Nanotechnology is a far-reaching high-tech in this century. Its emergence has spawned a large number of new disciplines. Such as nanophysics, nanochemistry, nanobiology, nanomaterials, etc. The research object of nanotechnology is the substance or structure of 1 ~ 100nm scale, including the manufacturing and processing technology, characteristics and application technology, characterization and measurement technology of nanomaterials. The so-called "nano" is actually a unit of measurement of length, but this unit is very small. 1nm = 10-9m, which is one billionth of a meter. Studies have found that substances at the nanoscale, due to their small size effect, surface and interface effects, and quantum effects, they (nanoparticles, nanowires, nanofilms) are in sound, light, electricity, magnetism and mechanics. Each aspect shows a series of distinctive specificities. For example, ordinary metals are mostly solid at room temperature, just like gold, silver, copper, and iron, all of which have high melting points. This is not the case with metals in the nanometer state. If gold nanoparticles are placed 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 components are pigment (dye), polymer binder, solvent and a small amount of additives. During ink formation, people put the above components into a special equipment at a certain ratio, and fully disperse to produce a uniform printing ink with a certain viscosity and thixotropy. The composition and manufacturing method of nano ink is no different from ordinary ink. If there is a difference, it is only the "pigment" particles used in these two inks, which are very different in particle size. The pigment particle size of ordinary ink is in the micrometer (Î¼m) level, while the "pigment" particle size of the nano ink is in the nanometer level. The difference between the two is about 1000 times. It is also understandable that the introduction of nano-pigments with specific energy will bring some changes in the ink manufacturing process to some extent. Although both nano inks and common inks are used for product printing, the former mainly focuses on the application of special functions, while the latter is often used for the printing of monochrome or color prints.
Recently, although the nano ink has just emerged, it has initially shown excellent performance in the fields of processing and installation of electronic components, decoration and decoration of high-end products, sterilization and testing in medicine, and anti-counterfeiting printing of special products. And great attraction. This article focuses on metal nano-ink and its application technology in the field of electronics. In addition, related theoretical issues are briefly discussed.
As mentioned above, nanoparticles are the core component of nanoink. From the perspective of their properties, the nanoparticles used in inks can be either organic or inorganic; they can be metallic or non-metallic; or their oxides. According to the different application fields of ink, people can choose freely. The particle size of the nanoparticles used in nano-ink is preferably several nanometers. It should be noted that the particle size of nanoparticles often refers to the average value. Even for products of the same production batch, the particle size of each particle is difficult to be completely consistent, but only an aggregate with different size distributions. . It should be emphasized here that not all nanoparticles can be used as "pigments" of nanoinks. The reason is very simple: ordinary nanoparticles have large surface activity and high energy, and particles are very prone to "agglomeration" in the population. Once the particles are agglomerated, it is difficult to disperse them by ordinary methods. Nanoparticles for nano-inks have special requirements, that is, each particle should have monodispersity, which is the key to the success of nano-ink manufacturing technology.
1. Monodisperse nanoparticles
The typical characteristic of nanoparticles is that the particle size is extremely small, and the surface area is very large. With the sudden increase of the surface energy of the particles, the melting point is greatly reduced. For example, under normal conditions, the melting point of gold is 1063 Â° C. However, when the solid gold becomes gold nanoparticles with a diameter of 2 nm, the melting point has changed significantly. It has dropped from 1063 Â° C to near room temperature. People use this feature to easily sinter it into metallic conductors even at very low temperatures.
The fundamental difference between monodisperse nanoparticles and ordinary nanoparticles is that the surface of the former has been covered with a thin layer of special coating agent. In this way, the surface activity of the particles and the melting point of the particles are temporarily suppressed to a moderate range Inside. Even in a solvent or resin solution, it can always maintain the characteristics of uniform dispersion. In other words, the monodispersity stability of the particles is excellent. This is exactly what is eagerly awaited in the manufacture or storage of nano inks. However, ordinary nanoparticles are completely different. There is no protective film on the surface, but they are directly exposed to the outside world. The original excessive surface activity and excessive surface energy keep the particles in an unstable state. Particles can easily reunite with each other.
The manufacturing methods of metal nanoparticles are roughly divided into two categories: physical method and chemical method. As one of the physical methods, the evaporation method is to add the metal to a molten state in a container filled with an inert gas, so that the vaporized metal quickly solidifies into metal nanoparticles. The advantage of this method is the high purity of the product, the disadvantage is that the continuous production of nanoparticles is poor. The production of metal nanoparticles by chemical methods can be divided into dry method and wet method. Regardless of the former or the latter, their respective product purity is not very high, often with a small amount of impurities such as alkali or sulfide. In this regard, complex purification processes have to be added in order to purify the newly produced metal nanoparticles. 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 compare and carefully select the most reasonable process method.
2. Composition and characteristics of metal nano-ink
As mentioned above, the metal nano-ink is composed of metal nano-particles (monodisperse particles), resins, solvents, additives, etc., uniformly dispersed in a certain ratio.
Metal nanoparticles are one of the most important components in inks, and they are the only source of conductivity for printed graphics. The quality of its electrical conductivity mainly depends on the metal properties of the nanoparticles, that is, the lower the inherent resistance of the metal, the better the electrical conductivity, and vice versa. Such as silver, copper, gold, etc. have good conductivity. In theory, their nanoparticles can be used as nano ink raw materials. However, it is not the case in actual ink application, because we still have to consider the chemical activity of the metal used. Copper and other metals are relatively chemically active, and their particles, especially nanoparticles, are easily oxidized in the air and lose the characteristics of metals. At present, it is still difficult to solve this problem from a technical point of view. Therefore, practical examples of copper nanoparticles used in nano inks have not been reported so far. In contrast, gold and silver have stable chemical properties and good electrical conductivity, so their nanoparticles have become the main raw materials of choice for metal nano-inks today.
As another important component of metal nano-ink, thermosetting resin has two functions: one is to incorporate monodisperse metal nano particles into the resin system to become a uniform resin composition with certain viscosity and printing suitability; second Give the ink a certain adhesion function, so that the ink can be firmly attached to the substrate to be printed. Therefore, the chemical structure and molecular weight of the resin are critical and cannot be ignored. Commonly used resins are acrylate resin, phenol resin and epoxy resin. It is necessary to choose polymer resins with corresponding chemical structures according to different printing substrates. The main role of the solvent in the ink: First, it is used to dissolve the solid resin and make it into a resin solution, so as to match the ink; Second, it is used as a thinner to adjust the viscosity of the ink to meet the printing requirements. When the resin is selected, what solvent should be used? It depends on the solubility parameters of the resin and the solvent. If the two parameters are relatively close, it means that the solvent is a good solvent for this resin, that is, the solubility is good. On the contrary, it cannot be used as the solvent of the resin. The more commonly used organic solvents are toluene, xylene, cyclohexanone, methyl ethyl ketone, diacetone alcohol, glycol ether, and some higher alkanes. Today, all countries attach great importance to the relationship between the environment and people, environment and society, the toxicity of solvents and their impact on the surrounding environment are important issues that must be considered.
In order to improve some properties of the ink and improve the quality of the ink, a small amount of additives are often selected. Such as surfactants, leveling agents, plasticizers and defoamers. Although silver nano inks and ordinary conductive silver inks are both used for conductive printing, due to the different conductive particles used, the performance and application of these two inks are also significantly different.
3. Mixed silver ink (silver powder + silver nanoparticles)
Common conductive silver ink is often used for conductive printing of membrane switches and membrane circuits. However, in the face of printing electronic products with faster transmission speed and higher performance requirements, ordinary silver ink has major defects in both conductivity and reliability, which cannot meet the higher requirements of such electronic products for ink Of course, we can choose silver nano-ink to complete this task. However, if the product does not require ultra-fine printing, we rush to choose silver nano-ink to print, which is inappropriate from the perspective of business management. The reason is that this means that the selected raw materials have excessive performance, that is, people often say "large horse cart", which is an unnecessary waste. In response, people have developed a new product called a mixed silver ink with a small amount of silver nanoparticles modified metal silver powder (flaky or spherical). This product is very suitable for the printing and processing of electronic products with high conductivity, high reliability and no special fine requirements. Table 3 lists the composition and characteristics of the ink.
In the mixed silver ink, the total content of silver has a great influence on the conductivity of the ink. Experiments have shown that the silver content accounts for 80% to 95% of the solid components of the ink. If the silver content is too small, the conductivity of the ink is insufficient; otherwise, if the silver content is too large, the conductivity of the ink will be damaged due to other reasons. When the silver content is 85%, the volume resistance of the ink is within 10 Ã— 10-6 Î©cm. When the silver content rises to 90%, the volume resistance drops to the minimum value, which is 6 Ã— 10-6 Î©cm. After this, as the amount of silver in the ink increases, its volume resistance not only does not decrease, but also gradually increases. The reason here can be explained as follows: when the amount of silver in the ink increases to the maximum, in other words, the percentage of resin binder (linker) has reached the minimum. At this time, the resin content is further reduced (the silver content continues to increase), and the homogeneity of the ink film will be problematic during the sintering after drying, that is, some small cracks gradually occur inside the ink film, which hinders the electrons inside the ink film successfully passed. In the mixed silver ink, the proportion of silver nanoparticles in the total silver content is good. Practice has shown that it is appropriate to account for 10% to 20%. Too little modification effect is not obvious, too much will cause unnecessary waste. It should be noted that when we prepare the mixed silver ink, in addition to considering the percentage of silver, what type of resin structure is used, what is its molecular weight, and which additives and mixed solvents are cited will affect the ink. Printability (viscosity, thixotropy, adhesion, drying and curing characteristics, etc.).
From the comparison of data, the reliability of mixed silver ink is significantly higher than that of ordinary silver ink. If there is no special fineness requirement for printed graphics, it is undoubtedly the most suitable choice for the printing of high conductivity and high reliability electronic components. Another point to note is that the mixed silver ink has a relatively low cost, and the conductivity and reliability have been greatly improved (compared to ordinary silver ink). The ink can also be applied in many fields through the screen printing method. :
â‘´ Print electronic circuit on heat-resistant organic substrate;
â‘µForm BVU on the composite substrate;
â‘¶Printing and bonding of small electrode parts; â‘·In addition to the above applications, all kinds of uses that can be sintered at low temperature (200 ~ 250 â„ƒ) to obtain high conductivity and high reliability electronic components can be mixed silver ink through screen printing achieve.
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