pressure of about 5-30 MPa is preferably applied onto the mixture for the formation of green pellet. A compatible binder may optionally be added to the powder mixture to provide some cohesiveness in making up the green pellet.

[0042] The green pellet is exposed to high temperature and pressure (optional), simultaneously in an inert atmosphere or in vacuum. Under these optimized conditions, the mixture of powder materials react with each-other exothermally with a significant evolution of heat, forming MAX phase with inherent presence of an auxiliary phase. Several techniques like hot-isostatic pressing (HIP), vacuum hot pressing (HP), spark plasma sintering (SPS), self-propagating high temperature synthesis (SHS) etc. have been employed for the synthesis of MAX phase. However, it is understood that these techniques are complex, expensive and time consuming. MAX phases, because of their excellent metal-ceramic properties and serving as the starting material for the fabrication of the two-dimensional nano-material MXenes, it is imperative to develop simpler methods for their synthesis in bulk. In accordance with the present invention, the green body is subjected to an electric field applied across the sample, which is sufficient to promote the propagation of combustion process termed as Flash Sintering. It can be defined as a field assisted sintering technique characterized by a very rapid flow of current within the specimen, followed by light emission, a drop in electrical resistivity and a quick increase in temperature of the specimen by Joule effect. The experimental setup for this novel technique comprises of the following: [0043] (i) an electric furnace for supply of external heat to the specimen; [0044] (ii) a chamber for the combustion synthesis in an inert atmosphere or in vacuum; [0045] (iii) a power source for application of electric field to the specimen; and [0046] (iv) two electrodes for holding the specimen.

[0047] In the experimental setup as shown in FIG. 1, the positive and negative terminals of DC/AC power supply are connected to the two flat graphite electrodes, respectively. The green pellet is sandwiched between the two flat electrodes to ensure a good contact. In order to improve the contact between the specimen and the electrodes, a conductive paste (i.e., Silver/Platinum paste) may be used. It would be advantageous to use the lowest voltage for ignition of the precursor material for which pre-heating of the green pellet is a much-needed act. The heat-energy supplied externally to initiate the reaction via an electric furnace varies in a range of about 200? ? C. to 400? C., to ensure a complete evaporation of water used in binder before the application of the electric field. The voltage applied across the green pellet through a DC/AC power source is in a range of 20-60 V/cm. A strong light emission is observed confirming the flash event with a quick increase in current flow and temperature with a heating rate of about 300? C./sec within the specimen. The localized temperature of the pellet is measured to be in a range of 1200-1400? C. using an optical pyrometer or a thermocouple. Current in a range of 100-300 mA/mm.sup.2 is used for the formation of different MAX phases. Because of very short reaction time, there is no noticeable change in sample dimension during formation of MAX phases unlike that observed in SHS synthesis method.

[0048] The present invention demonstrates the synthesis of MAX phases in air, vacuum and in an inert atmosphere (especially argon) using an electric furnace for external heat supply (FIG. 2). Several conventional techniques reported the synthesis of MAX phase only in vacuum or in an inert atmosphere (Ar, N.sub.2, H.sub.2 etc.). However, as stated in a particular embodiment of the present invention, the synthesis of MAX phases can be carried out in air too. A very short residence time of the flashing event dramatically increases the formation of MAX phase by suppressing the oxidation of material both at low and high temperature.

[0049] In an aspect, the present invention discloses a process for synthesis of a dense and pure MAX phase, comprising the following steps: a) forming a mixture of a transition metal (M), a post-transition metal (A) and a non-metal (X) in a certain stoichiometric ratio, wherein the transition metal is titanium, wherein the post-transition metal is selected from tin and silicon, wherein the non-metal is carbon; b) wet-milling the mixture in a solvent followed by vacuum drying to form a compact disc; c) igniting exothermally the compact disc sandwiched between two flat graphite electrodes, to initiate combustion on an application of current having an electric field is in a range of 20-60 V/cm, thereby promoting propagation of the combustion in an extremely short time; d) terminating the application of current to obtain the MAX phase; wherein the MAX phase is a three-dimensional (3D) layered transition metal carbides or nitrides or carbonitrides.

[0050] In a feature of the present invention, the mixture has a M:A:X molar ratio of about 2:0.2-1.0:1 for synthesis of Ti.sub.2SnC as MAX phase.

[0051] In a feature of the present invention, the mixture has a M:A:X molar ratio of about 3:1.0-1.8:2 for synthesis of Ti.sub.3SiC.sub.2 as MAX phase.

[0052] In a feature of the present invention, a pressure of about 5 MPa to 30 MPa is employed onto the mixture to form a compact disc.

[0053] In a feature of the present invention, pre-heating of the compacted disc is done in a range of 200-400? C.

[0054] In a feature of the present invention, the application of current is in a continuous mode and is in an optimized range of 100-300 mA/mm.sup.2.

[0055] In a feature of the present invention, temperature of the compact disc during flash event is about 1200-1400? C. and duration of the flash sintering process is less than 10 seconds.

[0056] In a feature of the present invention, the process is conducted in vacuum, or in an atmosphere of an inert gas.

[0057] In a feature of the present invention, the process is conducted in air without oxidation of the transition metal, a post-transition metal and a non-metal.

[0058] In a feature of the present invention, the MAX phase formed is grey in colour and is easily pulverizable.

[0059] The invention can be better understood with the following examples, which is intended for the purpose of illustration only and are not to be construed as a limitation thereon.

EXAMPLES

[0060] Following examples are given by way of illustration, and therefore should not be construed to limit the scope of the invention.

Example-1

[0061] The materials used in this example were elemental powders of titanium (99% purity, John Baker Inc., Colorado, USA), tin (60 mesh, 99% purity, Central Drug House Pvt. Ltd., India) and graphite powder (150 mesh, 99.5% purity, Central Drug House Pvt. Ltd., India) as precursor materials. The powder materials were mixed in a stoichiometric ratio of 2:0.2-1:1 of Ti:Sn:C, and wet-milled for 12 hours in ethanol using 2 mm zirconia balls. The milled powder was then vacuum dried, and a green cylindrical compact disc was formed.

[0062] The resulting compact pellet was sandwiched between two flat graphite electrodes. The electric field of 20-45 V/cm was applied using a DC/AC power source followed by pre-heating of the compact powder at 300? C. During the flashing event, the material gets ignited showing a quick increase in current up to 150 mA/mm.sup.2 with strong emission of light. Application of voltage was then terminated. The temperature of the compact pellet during flash event was measured to be about 1200? C. by an optical pyrometer. The resulting product was single phase Ti.sub.2SnC, with a minor amount of TiC as an auxiliary phase. A trace amount of Sn metal powder was also present in the final product. This experiment was carried out in both air as well as in vacuum inside a vacuum chamber to ensure the formation of MAX phase. The photographic image of the compact powder before and after sintering has been shown in FIG. 3 which represents that the reaction has occurred.

[0063] An X-ray diffraction study ascertained the crystallinity and phase purity of the sample. The X-ray diffractograms of Ti.sub.2SnC MAX phase is presented in FIG. 4. The presence of diffraction lines at 2??12.9?, 26?, 38.4?, and 39.5? confirms the formation of pure Ti.sub.2SnC MAX phase (JCPDS ref. code: 01-089-5590). The presence of TiC as an auxiliary phase is also observed in the sample (20=35.9? and 41.8?). An additional peak of unreacted Sn is also apparent from the XRD pattern. FIG. 5 represents the FESEM image of pure Ti.sub.2SnC MAX phase which exhibits stacked and compacted plate-like morphology. Elemental analysis of the sample was done using EDAX spectroscopy revealing the presence of Ti, Sn and C in different atomic and weight percentages (Inset).

Example-2

[0064] A mixture of elemental powders of titanium, silicon (60 mesh, 99% purity, Central Drug House Pvt. Ltd., India) and graphite powder was prepared in a stoichiometric ratio of 3:1-1.8:2 with the replacement of tin with silicon metal powder as depicted in Example-1. A green pellet was prepared and electrified with an applied voltage in a range of 25-55 V/cm, followed by pre-heating of the specimen at about 300? C. During the flashing event, the current was measured to be about 220 mA/mm.sup.2 and the temperature of the sample was measured to be about 1400? C. The product was primarily Ti.sub.3SiC.sub.2 MAX phase along with TiC which can be confirmed from the XRD analysis in FIG. 6. The diffraction lines of Ti.sub.3SiC.sub.2 MAX phase are in accordance with the JCPDS reference no.: 03-065-3559. The presence of TiC during the synthesis of Ti.sub.3SiC.sub.2 MAX phase can be either an intermediate product prior to the formation of Ti.sub.3SiC.sub.2 MAX phase or a result of its decomposition. The layered crystalline characteristics of synthesized Ti.sub.3SiC.sub.2 MAX phase can be seen in FIG. 7 with elemental analysis (EDAX) (Inset) confirming the elemental composition of Ti.sub.3SiC.sub.2 MAX phase.

Advantages of the Invention

[0065] 1. Use of a novel, simple and cost-effective sintering technique for the synthesis of pure MAX phase. [0066] 2. Synthesis of major MAX phases include but not limited to examples such as Ti.sub.2SnC, Ti.sub.3SiC.sub.2, but includes different MAX phases like Zr.sub.2AlC, V.sub.4AlC.sub.3, Ti.sub.3AlC.sub.2 etc. [0067] 3. This technique delivers dense, crystalline and pure MAX phase in extremely short time in comparison to other conventional techniques. [0068] 4. This invention introduces the synthesis of MAX phases in air by suppressing the oxidation of material both at low and high temperature. [0069] 5. This technique synthesizes MAX phase in bulk for its extensive study in different applications and for the fabrication of excellent two-dimensional nano-material called MXenes.