Yang Huaming Hu Yuehua Zhang Huihui
(School of Resource Biology, Central South University, Changsha 410083, Hunan)
Abstract The composite conductive powder was prepared by surface coating doped tin oxides with barite powder as the matrix using the chemical **** precipitation technique. Orthogonal experimental design method was used to determine the optimized conditions for the preparation of composite conductive powder, and the composite conductive powder with an average particle size of 4.3 μm and resistivity of 8.1 Ω-cm was produced. The effect of the amount of conductive powder on the resistivity of the coating was investigated, and the resistivity of the prepared conductive coating was only 10 Ω-cm when the amount of powder added was from 20% to 45%.The conductive coating can achieve a moderate shielding value (40 dB) for electromagnetic waves with a frequency less than 100 MHz, and can be applied to the anti-electromagnetic interference in the fields of cellular phones, laptop computers, electronic medical equipment, and military facilities. The conductive network and endowment state of conductive powder in conductive coatings are discussed, and it is concluded that good dispersion of conductive powder in coatings and the formation of network structure are the keys to ensure the conductivity of coatings.
Keywords Ultrafine barite powder; composite conductive powder; doping; conductive coatings; shielding properties
First author's profile: Huaming Yang, male, 38 years old, Shaoxing City, Zhejiang Province, Ph.D., professor, doctoral supervisor, the main research areas are mineral resources refining, functional mineral materials and inorganic non-metallic materials. Tel: 0731-8830549; E-mail: hmyang@mail.csu.edu.cn.
I. Preface
Conductive powders have been widely used in the fields of electronic and electrical appliances, aerospace, military, electromagnetic shielding and antistatic, etc. The traditional conductive powders are metal powders, carbon black, graphite, carbon fiber, metal fiber and metal oxides and so on. Among them, the metal powder is more expensive, easy to oxidize and reduce the conductive properties; metal fiber mixed dispersion is not uniform, easy to break during processing and orientation, only for conductive properties require particularly good electromagnetic wave shielding and other occasions; graphite requires a larger amount of additive (wB = 30%), so that the product's performance becomes brittle; carbon fiber is expensive; the conductivity of the metal oxide powder is poor; the actual application of the more is the Carbon black, although carbon black can give the material excellent conductive properties, but its single color, poor coloring shortcomings are difficult to meet the requirements of different fields and users. Generally used for conductive materials conductive powder, the requirements of its electrical resistivity is less than 10Ω-cm. through certain physicochemical treatment, the inexpensive natural minerals processed into a composite conductive powder, will promote the development of conductive composites, but also for the development of high value-added minerals to provide a new way. In this paper, barite powder as a matrix, chemical **** precipitation technology to prepare composite conductive powder, and apply orthogonal experimental methods to optimize the preparation process.
Second, the experimental method
The barite mineral raw materials for purification, classification, crushing, the use of stirred ball mill, φ30mm ZrO2 ball as a medium, fine grinding to get different particle size of ultrafine barite powder. The reagents used were tin tetrachloride (SnCl4-5H2O), antimony trichloride (SbCl3), sodium hydroxide and hydrochloric acid, all of which were analytically pure. The particle size of the powder was measured by laser method and the whiteness of the powder was measured by whiteness meter.
The basic process of the preparation of conductive powder: take an appropriate amount of ultrafine barite powder in a 500 mL beaker, add distilled water (solid concentration of 40%) stirring, heated to a set temperature, will be proportional to the proportion of the prepared SnCl4/SbCl3 hydrochloric acid solution and 40% of the NaOH solution, respectively, uniformly titrated, and maintained at the corresponding pH value. The titration should be controlled to be completed within 15-20 min, and continue stirring for 10 min. After removing the beaker and leaving it for 20-30 min, the free Cl- was removed by filtration, washing with distilled water for several times and removing the free Cl- (can be examined with AgNO3 solution until no white precipitate is generated). The filter cake is dried and ground into powder, then placed in a porcelain boat and roasted at a set temperature for 30 min, and then cooled.
Powder resistivity test: the powder is loaded into the plexiglass mold, the resistance meter to measure the powder resistance R, and then by the formula ρ = R × A/H [where ρ for the powder resistivity (Ω - cm), R for the measured resistance (Ω), A for the cross-sectional area of the powder column (cm2), H for the height of the powder column (cm), the experiments in the A = 0.915 cm2], the calculation to get the powder resistivity ρ.
The instruments used were: ZJM-20 stirred ball mill, DT890A resistance tester (0-200 MΩ), 5-12 box-type resistance furnace, Hydro-2000MU powder particle size analyzer (Malverm), Autosorb-I specific surface area tester, and WSD-III whiteness meter.
Based on the results of previous related studies, the main factors affecting the electrical resistivity of conductive powders are SnCl4-5H2O/SbCl3 molar ratio, hydrolysis pH, SnCl4-5H2O dosage, hydrolysis temperature and roasting temperature. In this paper, orthogonal experimental design was used: the orthogonal table L16(45) with 5 factors and 4 levels is shown in Table 1.
Table 1 Factors and levels of orthogonal experiments
Three, preparation of composite electrically conductive powders
(I) Results of orthogonal experiments
Experimental studies were carried out according to the conditions set in Table 1, and the resistivity ρ of the resulting powders was calculated as shown in Table 2.
Table 2 Results of orthogonal experiments
Note: The average particle size of ultrafine barite powder matrix is 4.1 μm, whiteness is 82.4%, and specific surface area is 71.7 m2-g-1.
(II) Extreme variance analysis
The extreme variance analysis of orthogonal experiments is shown in Table 3, and Kij is used to indicate the sum of experimental results of the groups of test results with the level number of i on the jth column, and Kij is used to indicate the mean value of experimental indexes (average resistivity) for the i-th level of the factor j. The average value of the experimental index of factor j at the ith level (average resistivity), and the extreme difference Sj indicates the difference between the maximum and minimum values of the average experimental index of the four levels of factor j. By the following formula:
Table 3 orthogonal experiments of the polar analysis
China's non-metallic mining industry
Calculation of , Sj, the results are included in Table 3. the experimental indicators of the experiment that is the powder resistivity ρ, and the smaller the better the resistivity is required, that is, the min ｛ ｝ corresponding to the level of the optimal conditions of the factor j. The results are summarized in Table 3.
From the intuitive analysis, the optimal process conditions for the preparation of barite-based composite conductive powder are A1B2C4D1E3, i.e., hydrolysis pH 1.5, SnCl4-5H2O/SbCl3 molar ratio of 15:1, SnCl4-5H2O is 15%; hydrolysis temperature of 50 ℃; and roasting temperature of 600 ℃. The larger the extreme difference Sj, the greater the influence of the factor on the experimental results, Table 3 shows that the relationship between the factors is C＞B＞A＞E＞D, i.e., the influence of each factor on the powder resistivity is SnCl4-5H2O dosage, SnCl4-5H2O/SbCl3 molar ratio, hydrolysis pH, roasting temperature, hydrolysis temperature, in descending order.
(iii) Effect of SnCl4-5H2O dosage on powder resistivity
From the above orthogonal experiments, it was shown that SnCl4-5H2O dosage had the greatest effect on powder resistivity. The electrical conductivity of the conductive powder mainly depends on the antimony-doped SnO2 coating layer on the surface of the powder matrix. When the material ratio is certain, the amount of SnCl4-5H2O is too small, the surface of the substrate can not be completely covered or the coating layer is very thin and it is difficult to form a continuous conductive network structure, and the exposure point of the conductive powder substrate increases. With the increase in the amount of hydrolysis reaction material, the conductive network structure is gradually formed and perfected. At this time, the surface of the conductive particles can be regarded as a small resistance by countless parallel connection, the powder resistivity decreases. When the amount of powder reaches a certain amount, the conductive network has been formed, and the resistivity of the conductive powder tends to be unchanged with the increase in the amount of material added. In the case of a certain ratio of SnCl4-5H2O/SbCl3, with the increase of the amount of SnCl4-5H2O, Sb5+ replaces Sn4+ and enters the crystal lattice, which makes the appearance of the conductive powder darker. In this experiment, other conditions (all the best conditions for orthogonal experiments) were fixed to study the effect of SnCl4-5H2O dosage on the experimental results, as shown in Table 4. As can be seen from Table 4, with the increase of SnCl4-5H2O addition, the conductivity of the conductive powder increases, and this result is consistent with the conclusion of the orthogonal experiments mentioned above, but the whiteness of the powder decreases and the color deepens.
Table 4 Effect of SnCl4-5H2O dosage on powder resistivity
Note: The ultrafine barite powder used as the matrix has an average particle size of 4.1 μm, whiteness of 82.4%, and a specific surface area of 71.7 m2-g-1
(D) The effect of raw material particle size on the resistivity of the powder
The raw material particle size also has a great influence. Powder resistivity determination has the following relationship:
China's non-metallic mining industry
The formula: Rs for the determination of powder resistance; R for the powder particle resistance; R1 for the contact resistance between the particles; R2 for the micropores inside the particles generated by the resistance; R3 for the specimen and the pressure column between the resistance.
Usually, the values of R2 and R3 are small and negligible, and the value of R is constant for a certain particle. Therefore, the test resistance value Rs depends mainly on the R1 value. The smaller the particle size, the more the number of particles per unit volume of the specimen increases, and the contact resistance between the particles R1 value increases. On the other hand, too large a particle size is detrimental to the uniform adsorption of hydrolysis products and does not meet the requirements of applications in certain fields. Fixing the optimal conditions determined by orthogonal analysis while changing the fineness of barite matrix particles were tested, and the results are shown in Table 5. When the average particle size of the raw material is 4.1 μm, the volume resistivity of the product is the lowest (8.1 Ω-cm), which meets the standard of similar products.
Table 5 Effect of raw material particle size on the resistivity of conductive powder
Note: d0 and S represent the average particle size and specific surface area of barite powder; ρ and d represent the resistivity and average particle size of conductive powder.
Four, antimony doped tin oxide defects and energy level effects
Using the natural properties of barite, the barite surface covered with a layer of antimony-doped SnO2 layer can be produced conductive properties of light-colored conductive powder. Pure SnO2 conductivity is poor, after appropriate doping treatment has semiconductor properties. According to the energy band theory of the energy level model, when the forbidden band width down to a certain extent, at room temperature, thermal energy (lattice vibration) can make the electron jump to the conduction band and realize the conductive. In semiconductors the number of carriers per unit volume is generally much less than the number of atoms per unit volume. The energy level model for insulators, semiconductors and conductors is shown in Figure 1.
Figure 1 Energy band model of insulators, semiconductors and conductors
Shading is the range occupied by electrons
The insulator forbidden band is wider, generally greater than 4eV, full band of electrons on the chances of excitation is very small, can not conduct electricity; and semiconductors of the forbidden band of the width of the smaller, the chances of electrons to be excited by a larger, there is a certain conductive ability. Figure 2 shows the relationship between the Fermi energy level and temperature. Composite conductive powder preparation process, the use of antimony doped tin oxide, and through the doping with a view to achieving access to high-performance products, antimony doped oxide Sb: SnO2 energy levels are shown in Figure 3.
Figure 2 Schematic diagram of the Fermi energy levels as a function of temperature
1- the donor semiconductor; 2- the recipient semiconductor; 3 - electron simple merger in the conduction band
Figure 3 Energy levels of doped tin oxide Sb:SnO2
Doping can increase the conductivity of the substrate, which is the main reason for the semiconductor's electrical conductivity; at the same time, the presence of doping defects cause the generation of non-integer ratio compounds, which have a great impact on the conductivity of the material . According to crystallography, it can be seen that the defect reaction in the gap-filling type of existence is unlikely, and due to the size of the ionic radius, the formation of oxygen vacancies is unlikely.
Because of the large oxygen ion radius, antimony doped tin oxide the most important doping reactions are:
China's non-metallic mining industry
Fifth, composite conductive powder in the paint conductive network
Anti-static and conductive coatings are the largest application areas of composite conductive powder, environmentally friendly, functional coatings have become the focus of research at home and abroad. Conductivity depends mainly on the conductive powder in the polymer matrix in the dispersion state, if the dispersion is not uniform, the whole composite system does not have conductivity, when the powder particles are uniformly distributed in the polymer matrix, forming a network, the whole composite system will have conductivity. There are two key factors affecting the conductive ability of the coating: one is the number of contacts between particles, which is actually the number of conductive channels; the second is the degree of proximity between particles. Figure 4 represents the presence of conductive powder in the form of conductive coatings, Figure 5 represents the conductive coating of powder particles in the coating film formation process may occur in the contact state and equivalent circuit.
Figure 4 Conductive powder in conductive coatings in the endowment state
Figure 5 Powder particles in the contact state and equivalent circuit
Six, composite conductive powder for the production of shielding coatings
Composite conductive powder is an important use is the production of anti-static and shielding coatings are widely used in oil storage facilities, electronic component encapsulation, ultra-purified environments, defense facilities, anti-corrosion engineering and electromagnetic interference (electromagnetic interference, EMI) and other fields. With the ever-changing industry, the requirements of conductive coatings are also increasingly high, the earlier development of electrostatic coatings in the use of many problems exposed to: metal-based powder (such as gold, silver, nickel powder) based on conductive coatings of conductivity is good, but it is expensive, the use of the value is not great; copper powder is inexpensive, but is easy to be oxidized; carbon based on the conductivity of conductive coatings of conductive powder and corrosion resistance is good, but the oil resistance and Poor adhesion, and the color of the paint is darker. In order to solve the above problems, Germany, France, Japan and other countries were developed in the 1990s to metal oxides as conductive powder light color, white conductive coatings, but the cost is high.
Figure 6 Effect of conductive powder filling amount on the resistivity of shielding coatings
Figure 7 Shielding effect of shielding coatings
Using the high-performance composite conductive powder developed in this project, the effect of its dosage on the resistivity of the coating was examined by examining its effect on the resistivity of the coating (Fig. 6), which was detected by the resistivity tester of the ACL 385 type. The resistivity of the prepared acrylic shielding coating coating is less than 10 Ω-cm when the powder addition is 20%-45%, while the resistivity of the pure acrylic coating is greater than 105 Ω-cm.
According to Schelunoff's theory, the sum of the shielding effect S is
China Nonmetallic Mining
When the dosage of composite conductive powder (ρ resistivity) is 20% and the coating thickness (t) is 40 μm, the relationship between the effect of the shielding coating and the frequency (f) is shown in Fig. 7, and this coating can achieve a better shielding value (35-40 dB) for electromagnetic waves with a frequency of less than 100 MHz, and it can be applied to cell phones, laptop computers, e-medical equipment, electronic metrology, and other electronic products for anti-electromagnetic wave interference.
Preparation and Application of Advanced Barite-matrix Composite Conductive Powders
Yang Huaming, Hu Yuehua, Zhang Huihui
( School of Resources Processing and Bioengineering, Central South University, Changsha 410083, China)
Absract: Barite-matrix composite conductive powder coated with antimony-doped tin oxide (Sb-SnO2/BaSO4, SSB) has been successfully prepared by chemical co-precipitation technology. The optimum processing parameters of preparing composite conductive powder are determined by orthographic method.The conductive powder with 4.1μm The conductive powder with 4.1μm of average particle diameter and 8.1Ω-cm of volumetric electric resistivity is prepared under the optimum parameters.Effect of SSB percentage on The effect of SSB percentage on the coated layer was investigated.The resistivity of acrylic acid paint was only 10Ω-cm when percentage of SSB reaches 20%-45%, which The resistivity of acrylic acid paint was only 10Ω-cm when percentage of SSB reaches 20%～45%, which indicates interesting application potential in several fields.Conductive paint with SSB as filler shows excellent shield value (40dB) to Conductive paint with SSB as filler shows excellent shield value (40dB) to electromagnetic waves of less than 100MHz.It can be widely applied in the fields of mobile phone, notebook PC, medical facility, electronic measuring It can be widely applied in the fields of mobile phone, notebook PC, medical facility, electronic measuring apparatus and military establishments for antijamming of electromagnetic waves.Conductive network and condition of conductive powder in conductive paint were primarily discussed, it is Conductive network and condition of conductive powder in conductive paint were primarily discussed, it is indicated that well dispersion and network formation of conductive powder in paint is the key to keep better Conductive network and condition of conductive powder in conductive paint were primarily discussed.it is indicated that well dispersion and network formation of conductive powder in paint is the key to keep better conductive property of paint.
Key words: ultrafine barite powder, composite conductive powder, doping, conductive paint, shield property.