Metallic microlattice
Encyclopedia
A metallic microlattice is a synthetic porous metallic material, consisting of an ultra-light
form of metal foam
. Its creators claim it is the "lightest structural material" known, with a density
as low as 0.9 mg/cm3. It was developed by a team of scientists from HRL Laboratories, in collaboration with researchers at University of California, Irvine
and Caltech, and was first announced in November 2011. The prototype samples were made from a nickel
-phosphorus
alloy.
template was first prepared using a technique based on self-propagating waveguide
formation, though it was noted that other methods can be used to fabricate the template. The process passes UV light through a perforated mask into a reservoir of UV-curable resin
. Fiber-optic-like “self-trapping” of the light
occurs as the resin cures under each hole in the mask, forming a polymer fiber along the path of the light. By using multiple light beams, multiple fibers can then interconnect to form a lattice. The process is similar to photolithography
in that it uses a 2D mask to define the starting template structure, but differs in the rate of formation: where stereolithography
might take hours to make a full structure, the self-forming waveguide process enables templates to be formed in 10–100 seconds. In this way, the process enables large free-standing 3D lattice materials to be formed quickly and scalably. The template is then coated with a thin layer of metal by electroless nickel plating
, and the template is etched away, leaving a free-standing, periodic
porous metallic structure. Nickel
was used as the microlattice metal in the original report. Owing to the electrodeposition process, 7% of the material consisted of dissolved phosphorus atoms, and it contained no precipitates
.
s in diameter, with a wall 100 nanometre
s thick. The completed structure is about 99.99% air by volume, and by convention, the mass of air is excluded when the microlattice density is calculated. Allowing for the mass of the interstitial air, the true density of the structure is approximately 2.1 mg/cm3 (2.1 kg/m3), which is only about 1.76 times the density of air itself at 25 degrees Celsius
. The material is described as being 100 times lighter than Styrofoam
.
Metallic microlattices are characterized by very low densities, with the current record of 0.9 mg/cm3 being the lowest value for any solid
yet discovered. The previous record of 1.0 mg/cm3 was held by silica aerogel
s. Mechanically, these microlattices are behaviorally similar to elastomer
s and almost completely recover their shape after significant compression. This gives them a significant advantage over current aerogels, which are brittle, glass-like substances. This elastomeric property in metallic microlattices furthermore results in efficient shock absorption. Their Young's modulus
E exhibits different scaling, with the density ρ, E ~ ρ2, compared to E ~ ρ3 in aerogel
s and carbon nanotube
foams.
Ultralight material
Ultralight materials are solids with a density of less than 10 mg/cm3, including silica aerogels, carbon nanotube aerogels, metallic foams, polymeric foams, and metallic microlattices. The density of air is about 1.275 mg/cm3, which means that the air in the pores contributes significantly to the...
form of metal foam
Metal foam
A metal foam is a cellular structure consisting of a solid metal, frequently aluminium, containing a large volume fraction of gas-filled pores. The pores can be sealed , or they can form an interconnected network . The defining characteristic of metal foams is a very high porosity: typically...
. Its creators claim it is the "lightest structural material" known, with a density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
as low as 0.9 mg/cm3. It was developed by a team of scientists from HRL Laboratories, in collaboration with researchers at University of California, Irvine
University of California, Irvine
The University of California, Irvine , founded in 1965, is one of the ten campuses of the University of California, located in Irvine, California, USA...
and Caltech, and was first announced in November 2011. The prototype samples were made from a nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
-phosphorus
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
alloy.
Synthesis
To produce the metallic microlattice, a polymerPolymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
template was first prepared using a technique based on self-propagating waveguide
Waveguide
A waveguide is a structure which guides waves, such as electromagnetic waves or sound waves. There are different types of waveguides for each type of wave...
formation, though it was noted that other methods can be used to fabricate the template. The process passes UV light through a perforated mask into a reservoir of UV-curable resin
Resin
Resin in the most specific use of the term is a hydrocarbon secretion of many plants, particularly coniferous trees. Resins are valued for their chemical properties and associated uses, such as the production of varnishes, adhesives, and food glazing agents; as an important source of raw materials...
. Fiber-optic-like “self-trapping” of the light
Total internal reflection
Total internal reflection is an optical phenomenon that happens when a ray of light strikes a medium boundary at an angle larger than a particular critical angle with respect to the normal to the surface. If the refractive index is lower on the other side of the boundary and the incident angle is...
occurs as the resin cures under each hole in the mask, forming a polymer fiber along the path of the light. By using multiple light beams, multiple fibers can then interconnect to form a lattice. The process is similar to photolithography
Photolithography
Photolithography is a process used in microfabrication to selectively remove parts of a thin film or the bulk of a substrate. It uses light to transfer a geometric pattern from a photomask to a light-sensitive chemical "photoresist", or simply "resist," on the substrate...
in that it uses a 2D mask to define the starting template structure, but differs in the rate of formation: where stereolithography
Stereolithography
Stereolithography is an additive manufacturing technology for producing models, prototypes, patterns, and in some cases, production parts.-Technology description:...
might take hours to make a full structure, the self-forming waveguide process enables templates to be formed in 10–100 seconds. In this way, the process enables large free-standing 3D lattice materials to be formed quickly and scalably. The template is then coated with a thin layer of metal by electroless nickel plating
Electroless nickel plating
Electroless nickel plating is an auto-catalytic chemical technique used to deposit a layer of nickel-phosphorus or nickel-boron alloy on a solid workpiece, such as metal or plastic. The process relies on the presence of a reducing agent, for example hydrated sodium hypophosphite which reacts with...
, and the template is etched away, leaving a free-standing, periodic
Periodic function
In mathematics, a periodic function is a function that repeats its values in regular intervals or periods. The most important examples are the trigonometric functions, which repeat over intervals of length 2π radians. Periodic functions are used throughout science to describe oscillations,...
porous metallic structure. Nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
was used as the microlattice metal in the original report. Owing to the electrodeposition process, 7% of the material consisted of dissolved phosphorus atoms, and it contained no precipitates
Precipitation (chemistry)
Precipitation is the formation of a solid in a solution or inside anothersolid during a chemical reaction or by diffusion in a solid. When the reaction occurs in a liquid, the solid formed is called the precipitate, or when compacted by a centrifuge, a pellet. The liquid remaining above the solid...
.
Properties
Metallic microlattices are composed of a network of interconnecting hollow struts. In the least-dense microlattice sample reported, each strut is about 100 micrometreMicrometre
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...
s in diameter, with a wall 100 nanometre
Nanometre
A nanometre is a unit of length in the metric system, equal to one billionth of a metre. The name combines the SI prefix nano- with the parent unit name metre .The nanometre is often used to express dimensions on the atomic scale: the diameter...
s thick. The completed structure is about 99.99% air by volume, and by convention, the mass of air is excluded when the microlattice density is calculated. Allowing for the mass of the interstitial air, the true density of the structure is approximately 2.1 mg/cm3 (2.1 kg/m3), which is only about 1.76 times the density of air itself at 25 degrees Celsius
Celsius
Celsius is a scale and unit of measurement for temperature. It is named after the Swedish astronomer Anders Celsius , who developed a similar temperature scale two years before his death...
. The material is described as being 100 times lighter than Styrofoam
Styrofoam
Styrofoam is a trademark of The Dow Chemical Company for closed-cell currently made for thermal insulation and craft applications. In 1941, researchers in Dow's Chemical Physics Lab found a way to make foamed polystyrene...
.
Metallic microlattices are characterized by very low densities, with the current record of 0.9 mg/cm3 being the lowest value for any solid
Solid
Solid is one of the three classical states of matter . It is characterized by structural rigidity and resistance to changes of shape or volume. Unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire volume available to it like a...
yet discovered. The previous record of 1.0 mg/cm3 was held by silica aerogel
Aerogel
Aerogel is a synthetic porous material derived from a gel, in which the liquid component of the gel has been replaced with a gas. The result is a solid with extremely low density and thermal conductivity...
s. Mechanically, these microlattices are behaviorally similar to elastomer
Elastomer
An elastomer is a polymer with the property of viscoelasticity , generally having notably low Young's modulus and high yield strain compared with other materials. The term, which is derived from elastic polymer, is often used interchangeably with the term rubber, although the latter is preferred...
s and almost completely recover their shape after significant compression. This gives them a significant advantage over current aerogels, which are brittle, glass-like substances. This elastomeric property in metallic microlattices furthermore results in efficient shock absorption. Their Young's modulus
Young's modulus
Young's modulus is a measure of the stiffness of an elastic material and is a quantity used to characterize materials. It is defined as the ratio of the uniaxial stress over the uniaxial strain in the range of stress in which Hooke's Law holds. In solid mechanics, the slope of the stress-strain...
E exhibits different scaling, with the density ρ, E ~ ρ2, compared to E ~ ρ3 in aerogel
Aerogel
Aerogel is a synthetic porous material derived from a gel, in which the liquid component of the gel has been replaced with a gas. The result is a solid with extremely low density and thermal conductivity...
s and carbon nanotube
Carbon nanotube
Carbon nanotubes are allotropes of carbon with a cylindrical nanostructure. Nanotubes have been constructed with length-to-diameter ratio of up to 132,000,000:1, significantly larger than for any other material...
foams.