What are the Types and Dielectric of Ceramic Capacitors?

Ⅰ Introduction

A ceramic capacitor is a general term for capacitors with ceramic material as the dielectric. There are many varieties, and the dimensions vary greatly. According to the voltage, it can be divided into high voltage, medium voltage, and low voltage ceramic capacitors. According to the temperature coefficient, the dielectric constant can be divided into a negative temperature coefficient, positive temperature coefficient, zero temperature coefficient, high dielectric constant, and low dielectric constant. Also, there are classification methods for class 1, class 2, and class 3. Compared with other capacitors, general ceramic capacitors have the advantages of higher use temperature, large specific capacity, good humidity resistance, and small dielectric loss. The temperature coefficient of capacitance can also be selected in a wide range.  

ceramic capacitor

Figure1. ceramic capacitor

Ⅱ Ceramic capacitor types

1. Semiconductor ceramic capacitors

(1) Surface-layer ceramic capacitors. Micro-miniaturized capacitor, that is, the capacitor gets the largest possible capacity in the smallest possible volume, which is one of the trends in capacitor development.  

For separation capacitor components, there are two basic approaches to miniaturization:

① make the dielectric constant of the dielectric material as high as possible;

② make the thickness of the dielectric layer as thin as possible.

In ceramic materials, the dielectric constant of ferroelectric ceramics is very high, but when ferroelectric ceramics are used to make common ferroelectric ceramic capacitors, it is difficult to make the ceramic dielectric thin. Firstly, ferroelectric ceramics have low strength and are easy to crack when it is thinner, which makes it difficult to carry out actual production operations. Secondly, when the ceramic medium is thin, it is easy to cause various structural defects, and the production process is very difficult.

Surface layer ceramic capacitors use a thin insulating layer formed on the surface of a semiconductor ceramic such as BaTiO3 as a dielectric layer, and the semiconductor ceramic itself can be regarded as a series circuit of a dielectric. The thickness of the insulating surface layer of the surface layer ceramic capacitor varies depending on the formation method and conditions and ranges from 0.01 to 100 μm. In this way, not only the high dielectric constant of the ferroelectric ceramic is used, but also the thickness of the dielectric layer is effectively reduced, which is an effective solution for preparing micro-small ceramic capacitors.

The following figure shows the general structure of a surface layer ceramic capacitor, and (b) its equivalent circuit.

Structure of surface layer ceramic capacitor and its equivalent circuit 

Figure2. Structure of surface layer ceramic capacitor and its equivalent circuit

(2) Grain boundary layer ceramic capacitors. The surface of BaTiO3 semiconductor ceramics with relatively well-developed grains is coated with an appropriate metal oxide (such as CuO or Cu2O, MnO2, Bi2O3, Tl2O3, etc.). Heat treatment under appropriate temperature and oxidation conditions, the coated oxide will form a eutectic phase with BaTiO3, and a thin solid solution insulation layer will be formed on the grain boundaries. The resistivity of this thin solid solution insulation layer is very high (up to 1012 ~ 1013Ω·cm). Although the crystal grains of the ceramic are still semiconductors, the entire ceramic body shows a significant dielectric constant as high as 2 × 104 to 8 × 104 insulator dielectric. Capacitors made with this porcelain are called boundary layer ceramic capacitors, or BL capacitors for short.

2. High-voltage ceramic capacitors 

With the rapid development of the electronics industry, it is urgent to develop high-voltage ceramic capacitors with high breakdown voltage, small loss, small size, and high reliability. In the past 20 years, high-voltage ceramic capacitors successfully developed and have been widely used in power systems, laser power supplies, videotape recorders, color TVs, electronic microscopes, photocopiers, office automation equipment, aerospace, missiles, and navigation.

The ceramic materials of high-voltage ceramic capacitors are mainly two types: barium titanate-based and strontium titanate-based. 

Barium titanate-based ceramic materials have the advantages of high dielectric constant and good AC withstand voltage characteristics, but also have disadvantages such as the capacitance change rate increases with the medium temperature and the insulation resistance decreases.

The curie temperature of strontium titanate crystals is -250 °C, and it has a cubic perovskite structure at a normal temperature. At high voltages, the strontium titanate-based ceramic material has a small change in dielectric coefficient, a small tgδ, and a small rate of change in capacitance. These advantages make it very advantageous as a high voltage capacitor dielectric.

The main points of the manufacturing process

(1) Raw materials must be selected

Factors affecting the quality of high-voltage ceramic capacitors, in addition to the composition of the ceramic material, are also optimized process manufacturing and strict process conditions. Therefore, it is necessary to consider both the cost and the purity of the raw materials. When selecting industrial pure raw materials, we must pay attention to the applicability of the raw materials. 

(2) Preparation of frit

The quality of the frit preparation has a great impact on the ball grinding fineness and firing of the porcelain. If the frit synthesis temperature is low, the synthesis is insufficient. Detrimental to subsequent processes. If Ca2 + remains in the composite, it will hinder the rolling process. If the synthesis temperature is too high, the frit will be too hard, which will affect the ball milling efficiency. The introduction of impurities in the grinding medium will reduce the activity of the powder and cause the firing temperature of the porcelain to increase.

(3) Molding process

When molding, it is necessary to prevent uneven pressure in the thickness direction, and there are too many pores in the closed body. If there are large pores or layer cracks, it will affect the electrical strength of the porcelain.

(4) Firing process

The firing system should be strictly controlled, and temperature control equipment with good performance and kiln furniture with good thermal conductivity should be adopted.

(5) Encapsulation

The selection of the encapsulant, the control of the encapsulation process, and the cleaning of the surface of the porcelain have a great impact on the characteristics of the capacitor. Therefore, it is necessary to choose an encapsulation material with good moisture resistance, which is closely combined with the surface of the porcelain body and has high electrical strength.

To improve the breakdown voltage of ceramic capacitors, coating a layer of glass glaze around the edges of the interface between the electrode and the dielectric surface can effectively improve the withstand voltage and high-temperature load performance of ceramic capacitors used in high-voltage circuits such as televisions.

3. Multilayer ceramic capacitors

Multilayer ceramic capacitor (MLCC) is the most widely used type of chip component. It is an internal electrode material and a ceramic body stacked alternately in parallel in multiple layers and co-fired into a whole, also known as a chip monolithic capacitor. It has the characteristics of small size, high specific volume, and high precision. It can be mounted on printed circuit boards (PCB) and hybrid integrated circuit (HIC) substrates, effectively reducing the size of electronic information terminal products (especially portable products). And weight to improve product reliability. It complies with the development direction of miniaturization, lightweight, high performance, and multi-function of the IT industry. It not only has simple packaging and good sealing performance but also can effectively isolate the opposite electrode. MLCC can play the role of storing electric charge, blocking DC, filtering, combining, distinguishing different frequencies, and tuning circuits in electronic circuits. It can partially replace organic film capacitors and electrolytic capacitors in high-frequency switching power supplies, computer network power supplies, and mobile communication equipment. It can greatly improve the filtering performance and anti-interference performance of high-frequency switching power supplies.

1. Miniaturization

For compact electronic products such as camcorders and mobile phones, more compact MLCC products are needed. On the other hand, due to advances in precision printed electrodes and lamination processes, ultra-small MLCC products have also gradually appeared and obtained applications. Taking the development of the Japanese rectangular MLCC as an example, the external dimensions have been reduced from 3216 in the early 1980s to 0603 today.

2. Cost reduction-base metal inner electrode MLCC

Because the traditional MLCC uses an expensive palladium electrode or palladium-silver alloy electrode, 70% of its manufacturing cost is occupied by the electrode material. New-generation MLCCs, including high-voltage MLCCs, use cheap base metal materials nickel and copper as electrodes, which greatly reduces the cost of MLCCs. However, the base metal internal electrode MLCC needs to be sintered at a lower oxygen partial pressure to ensure the conductivity of the electrode material, and a lower oxygen partial pressure will bring the semiconducting tendency of the dielectric ceramic, which is not conducive to the insulation and reliability. Murata has developed several anti-reduction ceramics, which are sintered in a reducing atmosphere. The reliability of the capacitors is comparable to that of capacitors using noble metal electrodes. At present, the sales of base metalized Y5V capacitors have accounted for about half of the MLCCs in this group.

3. Large capacity and high frequency

On the one hand, with the low-voltage driving and low power consumption of semiconductor devices, the operating voltage of integrated circuits has been reduced from 5 V to 3 V and 1.5 V; On the other hand, miniaturization of power supplies requires small, large-capacity products to replace bulky aluminum electrolytic capacitors. To meet the development and application of such low-voltage and large-capacity MLCCs, in terms of materials, relaxation-type high-dielectric materials with a relative dielectric constant that is 1 to 2 times higher than BaTiO3 have been developed. In the process of developing new products, three key technologies have been developed at the same time, namely ultra-thin green sheet powder dispersion technology, improved green film formation technology, and internal electrode and ceramic green sheet shrinkage matching technology. Recently, Japan's Matsushita Electronic Components Co., Ltd. successfully developed a large-capacity MLCC with a maximum capacitance of 100 μF and a maximum withstand voltage of 25 V. This product can be used for liquid crystal display (LCD) power lines.

Ⅲ Ceramic capacitor dielectric

Ceramic materials have superior electrical, mechanical, and thermal properties, and can be used as capacitor dielectrics, circuit substrates, and packaging materials. 

1. The Microstructure of ceramic materials

Ceramic materials are materials that are made from oxides or other compounds and then fired at high temperatures which are close to melting temperature. Ceramic is a complex polycrystalline and multiphase system, which is generally composed of crystalline phase, glass phase, gas phase, and phase boundary. The characteristics, composition, relative content, and distribution of these phases determine the basic properties of the ceramic.

The crystal phase in ceramics usually refers to those crystal grains with different sizes, shapes, and random orientations. The diameter of the crystal grains is usually several micrometers to several tens of micrometers. The crystal phases may belong to the same compound or crystal system, or they may be different compounds or different crystal systems. If there are two or more grains with different compositions and structures in ceramics, they are called polycrystalline phase ceramics. The product phase with the most relative content is called the main crystal phase, and the other is called the by-product phase. Among them, the properties of the main crystalline phase determine the properties of the material, such as the relative f-constant, electrical conductivity, loss, and thermal expansion coefficient.

The gas phase is generally distributed in the grain boundaries, the recrystallized crystals, and the glass phase, and it is an inevitable part of the ceramic structure. It originates from the fact that it is impossible to achieve a complete close setting between the individual crystal grains during the firing process, and the glass phase cannot fill the voids of the individual crystal grains; it may also be a pore formed due to the release of gas during the sintering of the billet. The gas phase can seriously affect the electrical, mechanical, and thermal properties of ceramic materials. It is generally desired that the less the gas-phase content in the ceramic, the better.

2. Characteristics and classification of capacitor porcelain

A Ceramic capacitor is made by soldering leads after forming metal layers on both sides of the ceramic substrate. These ceramic materials used as capacitors are called porcelain.

ceramic capacitor Figure3. ceramic capacitor

(1) Compared with other capacitor dielectric materials, dielectric ceramics have the following characteristics:

① The dielectric constant and the temperature coefficient of the dielectric constant, as well as the mechanical and thermophysical properties, can be adjusted, and the dielectric constant is also large.

②The dielectric constant of some dielectric ceramics (strong dielectric ceramics, mainly ferroelectric ceramics) can change with the strength of the electric field. It can be used to make non-linear capacitors, sometimes called varistor capacitors.

③ Abundant raw materials, low cost, and easy mass production.

(2) There are several classification methods for capacitor porcelain.

According to the application, it can be divided into Class 1 porcelain, used for manufacturing Class 1 (high frequency) ceramic dielectric capacitors; Class 2 porcelain, used for manufacturing Class 2 (ferroelectric) ceramic dielectric capacitors; Class 3 porcelain, used for manufacturing Class 3 ( Semiconductor) ceramic dielectric capacitors.

Among them, class 1 porcelain with a large relative dielectric constant (ε = 12 to 600) is called high dielectric porcelain; and class 2 porcelain with a higher relative dielectric constant (ε = 103 to 104) is called strong dielectric porcelain; and the class 3 porcelain with low relative dielectric constant (ε <10.5) are called low dielectric porcelain. The tanδ of high dielectric ceramic and low dielectric ceramic is very small, which is suitable for manufacturing capacitors in high-frequency circuits, so it is called high-frequency ceramic. Because the tanδ of the strong dielectric porcelain is large, it is only suitable for manufacturing capacitors used in low-frequency circuits, and it is also called low-frequency porcelain. Generally, a mixed classification method is adopted in engineering to divide capacitor porcelain into high-media porcelain, strong-media porcelain, monolithic porcelain, and semiconductor grain boundary porcelain.

With the development of hybrid ICs, computers, and portable electronic devices, ceramic capacitors have become an indispensable component in electronic devices. The total number of ceramic dielectric capacitors now accounts for about 70% of the capacitor market.

 

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