Nano Ink and Its Application Technology (I)

Foreword

Nanotechnology is a far-reaching high technology in this century. Its appearance has spawned a large number of new disciplines. Such as nano-physics, nanochemistry, nanobiology, nanomaterials, and so on. The research object of nanotechnology is a 1-100nm-scale material or structure, including the manufacturing and processing technology, characteristics and application technology, characterization and measurement technology of nanomaterials. The so-called "nano" is actually a measure of length, but this unit is very small. 1nm = 10-9m, which is one billionth of a meter.

People have discovered that at the nanometer scale, due to its small size effect, surface and interface effects, and quantum effects, they (nanoparticles, nanowires, and nanofilms) have a lot of acoustic, optical, electrical, magnetic and mechanical properties. Each aspect shows a series of distinctive specific energy. For example, ordinary metals are mostly solid at room temperature. Like gold, silver, copper, and iron, they all have high melting points. The situation in the nano-metal is not the case. If we put gold nanoparticles in the palm of our hand, we can find that the particles will melt like ice and become a paste. Furthermore, some metal nanoparticles spontaneously ignite due to strong oxidation even in normal air.

As we all know, ordinary ink is a complex polymer composition, it has a specific viscosity and excellent printability. The main ingredients are pigments (dye), polymer binders, solvents, and small amounts of additives. When ink is formed, people put the above-mentioned components into a special apparatus according to a certain proportion, and are sufficiently dispersed to obtain a uniform printing ink having a certain viscosity and thixotropy.

Nano ink composition and manufacturing methods are no different from ordinary ink. If there is a difference, it is only the "pigment" particles used in the two inks, which vary greatly in particle size. The pigment size of conventional inks is in the micron (μm) grade, while the "pigment" size of the nanoinks is nanoscale. The difference between the two sizes is about 1,000 times. The introduction of nano-specific “pigments” with specific energy will give moderate changes to the ink manufacturing process, which is understandable.
Although nano inks and common inks are used for printing products, the former mainly focuses on the application of special functions, while the latter is often used for printing monochrome or color prints.

Recently, although nano inks have just emerged, they have shown excellent performance in the fields of processing and installation of electronic components, decoration and decoration of high-end products, sterilization and detection of pharmaceuticals, and anti-counterfeit printing of special products. And great attraction.

This article focuses on metal nano inks and their application technology in the field of electronics. In addition, it also makes a brief discussion on related theoretical issues.

First, metal nano ink

As mentioned above, nanoparticles are the core component of nanoinks. Nanoparticles used in inks can be either organic or inorganic in view of their properties; they can be metallic or non-metallic; or they can be oxides. According to the different applications of ink, people can choose freely. Nanoparticles for nanoinks have a particle size of several nanometers. It should be noted that the particle size of nanoparticles often refers to the average value, even for the same production batch product, the particle size of each particle is difficult to be exactly the same, but only the size distribution of different aggregates (Figure 1 shows the particle size distribution of gold nanoparticles produced by a company).

What needs to be emphasized here is that not all nanoparticles can be used as "pigments" for nanoinks. The reason is simple: ordinary nano-particles have large surface activity and high energy, and particles are prone to "agglomeration" in the population. Once agglomeration of particles occurs, it is difficult to disperse them by ordinary methods. Nanoparticles for nanoinks have special requirements that each particle should have monodispersity, which is the key to the success of nanoinks.

1. Monodisperse nanoparticles

Nanoparticles have the typical characteristic that the particle size is extremely small, and the surface area is very large. With a sharp increase in the surface energy of the particles, the melting point is greatly reduced. As in the normal state, the melting point of gold is 1063°C. However, when the solid gold becomes a gold nanoparticle with a diameter of 2 nm, the melting point has undergone a significant change, and it has dropped from 1063°C to near room temperature. People use this feature to easily sinter metal conductors even at very low temperatures.
The fundamental difference between monodisperse nanoparticles and normal nanoparticles is that the former surface has been coated with a thin layer of a special coating agent, so that the surface activity of the particles and the decrease in the melting point of the particles are temporarily suppressed in a moderate range. Inside. The coated nanoparticles then have a monodisperse function (see Figure 2a). Even in solvent or resin solution, it can always maintain the characteristics of uniform dispersion. In other words, the monodisperse stability of the particle is very good. This is exactly what nano-inks are eagerly looking for in their manufacture or storage. However, ordinary nanoparticles are completely different, and their surface is not protected by any protective film, but is directly exposed to the outside world. The original large surface activity and excessive surface energy make the particles always in an unstable state. Particles and particles can easily agglomerate with each other (see Figure 2b).

There are generally two types of methods for producing metal nanoparticles: physical methods and chemical methods. One of the physical methods of evaporation is to add metal to the molten state in a vessel filled with an inert gas, so that the vaporized metal rapidly solidifies into metal nanoparticles. The advantage of this method is that the product is of high purity, with the disadvantage that the continuous production of nanoparticles is poor. Chemically produced metal nanoparticles can be divided into dry and wet methods. Regardless of the former or the latter, the purity of their respective products is not very high, and they often contain small amounts of impurities such as alkalis or sulfides. In this regard, complex purification processes have to be added in order to purify the metal nanoparticles that have just been produced. Although the chemical method can be continuously produced in batches, the total cost of the product after purification is too high. In short, in order to obtain high-purity metal nanoparticles, it is necessary to carefully and carefully select the most reasonable process method.