18
S. Gupta
correction, heart-lung machines, urinary tract reconstruction, and cardiac simulator
[1–3, 5]. Some of the important characteristics of biomaterials are biocompatibility,
mechanical performance, corrosion and wear resistance, and osteointegration [2]. In
order to further improve the durability and performance of biomaterials, there is a
need to develop engineered materials with tailored properties.
MAX and MAB Phases
MAX phases are novel ternary and quaternary solids with layered structures where
MAX acronym for M n+1 AX n were M = Early Transitional Metal, A, Group A
element, X is C or N, and n = 1–3 [6–12]. These stoichiometries are also referred
to as 211, 312, and 413. Due to their layered atomic structure, these solids are
bestowed with promising properties like non-linear elasticity, high electrical and
thermal conductivity, low hardness, damage tolerance, and triboactive behavior. The
readers can refer to Barsoum [6] for detailed definition of these phases. Sokol and
co-workers [9] had summarized that there are at least ~155 different compositions
in different formulations of 16 A elements and 14 M elements.
In these structures, smaller X atoms fill the octahedral voids in closely packed
M layers, and these packed layers are then sandwiched with A group elements to
form layered or nanolaminate structure at the microstructural level. In 211, 312, and
413, the number of A-layers is 2, 3, and 4, respectively. Different types of adjacent
stacking of M-X blocks can lead to polymorphism in these structures. In M 2 AX, there
is one polymorph, whereas in M 3 AX 2 and M 4 AX 3 , there are 2 (α and β) and 3 (α, β,
and γ) polymorphs, respectively [9, 10]. These unit cells can further combine with
each to form hybrid structures (h-MAX). Palmquist and co-workers [11] observed
Ti 5 Si 2 C 3 (combination of half cells of 312 and 211) and Ti 7 Si 2 C 5 (combination of
unit cells of 312 and 413) in thin films. Lane and co-workers [12] observed Ti 5 Al 2 C 3
with trigonal structure (space group P3m1). Each unit consisted of three formula
units with cell parameters of a = 3.064 Å and c = 48.23 Å.
Recently, different groups are exploring the ordering within the crystal structures of MAX phases. Quaternary MAX phases (M
,M
) n+1 AlC n can show out
of plane ordering which are referred to o-MAX [9]. Some examples of o-MAX
are (Cr 2/3 Ti 1/3 ) 3 AlC 2, (Cr 0.5 V 0.5 ) n+1 AlC n (n = 2, 3), Mo 2 TiAlC 2 , Mo 2 Ti 2 AlC 3 and
Mo 2 ScAlC 2 [13–16]. The crystal structures of MAX phases can be further tailored
by in-phase ordering (i-MAX), for example, (Mo 2/3 Sc 1/3 ) 2 AlC, (Mo 2/3 Sc 1/3 ) 2 GaC
and (Mo 2/3 Y 1/3 ) 2 GaC, (V 2/3 Zr 1/3 ) 2 AlC and (Mo 2/3 Y 1/3 ) 2 AlC [17–19]. By etching
with Hydrofluoric Acid (HF) [20] or HF generated by reaction of Lithium Fluoride
(LiF) and Hydrochloric Acid (HCl) in the solution during the manufacturing process
[21], it is possible to completely leach A-group elements, and 2D MXenes can be
produced [20, 21].
Ade and co-workers [22] categorized B-containing ternaries with the stoichiometry of (MB) 2 Al y (MB 2 ) x as MAB phases. These solids crystallized in orthorhombic
space groups. Some examples are MoAlB, Mn 2 AlB 2 , Cr 2 AlB 2 , Fe 2 AlB 2 , among
S. Gupta
correction, heart-lung machines, urinary tract reconstruction, and cardiac simulator
[1–3, 5]. Some of the important characteristics of biomaterials are biocompatibility,
mechanical performance, corrosion and wear resistance, and osteointegration [2]. In
order to further improve the durability and performance of biomaterials, there is a
need to develop engineered materials with tailored properties.
MAX and MAB Phases
MAX phases are novel ternary and quaternary solids with layered structures where
MAX acronym for M n+1 AX n were M = Early Transitional Metal, A, Group A
element, X is C or N, and n = 1–3 [6–12]. These stoichiometries are also referred
to as 211, 312, and 413. Due to their layered atomic structure, these solids are
bestowed with promising properties like non-linear elasticity, high electrical and
thermal conductivity, low hardness, damage tolerance, and triboactive behavior. The
readers can refer to Barsoum [6] for detailed definition of these phases. Sokol and
co-workers [9] had summarized that there are at least ~155 different compositions
in different formulations of 16 A elements and 14 M elements.
In these structures, smaller X atoms fill the octahedral voids in closely packed
M layers, and these packed layers are then sandwiched with A group elements to
form layered or nanolaminate structure at the microstructural level. In 211, 312, and
413, the number of A-layers is 2, 3, and 4, respectively. Different types of adjacent
stacking of M-X blocks can lead to polymorphism in these structures. In M 2 AX, there
is one polymorph, whereas in M 3 AX 2 and M 4 AX 3 , there are 2 (α and β) and 3 (α, β,
and γ) polymorphs, respectively [9, 10]. These unit cells can further combine with
each to form hybrid structures (h-MAX). Palmquist and co-workers [11] observed
Ti 5 Si 2 C 3 (combination of half cells of 312 and 211) and Ti 7 Si 2 C 5 (combination of
unit cells of 312 and 413) in thin films. Lane and co-workers [12] observed Ti 5 Al 2 C 3
with trigonal structure (space group P3m1). Each unit consisted of three formula
units with cell parameters of a = 3.064 Å and c = 48.23 Å.
Recently, different groups are exploring the ordering within the crystal structures of MAX phases. Quaternary MAX phases (M
,M
) n+1 AlC n can show out
of plane ordering which are referred to o-MAX [9]. Some examples of o-MAX
are (Cr 2/3 Ti 1/3 ) 3 AlC 2, (Cr 0.5 V 0.5 ) n+1 AlC n (n = 2, 3), Mo 2 TiAlC 2 , Mo 2 Ti 2 AlC 3 and
Mo 2 ScAlC 2 [13–16]. The crystal structures of MAX phases can be further tailored
by in-phase ordering (i-MAX), for example, (Mo 2/3 Sc 1/3 ) 2 AlC, (Mo 2/3 Sc 1/3 ) 2 GaC
and (Mo 2/3 Y 1/3 ) 2 GaC, (V 2/3 Zr 1/3 ) 2 AlC and (Mo 2/3 Y 1/3 ) 2 AlC [17–19]. By etching
with Hydrofluoric Acid (HF) [20] or HF generated by reaction of Lithium Fluoride
(LiF) and Hydrochloric Acid (HCl) in the solution during the manufacturing process
[21], it is possible to completely leach A-group elements, and 2D MXenes can be
produced [20, 21].
Ade and co-workers [22] categorized B-containing ternaries with the stoichiometry of (MB) 2 Al y (MB 2 ) x as MAB phases. These solids crystallized in orthorhombic
space groups. Some examples are MoAlB, Mn 2 AlB 2 , Cr 2 AlB 2 , Fe 2 AlB 2 , among