TECHNICAL FIELD
5 The present application relates to multi-component oxide for abrasive applications
designed and synthesized using high entropy concept. The application also relates
to the bulk powder synthesis of high entropy oxide (HEO) powders by electric arc
fusion route.
10 BACKGROUND AND PRIOR ART
Discovering new material systems to cater the advanced application needs has
always been the fundamental driving force in the material science field. The most
traditional method of making new materials is by mixing two components or
alloying. The mixture can yield properties average or superior than the individual
15 components or it can show complete new set of properties. The discovery of high
entropy alloys (HEAs) proposed by Yeh et al in Advanced Engineering Materials,
6 (2004) 299-303 opened richer compositional space to design and develop new
materials. HEAs generally have five or more principal elements with equi-atomic
or newer equip-atomic concentrations and therefore associated with larger
20 configuration entropy. In multi component systems like HEA, high entropy can
lead to unique attributes including stabilisation of single-phase solid solution with
simple crystal structure and collection of properties surpassing the individual
constituents in the system.
25 High entropy ceramics (HEC) are derived from the field of HEAs. Like HEAs,
HECs are also composed of multiple component elements which form a single
phase, and their formation is due to their high conformation entropy. The
advantage of HECs is their structural diversity. HECs have a crystal structure
composted of a cation sub-lattice and an anion sub-lattice, which enables the
30 coexistence of cations and anions in the same crystal structure, thereby increasing
configuration entropy. However, HECs gained much attention when High
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Entropy Oxide system (Mg, Co, Ni, Cu, Zn)O with high configuration entropy
with single phase rock salt crystal structure was prepared. Since then, brides,
carbides, sidelines, concatenates, phosphates, and nitrates have been extensively
investigated. HEO basically contains one cation sub-lattice with five or more metals
5 and oxygen sub-lattice which together form single phase oxide solid solution. HEOs
have severe lattice distortion and lattice defects due to the non-psychometric in
oxygen sub-lattice. This reduces thermal conductivity and increases the toughness
of the material. Hence, HEOs are considered for high temperature applications.
10 Although theoretically HEC sound very tempting it has its own challenges when it
comes to synthesis. To achieve a single-phase solid solution of HEC, diffusion at
the atomic scale is very much essential. Diffusivity of the atoms exponentially
increases with the temperature. Generally, the constituent elements in HEC systems
are ceramics which possess very high melting points. Therefore, to achieve high
15 diffusion rate in these systems high temperature is needed. The most common
synthesis methods of HEC reported so far are solid state reaction, wet chemical
route for powder synthesis and solidification, field assisted wintering or spark
plasma wintering to make wintered compacts. In wet chemical synthesis flame
hydrolysis, hydrothermal synthesis, sol-gel method, combustion synthesis are widely
20 used. In solid state synthesis, the most used process is ball milling of the powders
followed by spark plasma wintering or conventional wintering at high temperatures.
The first ever patent application on high entropy alloys was US20020159914 filed
by Yeh Jien-Wei which defined high entropy multi component alloys and explained
25 the concept of high entropy. High entropy multi-component alloy was made using
5 to 11 metallic elements from beryllium, magnesium, aluminium, scandium,
titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc,
yttrium, zirconium, niobium, molybdenum, ruthenium, rhodium, palladium, silver,
hafnium, tantalum, tungsten, platinum, gold, lanthanum, cerium, praseodymium,
30 neodymium, samarium, europium, gadolinium and terbium. The alloy was melted
in an electric arc furnace followed by a rapid solidification process. According to
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the claims, the basic characteristic of the high entropy alloy was very high hardness,
very high heat resistance and very high corrosion resistance.
Chinese patent no. CN113185305B, has disclosed preparation of high temperature
5 heat insulation high entropy oxide preparation by sol-gel chemical synthesis. The
chemical formula of the system is (Hf Zr Ce T Al) which has good high temperature
thermal stability, phase stability and heat insulating property. The method includes
preparation of precursor followed by drying to make a gel and heat treatment of the
gel at 900-1600°C for 24 hours to obtain HEO. The molar ratio of hafnium
10 tetra chloride to zirconium hydrochloride carbohydrate to cerium nitrate hydrate to
yttrium nitrate hydrate to an aluminium source is 0.15-0.3: 0.15-0.3: 0.15-0.3:
0.05-0.3: 0.05-0.3.
Prior art US20220134616, Chou et al, reported synthesis of high surface area of
15 HEO using sol-gel method. The composition of the HEO contains metal cations
from the first row of the transition metals with oxygen anions in equi-molar ratio.
In this sol-gel method, polymeric complex agent is dissolved in water to form
aqueous solution. Then the metal salts are dissolved in the aqueous solution
followed by drying to form a gel. The gel is subjected to calcification in oxygen
20 atmosphere to form high surface area HEO in a single-phase fluorite lattice
structure.
discloses a formation of single-phase spine type high entropy using
sol-gel combustion method. The high entropy oxide material system used in this
25 invention contains five transition metals Co, Cr, Fe, Mn, Ni and Zn is used to
replace any one of the Co, Cr, Fe, Mn, Ni metal cations. In the preparation method,
five out of the six nitrites are taken in equal quantity and mixed with glycerine
followed by blast drying in oven to form a viscous gel. Then the gel is heat treated
in a muffle furnace at 850°C to obtain single phase spine type HEO.
discloses a high entropy oxide method by ball milling and wintering
route. In this invention, ultra-fast synthesis of HEO is reported using joule heating
device which has an extremely high heating rate and very high temperature upper
limit. The material system taken was widely studied among the HEO system, i.e.
5 (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)O2. Prior to the wintering, all the metal oxides are
subjected to ball milling for 2 hours for homogenisation of the powders.
CN109987935 discloses synthesis of fluorite type (Zr Hf Ce Ti Zn)O2 HEO system
by ball milling and sintering route. According to the invention, the first step is to
10 mix the all five oxide powders in equi-molar ratio and subjected to high energy ball
mill for 10-30 hours. In the second step the powder is filled in a graphite mould and
subjected to wintering at 1200-1600°C in a discharge plasma furnace to obtain a
high-energy ceramic block.
15 CN114618503 discloses a HEO material designed for oxygen storage material. The
HEO precursor material is synthesized by hydrothermal method. To prepare the
precursor, all the metal nitrates are mixed with a complex agent preferably citric
acid. The metal nitrates include rare earth nitrates and/or transition metal nitrates.
The hydrothermal reaction of the prepared solution is carried out at 100-120°C for
20 1-5 hours. Finally, the reacted solution was subjected to calcification at 400-600°C
to obtain a high entropy oxide oxygen storage material. In the second part of the
invention, it is claimed that the synthesized HEO mainly composed of meso-pores
which is very beneficial for heterogeneous catalytic reaction.
25 Though the prior arts disclose single phase HEO by sintering methods which
involves solid to solid high temperature diffusion, it is difficult to scale such
methods to bulk production. Moreover, HEO systems contains anions and cations
in which during fusion, due to the exposure to atmosphere, some of the oxides can
evaporate based on the local partial pressure. It can create non psychometric oxides
30 with oxygen vacancies. These changes during fusion may disrupt the single crystal
6
lattice formation. Hence, the HEO system must be chosen to from a single high
entropy oxide phase after solidification.
Thus, it is an object of the present invention to provide for a method which offers
5 more homogeneity and is callable to commercial bulk manufacturing. It is also an
object of the present invention to provide for high entropy oxide which form a
stable, single high entropy phase after fusion.
SUMMARY OF THE INVENTION
10 Accordingly, the present invention provides high entropy oxide (HEO) and a
method for preparing the same by electric arc fusion method.