Nano-Powders

According to the forecasts of various analytical, scientific and technical centers, a significant development in the field of creating new materials, and developments based on high technologies, is associated, first of all, with the possibility of obtaining and use widely using highly dispersed powders of submicron sizes, including the so-called nanopowders. At present, the scientific and technical world is experiencing a real boom in nanopowders and nanotechnologies, with numerous scientists rightly believing that nanopowders are the fifth state of matter. In the microcosm, where the forces of Van der Waals are working with might and main, there is so much unknown, and so far inexplainable by science, that it is highly likely that more than one generation of researchers will have to devote their careers to this subject.

The properties of atoms in a crystal or liquid are very different from the properties of individual atoms in nanopowders - their behavior changes due to interaction with neighboring atoms.

If the number of neighbors of the same atom is different, then its properties are also different. Examples include atoms located inside a defined volume and atoms on the surface of a nanoparticle - their properties are completely different. In small bodies, not to mention large ones, the number of surface atoms is negligible compared to the number of atoms in the bulk of the substance. Surface atoms practically do not affect the visible properties of substances. In nanopowders with sizes on the order of tens or hundreds of angstroms, surface atoms make up about 30% of the total number of atoms (about 5,000) and they thus can significantly affect the properties of the nanomaterial.

The distinctive properties of ultra-small particles predetermine a wide range of their usages. Powders made from ultra-fine particles work much better as catalysts and sorbents than large powders from the same materials; the introduction of ultra-fine metal particles inside ceramic materials gives these materials unique mechanical properties, which makes them suitable for use in extreme space conditions. Mixtures of "ceramics + nanopowders" of a special compositions, are used to cover the fuselages of stealth aircrafts, making these aircraft invisible to radars: the radar beam, repeatedly reflected in a labyrinth of metal particles, is eventually absorbed by them.

The electronic properties of metal nanoparticles also depend on their size. The connection between them manifests itself through the dependence of the average kinetic energy of electrons (Fermi energy) on the particle size. In ensembles of small particles, their scatter in granulometric sizes is inevitable, and, consequently, in the values of the Fermi energy. If two nearby particles have different energy levels, then, in an effort to lower their energy, electrons from one particle will pass into another, just as the general level of a liquid is established in communicating vessels. The resulting electric fields will lower the energy of electrons in one particle and increase it in another. The migration of electrons will stop when the total energy of electrons equals the sum of the Fermi energy and the electrostatic energy of the nanoparticles. But in any case, both particles will be charged relative to each other. If the particles are charged relative to each other, then, according to Coulomb's law, electrostatic forces arise between them, which slowly decrease with increasing distance between them. Electrostatic forces resulting from mutual charge are especially noticeable when the particles are in a gas or liquid. Between neutral particles also act the forces of attraction of quantum nature - the van der Waals forces. These forces are very weak and decrease rapidly as the distance between the particles increases. However, they are very important in the processes of particle coagulation, determining its rate. If the particles are charged, then the Coulomb forces are added to the van der Waals forces, which accelerate the coagulation process.

Today in the world there is a situation when in high-tech countries the demand for ultrafine and nanopowders is much ahead of their supply.

Fields of application of nano metal powders and their oxides

Nano Metal Powders Application Examples

In the section below you can find examples with detailed explantion of the various metal powders being used to achieve a wide variety of industrial tasks ranging from water filtration to microwave absorption.

Microwave Absorption Properties of Nano-Iron Powder

Microwave absorption of nano iron oxide powder was characterized by heating curves Δφ(t) of samples of the same mass (1 g) placed in a quartz glass crucible (internal diameter 20 mm), which was placed in a special microwave oven of the AFKP type® MW 17.3 (AFK Deutschland GmbH, Hamburg) at 2.45 GHz, equipped with an Optris CT® IR sensor (Optris GmbH, Berlin). The measurements were repeated three times to determine temperature-induced changes (e.g. oxidation).

Iron Nano-Powder Use in Water Purification

Heavy metal ions can be extracted with an nano iron oxide coagulant. The wastewater treatment method is as follows; Dry powder of iron oxides is added to polluted wastewater (consumption - 6-7 grams of dry powder per gram of impurities) and mixed for 8-10 minutes. After completion of coagulation, the resulting suspension is passed through a self-cleaning filter (eg. AMIAD) with a mesh size of 10 μm. The quality of these waste waters meets the requirements of the Clean Water Act.

A production line with a capacity of 10 m3/hour can be located on an area of no more than 36 m2, and a production line of 50 m3/hour - no more than 80 m2. For optimal operation of the technology, the total concentration of heavy metal ions should not exceed 100 mg/l. If this limit is exceeded, cleaning also occurs, but the consumption of powders increases and the productivity of the equipment decreases.

Below are the results of the degree of extraction of heavy metal ions from galvanic drains by the iron oxide EED powder.

Aluminum Nano-Powder Iron Nano-Powder Use in Water Purification

Nano-sized aluminum powders can also be used for wastewater filtration. Below are the results of tests based on amorphous aluminum oxide in the treatment of wastewater from galvanic production at a pilot plant of the St. Petersburg Research Institute of Radio Engineering.

Lubricants alloyed with nanopowders

Lubricating media consisting of oils with additives of nanopowders of copper, brass and zinc provide anti-wear properties of the steel-steel friction pair under high load conditions better than commercial oils. The introduction of nanopowders into commercial oils makes it possible to somewhat improve the antifriction properties of the base oil. The reduction in wear and friction coefficient is determined by the type of base oil used, nanopowder and the hardness of the friction part.

Improvement in the anti-wear and anti-friction properties of a friction pair after introducing nanopowder additives into the base oil probably occurs due to the formation of nanopowder particles on the surface and introduction into the near-surface layers of the friction part.

A promising direction for improving the characteristics of commercial lubricating compositions is the use of nanopowders for alloying greases.

Application of nanopowders in high-energy materials and processes

For a long time, micron-sized aluminum powders have been widely used to improve the energy-mass and ballistic characteristics of high-energy condensed systems, including thermites, explosives, gunpowder, and rocket fuel. Due to their large specific surface area, nano-sized electro-explosive aluminum particles can provide a number of advantages over conventional aluminum powder, particularly in terms of burning rate.

Increasing the burning rate of standard rocket fuel increases thrust and the rate at which gases flow out of the rocket engine. Studies noted a doubling of the fuel burning rate when replacing micron-sized aluminum powder with nano-sized powder in conventional solid rocket propellants such as Al/AP/HTPB (aluminum/ammonium perchlorate/isobutylene-based binder). The increase in combustion rate occurs due to the smaller particle sizes of aluminum nanopowder. Models of combustion of aluminum particles in a rocket engine show that the lifetime of a combustion particle is proportional to the square of the particle diameter. From experimental data it follows that an aluminum particle with a size of 5 microns burns out in an engine in approximately 4 milliseconds. Extrapolation from these models shows that with a particle diameter of 100 nm, it will burn in approximately 0.6 microseconds, which is four orders of magnitude less than for a micron-sized particle. High-speed photography of the surface of a burning fuel confirms that the nano-sized aluminum particle burns completely on the surface of the burning fuel granule, and is not ejected into the exhaust stream, as is the case with micron-sized aluminum, i.e., the combustion of the particle is completed inside the engine, and not in the exhaust gases rockets.

The use of nano-sized aluminum powders can improve the performance of hybrid rocket engines. A typical hybrid engine uses liquid oxygen and rubber-based granules (such as HTPB), which either contain no oxidizer or contain enough to react with the granules. Pyrolysis of rubber produces organic molecules with low molecular weight, which enter the engine and react with liquid oxygen. If aluminum is introduced into a solid fuel such as HTPB, an increase in engine momentum is theoretically possible, but micron-sized aluminum does not burn efficiently in such an engine.

The addition of aluminum to kerosene increases the specific energy per unit volume of liquid rocket fuel. However, micron-sized aluminum does not burn completely in kerosene. The addition of nanopowder leads to complete combustion of the metal. Accordingly, high temperatures arising during the combustion of aluminum lead to an increase in the intensity of kerosene combustion.

One of the problems preventing the use of nano-sized aluminum powders in high-energy applications is their high reactivity. The metal can enter into chemical interaction with other components of pyrotechnic compositions. In order to prevent the chemical reaction, a procedure was developed to microencapsulate aluminum powder. During microencapsulation, layers of palmitic acid are applied to the surface of the particles, which protects the powder particles from contact with oxidizing environments

Synthesis of alloys and refractory chemical compounds

A promising application of electroexplosive nanopowders is the synthesis of intermetallic and high-temperature compounds. Research shows that particles of electroexplosive nanopowders have increased defectiveness. Probably, as a result of this, when heating some nanopowders, energy (Ee) is released that is not associated with chemical processes; a similar effect was observed when heating electroexplosive nanopowders of metals, in particular silver.

The high activity and peculiarities of the energy state of nanopowders make it possible to obtain compounds of metals with significantly different melting points, for example, iron-aluminum

Due to their high chemical activity, electroexplosive powders can also act as raw materials for the synthesis of refractory chemical compounds, for example, tungsten carbide.

Modification of epoxy adhesives

Nanofibers and a hardener were added to the epoxy resins, then the system was mixed for 15 minutes at a temperature of 52 ºC. The tests were carried out by gluing two aluminum plates (aluminum grade 2024-T3, made in the USA), previously degreased and primed with primer for aluminum BR 6747-1. The glue was placed between the plates, the plates were clamped at a pressure of 310 kPa. The glue hardened at a temperature of 177 ºС for 2 hours in an oven. The heating and cooling rate was 5 ºC/min. Five samples were used for each experiment. Shear strength testing was carried out on a Sintech rig in accordance with Boeing 7202 standard, applying a load at a constant lateral velocity of 0.254 mm/min. Peel resistance tests were also carried out on a Sintech test bench in accordance with standard 7206 type I.

The peeling force of both samples modified with nanofibers was higher than that of the control sample by approximately 30%, and was practically independent of the number of nanofibers.

Production of hydrogen usign aluminium nanopowder

When one kilogram of electroexplosive aluminum nanopowder interacts with water, 1244.5 liters of hydrogen are released, which, when burned, produces 13.43 MJ of heat. The efficiency of this process for producing hydrogen is higher than in the case of electrolysis - the oxidation of aluminum nanopowder proceeds 100%, i.e. the material used is completely used. The peculiarities of the thermal regime of the interaction of aluminum nanopowders with water lead to the emergence of new effects that were not known for the reaction involving large aluminum powders. First of all, this is the effect of self-heating of nanoparticles to temperatures exceeding the temperature of the surrounding water by hundreds of degrees.

When using industrial micron-sized aluminum powder, the hydrogen evolution rate is only 0.138 ml per second per 1 g of powder. At the same time, only 20...30% of the original powder is converted into the final product - a mixture of aluminum oxides and hydroxides. Research has shown that aluminum nanopowder is superior in its reactivity to conventional micron-sized industrial powders. At the same time, the rate of hydrogen evolution during the interaction of aluminum nanopowder with distilled water at 60 °C is 3 ml per second per 1 g of powder, at 80 °C - 9.5 ml per second per 1 g of powder, which exceeds the rate of hydrogen evolution during hydrothermal synthesis approximately 70 times. Another advantage of using nanopowder in this reaction is that the degree of aluminum conversion is 98...100% (depending on temperature). Moreover, the introduction of even small amounts of alkali into distilled water leads to a significant increase in the reaction rate: when the pH of the solution is increased to 12, the rate of hydrogen evolution increases to 18 ml per second per 1 g of powder at 25 °C. The rate of hydrogen evolution when micron-sized aluminum is dissolved in a solution containing 8 g/l NaOH at the same temperature is only 1 ml per second per 1 g of powder. The data presented show that electroexplosive aluminum nanopowders, in contrast to compact aluminum and large industrial powders, interact with water at high speed and a degree of conversion of ~100%, and it is their use that will make it possible to produce hydrogen at a sufficient speed under normal conditions.