DESC:TECHNICAL FIELD
[0001] The present disclosure relates to ternary oxide compounds. In particular, the present disclosure relates to synthesis of cuprous chromite having the delafossite phase, and synthesis of zinc niobate and magnesium niobate having the columbite phase.

BACKGROUND
[0002] The background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
[0003] Cuprous chromite (CuCrO2) belongs to delafossite (ABO2 with A=Cu1+) family, and niobate of magnesium (MgNb2O6) and niobate of zinc (ZnNb2O6) belong to columbite (AB2O6 with B=Nb5+) family. Both compounds have potential applications. They have a wide range of electrical properties, with conductivities ranging from insulating to metallic. Delafossite CuCrO2 is one of the promising p-type materials extensively used in Transparent Conducting Oxides (TCOs) in optoelectronic devices. Delafossite CuCrO2 is also used as a thermoelectric and catalytic material for water-splitting, and as a multiferroic material. The columbite compounds, zinc niobate and magnesium niobate, typically show considerable value of microwave dielectric constant, which makes them a potential candidate for resonators and filters in satellite and mobile communication systems. These delafossite and columbite compounds not only involve high temperatures, but also, as revealed in their phase diagrams, they exist over very narrow phase field regions: any change in the stoichiometry leads to multiphase mixtures. Synthesis of these delafossite and columbite compounds by the conventional solid-state reaction and sintering methods imposes a considerable energy budget due to the requirement of high temperatures of about 1000-1300°C over a long period of time of 12-36 hours.
[0004] In view of the above noted disadvantages of the conventional processes, there is a need for a new and improved synthesis method that is capable of producing delafossite and columbite phase oxides from solid precursors at low temperature and short duration of time. The present disclosure satisfies these needs and provides further related advantages.

OBJECTS
[0005] An object of the present disclosure is to provide a new and improved process that may overcome one or more limitations associated with the conventional processes for synthesis of delafossite and columbite phase oxides.
[0006] It is also an object of the present disclosure to provide an improved process for synthesis of delafossite cuprous chromite (CuCrO2) from solid precursors that reduces the process temperature and time.
[0007] Another object of the present disclosure is to provide an improved process for synthesis of columbite zinc niobate (ZnNb2O6) from solid precursors that reduces the process temperature and time.
[0008] Another object of the present disclosure is to provide an improved process for synthesis of columbite magnesium niobate (MgNb2O6) from solid precursors that reduces the process temperature and time.
[0009] Further object of the present disclosure is to provide a process for synthesis of delafossite and columbite phase oxides from solid precursors that is easy to follow, economical and industrially applicable.

SUMMARY
[00010] Aspects of the present disclosure relate to a new and improved synthesis method that is capable of producing delafossite and columbite phase oxides from solid precursors at low temperatures and in very short times compared to conventional methods. The present disclosure takes advantage of reactive flash sintering technique to produce delafossite and columbite phase oxides from solid precursors in very short times and at low temperatures. By using a reactive flash sintering method to produce delafossite and columbite phase compounds from solid precursors, the applicant has developed a process wherein the requirement of high processing temperature and long processing time, which is undesirable for reasons related to complications in manufacturing and energy usage, is avoided, thereby addressing a long felt need in the art for an alternative to conventional solid-state reaction and sintering methods.
[00011] Accordingly, one aspect of the present disclosure provides a method for synthesizing delafossite compounds, such as delafossite cuprous chromite (CuCrO2), and columbite compounds, such as columbite zinc niobate (ZnNb2O6) and columbite magnesium niobate (MgNb2O6), at low temperature and in short time, which comprises the steps of: providing a first metal oxide, providing a second metal oxide, mixing the first metal oxide, the second metal oxide and a binder to provide a mixture, uniaxially cold pressing the mixture to a compacted body, heating the compacted body to burn-off the binder, and flash sintering the compacted body to form a single-phase delafossite compound or a single-phase columbite compound. According to embodiments of the present disclosure, the step of flash sintering comprises applying to the compacted body a DC electrical field ranging from 100 V/cm to 1000 V/cm with a power supply under voltage control while heating the compacted body to a temperature ranging from 200 °C. to 800 °C, and switching the power supply from voltage control mode to current control mode upon incidence of flash.
[00012] Various objects, features, aspects and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS
[00013] The accompanying drawings are included to provide a further understanding of the present disclosure, and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the present disclosure and, together with the description, serve to explain the principles of the present disclosure.
[00014] FIG. 1 shows a flow chart for the synthesis of delafossite cuprous chromite (CuCrO2) by reactive flash method in accordance with an embodiment of the present disclosure.
[00015] FIG. 2 shows a flow chart for the synthesis of columbite zinc niobate (ZnNb2O6) and columbite magnesium niobate (MgNb2O6) by reactive flash method in accordance with an embodiment of the present disclosure.
[00016] FIG. 3 illustrates a reactive flash (RF) experimental set-up for the synthesis of delafossite cuprous chromite (CuCrO2), columbite zinc niobate (ZnNb2O6) and columbite magnesium niobate (MgNb2O6), in accordance with the teachings herein.
[00017] FIG. 4 is a graph showing current density vs. flash sintering time profile comprising 3 constant-j steps (in total) used for synthesis of columbite ZnNb2O6 & comprising 6 constant-j steps (in total) used for synthesis of delafossite CuCrO2.
[00018] FIG. 5 shows X-ray diffraction patterns of samples showing evolution of product phase (CuCrO2) from unreacted precursors (1-4) and the final samples matched with standard ICDD data of CuCrO2 (Card No: 039-0247).
[00019] FIG. 6 shows fraction of different phases in the samples (CuCrO2) at t=0 s (1); at t=30 s (2); at 270 s (3); after completion of RF experiments at 390 s (4).
[00020] FIG. 7 is SEM micrograph of fractured RF-sample (sample 4 - CuCrO2).
[00021] FIG. 8 shows X-ray diffraction pattern of columbite ZnNb2O6 synthesized by reactive flash (RF) method and comparison of the same with standard ICDD data of ZnNb2O6 (Card No: 037-1371).
[00022] FIG. 9A is secondary electron image of columbite ZnNb2O6 synthesized by reactive flash (RF) method.
[00023] FIG. 9B is backscattered electron image of columbite ZnNb2O6 synthesized by reactive flash (RF) method, showing uniform compositional contrast.

DETAILED DESCRIPTION
[00024] The following is a detailed description of embodiments of the present disclosure. The embodiments are in such detail as to clearly communicate the disclosure. However, the amount of detail offered is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure as defined by the appended claims.
[00025] The present disclosure is directed to a new and improved synthesis method that is capable of producing delafossite and columbite phase oxides from solid precursors at low temperatures and in very short times compared to conventional methods. The present disclosure takes advantage of reactive flash sintering technique to produce delafossite and columbite phase oxides from solid precursors in very short times and at low temperatures. By using a reactive flash sintering method to produce delafossite and columbite phase compounds from solid precursors, the applicant has developed a process wherein the requirement of high processing temperature and long processing time, which is undesirable for reasons related to complications in manufacturing and energy usage, is avoided, thereby addressing a long felt need in the art for an alternative to conventional solid-state reaction and sintering methods.
[00026] In one embodiment, the present disclosure provides a method for synthesizing delafossite compounds, such as delafossite cuprous chromite (CuCrO2), at low temperature and in short time, which comprises the steps of: providing a first metal oxide, providing a second metal oxide, mixing the first metal oxide, the second metal oxide and a binder to provide a mixture, uniaxially cold pressing the mixture to a compacted body, heating the compacted body to burn-off the binder, and flash sintering the compacted body to form a single-phase delafossite compound. The first metal oxide used in this method is Cu2O and the second metal oxide is Cr2O3. According to embodiments of the present disclosure, the step of flash sintering comprises applying to the compacted body a DC electrical field ranging from 100 V/cm to 1000 V/cm with a power supply under voltage control while heating the compacted body to a temperature ranging from 200 °C. to 800 °C, and switching the power supply from voltage control mode to current control mode upon incidence of flash.
[00027] The present disclosure also provides a method for synthesizing columbite compounds, such as columbite zinc niobate (ZnNb2O6) and columbite magnesium niobate (MgNb2O6), at low temperature and in short time, which comprises the steps of: providing a first metal oxide, providing a second metal oxide, mixing the first metal oxide, the second metal oxide and a binder to provide a mixture, uniaxially cold pressing the mixture to a compacted body, heating the compacted body to burn-off the binder, and flash sintering the compacted body to form a single-phase columbite compound. If columbite zinc niobate (ZnNb2O6) is to be synthesized by this method, ZnO can be used as the first metal oxide and Nb2O5 can be used as the second metal oxide. If columbite magnesium niobate (MgNb2O6) is to be synthesized by this method, MgO can be used as the first metal oxide and Nb2O5 can be used as the second metal oxide. According to embodiments of the present disclosure, the step of flash sintering comprises applying to the compacted body a DC electrical field ranging from 100 V/cm to 1000 V/cm with a power supply under voltage control while heating the compacted body to a temperature ranging from 200 °C. to 800 °C, and switching the power supply from voltage control mode to current control mode upon incidence of flash.
[00028] The reactants, i.e., the first metal oxide and the second metal oxide, can be mixed in stoichiometric quantities to form a reactant mixture. In one or more embodiments, a binder may be added to the reactant mixture, before the reactant mixture is uniaxially cold pressed, to produce some bonding between the particles. A binder which is thermally degradable is advantageous here. The binder in the reactant mixture can be removed before the flash sintering step by a pre-heat treatment at 500 °C to 600°C for a period of 30 minutes to 2 hours. In an embodiment, a polymer such as for example an aqueous polyvinyl alcohol (PVA) can be used as binder. In an embodiment, the mixture prior to cold pressing comprises 90 wt. % to 99.5 wt. % reactant mixture (first and second metal oxides) and 0.5 wt.% to 10 wt.% binder.
[00029] The reactant mixture can be uniaxially cold pressed at 1000 to 5000 psi for a period of time sufficient to compact the reactant mixture to a desired shape. The cold pressing can be carried out at room temperature. In one embodiment, the reactant mixture is uniaxially cold pressed to provide a compacted body in a cylindrical shape. In one embodiment, the reactant mixture is uniaxially cold pressed to provide a cold-pressed pellet.
[00030] As indicated above, a DC electrical field is applied to the compacted body with a power supply under voltage control while the compacted body is heated to an elevated temperature. In various embodiments, the electrical field ranges from 100 V/cm to 1000 V/cm, preferably from 150 V/cm to 600 V/cm. In various embodiments, the compacted body is heated to a temperature ranging from 200 °C. to 800 °C, preferably from 200 °C. to 400 °C. In an embodiment, the compacted body is heated at a heating rate of 5 ? /min. Preferably, the flash sintering is performed by applying an electric field to the compacted body in a furnace and increasing the temperature of the furnace at a constant rate until the onset of flash, and switching the power supply from voltage control mode to current control mode upon incidence of flash. The compacted body can be held in this state until a desired densification of the end product is reached. In one embodiment, the compacted body is held in the current control mode for a period of less than 8 minutes. In various embodiments, the duration of the flash sintering is less than 6 minutes. In one embodiment, the power supply reaches a predetermined level ranging from 1.0A to 5.0A in the current control mode.
[00031] In one exemplary embodiment, the flash sintering is carried out at an applied electric field of 160 V/cm at a constant heating rate of 5° C./min for the synthesis of delafossite cuprous chromite (CuCrO2).
[00032] In one exemplary embodiment, the flash sintering is carried out at an applied electric field of 560 V/cm at a constant heating rate of 5° C./min for the synthesis of columbite zinc niobate (ZnNb2O6).
[00033] In one exemplary embodiment, the flash sintering is carried out at an applied electric field of 800-1000 V/cm at a constant heating rate of 5° C./min for the synthesis of columbite magnesium niobate (MgNb2O6).
[00034] With reference to FIG. 1, a flowchart for an exemplary method for producing delafossite cuprous chromite (CuCrO2) by reactive flash sintering method according to an embodiment of the present disclosure is shown. The method involves solid-state synthesis, starting from the binary oxides of reactants in powder form. The method 100 includes providing copper and chromium oxide powders as reactants in appropriate ratio at 102. The method next includes mixing the reactant powders with a suitable binder and uniaxially cold-pressing to form discs of diameter 10 mm and thickness 3-5 mm at 104. The method includes heating the cold-pressed pellets to 500°C to remove the binder at 106. The method next includes attaching the cold-pressed pellets to platinum foils inside a customized vertical furnace attached to a DC power supply with a maximum power rating of 1.5 kW at 108. An optimized electric field of approximately 150 V/cm may be applied to the sample at 110. The furnace temperature may be ramped from room temperature at the rate of 5 OC/min at 112. At a certain furnace temperature, a bright luminescence of the sample may be observed accompanied by a sudden increase in the current which is termed as flash event at which power supply is changed into the current control mode at step 114. The condition of the sample may be sustained at this state for 6-8 minutes. The furnace temperature may be increased moderately by around 30-40°C at 116. After this, the electric field and the furnace may be switched off permitting the sample to cool down inside the furnace at step 118 to obtain delafossite cuprous chromite.
[00035] Referring to FIG. 2, there is provided a flowchart for an exemplary method for producing columbite zinc niobate (ZnNb2O6) or columbite magnesium niobate (MgNb2O6) by reactive flash sintering method according to an embodiment of the present disclosure. The method 200 includes providing zinc and niobium oxides or magnesium and niobium oxides as reactants in relative proportions to prepare the columbite compounds at 202. The method next includes mixing the reactant powders with a suitable binder and uniaxially cold-pressing to form discs with a diameter of 10 mm and thickness of 3-5 mm at 204. The method next includes heating the cold-pressed pellets to 500 0C to remove the binder at 206. These cold-pressed pellets may be attached to Pt foils inside a customized vertical furnace attached to a DC power supply with a maximum power rating of 1.5 kW at 208. An optimized electric field of ~375-450 V/cm may be applied to the sample for the preparation of columbites, (Zn/Mg)Nb2O6) at 210. The furnace temperature may be ramped from room temperature at the rate of 5 0C/min at 212. At a certain furnace temperature, a bright luminescence of the sample may be observed accompanied by a sudden increase in the current – termed the ‘flash’ event – at which the power supply is changed into current control mode at step 214. The condition of the sample may be sustained at this state for 6-8 minutes while the furnace temperature increased moderately by around 30-40°C at 216. After this, the electric field and the furnace may be switched off, permitting the sample to cool down inside the furnace at step 218 to obtain columbite zinc niobate (ZnNb2O6) or columbite magnesium niobate (MgNb2O6).
[00036] While the foregoing description discloses various embodiments of the disclosure, other and further embodiments of the invention may be devised without departing from the basic scope of the disclosure. The invention is not limited to the described embodiments, versions or examples, which are included to enable a person having ordinary skill in the art to make and use the invention when combined with information and knowledge available to the person having ordinary skill in the art.
EXAMPLES
[00037] The present disclosure is further explained in the form of following examples. However, it is to be understood that the foregoing examples are merely illustrative and are not to be taken as limitations upon the scope of the invention. Various changes and modifications to the disclosed embodiments will be apparent to those skilled in the art. Such changes and modifications may be made without departing from the scope of the invention.
Example 1: Reactive flash (RF) sample preparation for the synthesis of delafossite cuprous chromite (CuCrO2)
[00038] Stoichiometric mixtures of Cu2O and Cr2O3 were mixed with mortar and pestle to obtain a homogenous mixture of both oxides. The resultant powders were mixed with 5 wt. % binder (10 wt. % PVA in de-ionized water). The paste-like powders were placed in a hot-air oven for 1 hour at 100 to remove the water content. Approximately 0.9 gm of powders were pressed into cylindrical shapes (thickness=0.3 cm and diameter=1 cm) by applying uniaxial pressure of 370 MPa. The cold-pressed pellets were then heated to 500 OC for 1 hour to remove the binder. After binder removal, silver paste was applied on both surfaces of the samples in order to improve the electrical contact between the platinum foil and the sample. The final pellets were placed in a hot-air oven for 30 minutes at 140 OC to remove the organic component in the silver paste. The final pellets have been used for RF experiments.
Example 2: Reactive flash (RF) sample preparation for the synthesis of columbite zinc niobate (ZnNb2O6)
[00039] Stoichiometric mixtures of ZnO and Nb2O5 were mixed with mortar and pestle to obtain a homogenous mixture of both oxides. The resultant powders were mixed with 5 wt. % binder (10 wt. % PVA in de-ionized water). The paste-like powders were placed in a hot-air oven for 1 hour at 100 to remove the water content. Approximately 0.9 gm of powders were pressed into cylindrical shapes (thickness=0.3 cm and diameter=1 cm) by applying uniaxial pressure of 370 MPa. The cold-pressed pellets were then heated to 500 OC for 1 hour to remove the binder. After binder removal, silver paste was applied on both surfaces of the samples in order to improve the electrical contact between the platinum foil and the sample. The final pellets were placed in a hot-air oven for 30 minutes at 140 OC to remove the organic component in the silver paste. The final pellets have been used for RF experiments.
Example 3: Reactive flash (RF) sintering
[00040] All reactive flash (RF) experiments were conducted inside a customized, vertical loading furnace with Ag-pasted samples attached to Pt foils. The sample is placed at the centre of the furnace in the uniform temperature zone and Pt wires welded to the foils were used to apply the electric field/measure conductivity. The sample with attached Pt foils were held in place with a dense long cylindrical alumina rod, the other end of which was attached to a dilatometer capable of measuring shrinkage (resolution of 10 micrometre) as depicted in FIG. 3. The linear shrinkage is corrected by subtracting the LVDT reading of alumina, rod and frame during heating. Thermocouple measures the furnace temperature and sample temperature. The current flow across the sample was measured by a digital multimeter (Fluke make).
[00041] For the synthesis of CuCrO2, a fixed voltage corresponding to a field of 160 V/cm was applied, and furnace temperature was ramped from room temperature at the rate of 5 OC /min for all samples until flash was observed, at which the power supply was changed into current control mode from voltage control and reaching to a pre-set value of 5 A. For RF experiments, the total duration of the flash event was approximately 6 minutes. FIG. 4 shows a typical current density (j)profile adopted in the RF experiments after reaching to the current controlled regime. After the flash, the power supply and furnace were switched off and cooled to room temperature.
[00042] Similarly, for the synthesis of ZnNb2O6, fixed voltage corresponding to a field of 560 V/cm was applied, and furnace temperature was ramped from room temperature at the rate of 5 OC /min for all samples until the flash was observed, at which the power supply was changed into current control mode from voltage control and reaching to a pre-set value of 1 A. RF experiments were optimised such that total duration of the flash event was 4.5 minutes. FIG. 4 shows a typical current density (j)profile adopted in the RF experiments after reaching to the current controlled regime. After the flash, the power supply and furnace were switched off and sample was cooled to room temperature.
Results
[00043] The furnace temperature at flash for the delafossite CuCrO2 in repeated trials was around 270 OC. XRD of RF samples for the synthesis of CuCrO2 is shown in FIG. 5. Also, the reaction pathway from the precursor (CuO &Cr2O3) to the product (CuCrO2) were tracked by interrupting the RF experiments on stage III for different time intervals (samples labelled 1-4). At the point of flash onset (labelled 1) the phases are a mixture of unreacted precursor with small fraction of CuCrO2 (1.2 %). The fraction is increased to 6.9 % within 30 s (labelled 2) and emergence of minor secondary phases of CuO, Cr2O3 and CuCrO2 after 230 s (labelled 3). Then the overall phase transformation is completed within 390 s (labelled 4). The enlarged figure (2q between 30-40o) clearly demonstrates the appearance of CuCrO2 reflection and disappearance of precursor reflection with progression in time. The final RF sample (labelled 4) shows strong reflections from rhombohedral CuCrO2, and it’s matched with standard ICDD data (Card No: 039-0247) of CuCrO2.
[00044] Quantification of phase evolution was done by calculating the phase fraction of each sample from Rietveld refinement of subsequent X-ray powder diffraction data. The phase fraction bar diagram in FIG. 6 clearly demonstrates the increase in the phase fraction of CuCrO2 from 1.2 % (labelled 1) to 100 % (labelled 4) within 390 s.
[00045] FIG. 7 shows the microstructure of the final reacted samples showing fused grains around 1.4 mm. Hall measurements gave clear p-type conductivity, with[h]=3.62x1014/cm3.
[00046] The columbite sample ZnNb2O6 had a flash temperature in repeated trials of around 290 ?. XRD of RF samples for the synthesis of ZnNb2O6 is shown in FIG. 8. It shows phase pure ZnNb2O6 and matches with standard ICDD data reported for the columbite phase (Card No: 037-1371). We propose a liquid phase mediated mechanism for the formation of ZnNb2O6. In RF, the compound with the lowest melting point melts first. In this case Nb2O5 melts when the Joule-heating assisted temperature raises above 1450 C. The Nb-rich melt then permeates the microstructure, dissolving the ZnO particles. The melt stabilizes at the overall composition after progressing through the liquid phase boundary on the Nb-rich side. The local sample temperature being much greater than the external furnace temperature ensures swift diffusion and complete mixing of components. The liquid stabilizes at the overall composition of ZnNb2O6 and being a congruently crystallizing compound, solidifies below 1400 OC during cooling. FIG. 9A shows fully dense sintered microstructure of ZnNb2O6 with appearances of a melt phase. The backscattered electron image (FIG. 9B) also shows uniform composition of the microstructure.
[00047] A comparison of the temperature and time used in conventional sintering methods using same precursors and in the reactive flash method is shown in Table 1 below.

Table 1
System Conventional Method References

Reactive Flash Method
Furnace temperature
( )
Hold Time
(hours) Furnace Temperature at Flash
( )
Flash time

(minutes)
CuCrO2 1000
1200 30
12 [22]
[23]
270 6.5
ZnNb2O6 1230
1125 4
5 [16]
[24]
290 4.5

[00048] From the data shown in Table 1, it is evident that the reactive flash sintering method produces delafossite and columbite compounds in very short times and at low temperatures as compared to conventional methods, resulting in huge energy savings.
REFEENCES
[1] Transparent conductive CuCrO 2 thin films deposited by pulsed injection metal organic chemical vapor deposition: up-scalable process technology for an improved transparency/conductivity trade-off, J. Mater. Chem. C 4 (2016) 4278–4287. https://doi.org/10.1039/C6TC00383D.
[2] R. Manickam, K. Biswas, Double doping induced power factor enhancement in CuCrO2 for high temperature thermoelectric application, Journal of Alloys and Compounds 775 (2019) 1052–1056. https://doi.org/10.1016/j.jallcom.2018.10.083.
[3] C.E. Creissen, J. Warnan, D. Antón-García, Y. Farré, F. Odobel, E. Reisner, Inverse Opal CuCrO 2 Photocathodes for H 2 Production Using Organic Dyes and a Molecular Ni Catalyst, ACS Catal. 9 (2019) 9530–9538. https://doi.org/10.1021/acscatal.9b02984.
[4] I.C. Kaya, S. Akin, H. Akyildiz, S. Sonmezoglu, Highly efficient tandem photoelectrochemical solar cells using coumarin6 dye-sensitized CuCrO2 delafossite oxide as photocathode, Solar Energy 169 (2018) 196–205. https://doi.org/10.1016/j.solener.2018.04.057.
[5] R.-D. Zhao, Y.-M. Zhang, Q.-L. Liu, Z.-Y. Zhao, Effects of the Preparation Process on the Photocatalytic Performance of Delafossite CuCrO 2, Inorg. Chem. 59 (2020) 16679–16689. https://doi.org/10.1021/acs.inorgchem.0c02678.
[6] D. Bansal, J.L. Niedziela, A.F. May, A. Said, G. Ehlers, D.L. Abernathy, A. Huq, M. Kirkham, H. Zhou, O. Delaire, Lattice dynamics and thermal transport in multiferroic CuCrO 2, Phys. Rev. B 95 (2017) 054306. https://doi.org/10.1103/PhysRevB.95.054306.
[7] D.O. Scanlon, G.W. Watson, Understanding the p-type defect chemistry of CuCrO2, J. Mater. Chem. 21 (2011) 3655. https://doi.org/10.1039/c0jm03852k.
[8] J. Tate, H.L. Ju, J.C. Moon, A. Zakutayev, A.P. Richard, J. Russell, D.H. McIntyre, Origin of p -type conduction in single-crystal CuAlO 2, Phys. Rev. B 80 (2009) 165206. https://doi.org/10.1103/PhysRevB.80.165206.
[9] J. Schorne-Pinto, P. Chartrand, A. Barnabé, L. Cassayre, Thermodynamic and Structural Properties of CuCrO 2 and CuCr 2 O 4?: Experimental Investigation and Phase Equilibria Modeling of the Cu–Cr–O System, J. Phys. Chem. C 125 (2021) 15069–15084. https://doi.org/10.1021/acs.jpcc.1c04179.
[10] S. Kumar, S. Marinel, M. Miclau, C. Martin, Fast synthesis of CuCrO2 delafossite by monomode microwave heating, Materials Letters 70 (2012) 40–43. https://doi.org/10.1016/j.matlet.2011.11.091.
[11] P. Rattanathrum, C. Taddee, N. Chanlek, P. Thongbai, T. Kamwanna, Structural and physical properties of Ge-doped CuCrO 2 delafossite oxide, Ceramics International 43 (2017) S417–S422. https://doi.org/10.1016/j.ceramint.2017.05.244.
[12] A. Kan, H. Ogawa, H. Ohsato, Influence of microstructure on microwave dielectric properties of ZnTa2O6 ceramics with low dielectric loss, Journal of Alloys and Compounds 337 (2002) 303–308. https://doi.org/10.1016/S0925-8388(01)01950-8.
[13] A. Kudo, S. Nakagawa, H. Kato, Overall Water Splitting into H2 and O2 under UV Irradiation on NiO-loaded ZnNb2O6 Photocatalysts Consisting of d10 and d0 Ions, Chemistry Letters 28 (1999) 1197–1198. https://doi.org/10.1246/cl.1999.1197.
[14] B.C. Yadav, R. Srivastava, A. Yadav, T. Shukla, Synthesis and Characterization of ZnO/ZnNb 2 O 6 Nanocomposite and Its Application as Humidity and LPG Sensor, International Journal of Green Nanotechnology 3 (2011) 56–71. https://doi.org/10.1080/19430892.2011.574539.
[15] M. Hirano, T. Okamoto, Synthesis, morphology, and luminescence of ZnNb 2 O 6 nanocrystals by hydrothermal method, Nano-Structures & Nano-Objects 13 (2018) 139–145. https://doi.org/10.1016/j.nanoso.2015.11.001.
[16] Y.-C. Liou, Y.-L. Sung, Preparation of columbite MgNb2O6 and ZnNb2O6 ceramics by reaction-sintering, Ceramics International 34 (2008) 371–377. https://doi.org/10.1016/j.ceramint.2006.10.016.
[17] V.V. Deshpande, M.M. Patil, S.C. Navale, V. Ravi, A coprecipitation technique to prepare ZnNb2O6 powders, Bull Mater Sci 28 (2005) 205–207. https://doi.org/10.1007/BF02711248.
[18] R.R. Dayal, The binary system ZnO?Nb2O5, Journal of the Less Common Metals 26 (1972) 381–390. https://doi.org/10.1016/0022-5088(72)90087-2.
[19] M. Cologna, B. Rashkova, R. Raj, Flash Sintering of Nanograin Zirconia in <5 s at 850°C, Journal of the American Ceramic Society 93 (2010) 3556–3559. https://doi.org/10.1111/j.1551-2916.2010.04089.x.
[20] Q. Zuo, W. Liu, Y. Su, Y. Cao, K. Ren, Y. Wang, Synthesis of LiCoO2 cathode materials for Li-ion batteries at low temperatures, Scripta Materialia 233 (2023) 115511. https://doi.org/10.1016/j.scriptamat.2023.115511.
[21] G. Zhao, S. Cai, Y. Zhang, H. Gu, C. Xu, Reactive flash sintering of high-entropy oxide (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)2Zr2O7: Microstructural evolution and aqueous durability, Journal of the European Ceramic Society 43 (2023) 2593–2600. https://doi.org/10.1016/j.jeurceramsoc.2023.01.029.
[22] M. Lalanne, A. Barnabé, F. Mathieu, Ph. Tailhades, Synthesis and Thermostructural Studies of a CuFe 1- x Cr x O 2 Delafossite Solid Solution with 0 = x = 1, Inorg. Chem. 48 (2009) 6065–6071. https://doi.org/10.1021/ic900437x.
[23] P. Pokhriyal, A. Bhakar, M.N. Singh, H. Srivastava, P. Rajput, P. Sagdeo, A. Srivastava, N.P. Lalla, A.K. Sinha, A. Sagdeo, Possibility of relaxor-type ferroelectricity in delafossite CuCrO2 near room temperature, Solid State Sciences 112 (2021) 106509. https://doi.org/10.1016/j.solidstatesciences.2020.106509.
[24] H. Barzegar Bafrooei, E. Taheri Nassaj, T. Ebadzadeh, C.F. Hu, Reaction sintering of nano-sized ZnO–Nb2O5 powder mixture: sintering, microstructure and microwave dielectric properties, J Mater Sci: Mater Electron 25 (2014) 1620–1626. https://doi.org/10.1007/s10854-014-1774-9.
,CLAIMS:1. A method of synthesizing a delafossite compound or a columbite compound, comprising the steps of:
providing a first metal oxide;
providing a second metal oxide;
mixing the first metal oxide, the second metal oxide and a binder to provide a mixture;
uniaxially cold pressing the mixture to a compacted body;
heating the compacted body to burn-off the binder;
flash sintering the compacted body to form a single-phase delafossite compound or a single-phase columbite compound,
wherein the step of flash sintering comprises applying to the compacted body a DC electrical field ranging from 100 V/cm to 1000 V/cm with a power supply under voltage control while heating the compacted body to a temperature ranging from 200 °C. to 800 °C, and switching the power supply from voltage control mode to current control mode upon incidence of flash.
2. The method as claimed in claim 1, wherein the electrical field ranges from 150 V/cm to 600 V/cm.
3. The method as claimed in claim 1, wherein the compacted body is heated at a heating rate of 5 ? /min.
4. The method as claimed in claim 1, wherein the compacted body is heated to a temperature ranging from 200 °C. to 400 °C.
5. The method as claimed in claim 1, wherein, in the current control mode, the power supply reaches a predetermined level ranging from 1.0A to 5.0A.
6. The method as claimed in claim 1, wherein the compacted body is held in the current control mode until a desired densification is reached.
7. The method as claimed in claim 1, wherein the compacted body is held in the current control mode for a period of less than 8 minutes.
8. The method as claimed in claim 1, wherein a duration of the step of flash sintering is less than 6 minutes.
9. The method as claimed in claim 1, wherein the first metal oxide is Cu2O, the second metal oxide is Cr2O3 and the delafossite compound is delafossite cuprous chromite (CuCrO2).
10. The method as claimed in claim 1, wherein the first metal oxide is ZnO, the second metal oxide is Nb2O5 and the columbite compound is columbite zinc niobate (ZnNb2O6).
11. The method as claimed in claim 1, wherein the first metal oxide is MgO, the second metal oxide is Nb2O5 and the columbite compound is columbite magnesium niobate (MgNb2O6).
12. The method as claimed in claim 1, wherein the binder is a polymer.