Engineering Materials and Their Applications
Preparation and Characterization of Spherical AgPd Alloy Powder through Wet Chemical Reduction Method
2020 Volume 61 Issue 7 Pages 1409-1413
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2020 Volume 61 Issue 7 Pages 1409-1413
Spherical AgPd alloy powder was prepared through the reaction of metal nitrate and reducing agent. Natural gum Arabic was used as a dispersant. SEM observation showed that as-prepared particles had spherical morphology with narrow size distribution. Powder X-ray diffraction showed that as-prepared powder was crystallized regardless of molar ratio of Ag and Pd and alloyed. Fluorescent X-ray analysis showed that the atomic ratio of Ag and Pd in as-prepared powder was in good agreement with that of starting solution. The particle size (D50) of as-prepared powder was decreased with increasing the reaction temperature and the content of gum Arabic. The D50 increased with increasing starting solution concentration. It was assumed from the change of particle number density that the particle growth was due to the diffusion of Ag and Pd ions from starting solution. The thickness of gum Arabic layer on Ag90Pd10 and Ag40Pd60 alloy particles at gum Arabic content of 2 g/cm3 was 20 nm and 22 nm, respectively and increased to 34 nm and 31 nm, respectively, with increasing gum Arabic content. The specific resistivity of Ag90Pd10 paste was 6.2 × 10−8 Ω·m at 1000°C. The specific resistivity of Ag50Pd50 paste and Ag40Pd60 paste was 2.6 × 10−7 Ω·m at 1400°C.
Fig. 1 SEM images and particle size distribution of as-prepared powder with different atomic ratio of Ag and Pd, (a) Ag90Pd10, (b) Ag50Pd50 and (c) Ag40Pd60.
AgPd alloy powder has been used to electronic and dental material widely. The paste derived from AgPd alloy powder was used as the electrode for multilayer ceramic capacitor (MLCC),1–3) low temperature co-fired ceramic (LTCC),4,5) thick-film hybrid integrated circuits,6) surge resistor and inductor, etc. When the AgPd paste is screen printed and sintered, it is required to use AgPd alloy powder with particle characteristics such as uniform morphology, narrow size distribution, non-aggregation and high purity. So far various types of methods have been developed to synthesis AgPd alloy powder such as wet chemical reduction,7–11) hydrothermal method,12,13) spray pyrolysis.14–17) It was known that wet chemical reduction could easily prepare AgPd alloy powder in comparison with other method. In wet chemical reduction, the silver and palladium salt were reduced with reducing agent under the presence of dispersant. Since the precipitation rate of silver salt is different from palladium salt due to pH in the chemical reduction, silver and palladium particles are precipitated separately and then the chemical composition of AgPd alloy powder may be inhomogeneous. This problem was solved by making the strong acidic solution less than pH 1. Then, the silver salt and palladium salt will be precipitated at same rate. In this paper, synthesis and characterization of spherical AgPd alloy powder with various ratio of AgPd through wet chemical reduction was described. The influence of the condition of reducing such as concentration, type of reducing agent and temperature on the particle size of AgPd alloy powder was investigated. Also, the electrical property of AgPd paste was described.
Silver nitrate (AgNO3) and palladium nitrate (Pd(NO3)2) were used as the starting material. Ascorbic acid (L-C6H8O6) was used as the reducing agent. Silver nitrate and palladium nitrate were mixed at the molar ratio from 90/10 to 40/60 in a distilled water. The ascorbic acid was dissolved in a distilled water. The concentration of starting solution (mixture of silver and palladium nitrate) ranged from 0.1 to 0.5 mol/dm3. The concentration of ascorbic acid ranged from 0.1 to 0.5 mol/dm3. Gum Arabic (Acacia, Republic of the Sudan) was used as dispersant. Gum Arabic was added to aqueous silver nitrate solution and then mixed. The content of gum Arabic ranged from 2 to 6 g/dm3. The starting solution was reacted with ascorbic acid for 150 min. The reaction temperature ranged from 15 to 30°C. After the precipitation, the as-prepared powder was collected by the decantation. Then, the as-prepared powder was solvent substituted by acetone and the dried at 35°C for 12 h.
The particle morphology, microstructure, and agglomeration of the as-prepared powder were observed using a scanning electron microscope (SEM, JSM-6510, JEOL). The volume average particle size (D50) and standard deviation of as-prepared powder was determined by a dynamic light scattering method (MT3300EXII, Nikkiso). The crystal phase of the as-prepared powder was identified using powder X-ray diffraction (XRD, XRD-6100, Shimadzu). Thermal behavior of unreacted metal nitrate and gum Arabic adsorbed on as-prepared powder was observed by thermogravimetric and differential thermal analysis (DTA-TG, DTG60, Shimadzu). The chemical composition of as-prepared powder was determined by Fluorescent X-ray analysis (XRF, ZSX Primus III+, Rigaku). The specific surface area (SSA) of as-prepared powder was measured using the BET method with nitrogen gas absorption (FlowSorb III 2305, Shimadzu). The tap density of as-prepared powder was measured using tap denser (KYT-5000, Seishin enterprise).
2.2 Preparation and characterization of AgPd pasteThe electrical properties of AgPd paste prepared from as-prepared powder was evaluated. As-prepared powder was homogeneously blended with ethyl cellulose, butyl carbitol acetate using a three-roller mill at room temperature. The content ratio of as-prepared powder, ethyl cellulose, amine type polymeric dispersant (Solsperse) and butyl carbitol acetate was 78:19:1:2 mass%. The AgPd paste (0.5 m × 1 mm) was screen printed on alumina substrate and the comb pattern was formed. The AgPd pasted was sintered from 500 to 1400°C for 3600 s under the air atmosphere. The resistance of AgPd paste was measured at both ends of comb pattern using a multimeter and the specific resistivity (ρ) was determined according to eq. (1), where R was resistance (Ω), d was the cross section area (m2) of electrode and l was length (m).
| \begin{equation} \rho = \frac{R\times d}{l} \end{equation} | (1) |
Typical particle morphology and particle size distribution of as-prepared powder obtained at the different molar ratio of Ag and Pd were shown in Fig. 1. SEM images revealed that as-prepared particles had spherical morphology with non-aggregation and had narrow particle size distribution. The D50, specific surface area and tap density of as-prepared powder were summarized in Table 1. The D50 and tap density of them reduced when the Pd content increased. SSA increased with increasing Pd content. The standard deviation of as-prepared powder was more than 10% of D50. This suggests that as-prepared powder has a broad particle size distribution. The XRD patterns of as-prepared powder was shown in Fig. 2. The diffraction pattern revealed that the as-prepared powder was crystallized regardless of molar ratio of Ag and Pd and alloyed. The crystallite size of Ag90Pd10, Ag50Pd50 and Ag40Pd60 obtained from Debye-Scherrer’s equation was 373 nm, 142 nm and 92 nm, respectively. The crystallite size of AgPd powder increased with decreasing Pd content. It was found that the lower Pd content, the higher the crystallinity of as-prepared powder.
SEM images and particle size distribution of as-prepared powder with different atomic ratio of Ag and Pd, (a) Ag90Pd10, (b) Ag50Pd50 and (c) Ag40Pd60.
XRD patterns of as-prepared powder with different atomic ratio of Ag and Pd.
The chemical composition of as-prepared powder was listed in Table 2. The atomic ratio of Ag and Pd in as-prepared powder was in good agreement with that of starting materials regardless of the atomic ratio of Ag and Pd. DTA-TG curves of as-prepared powder was shown in Fig. 3. Exothermic peak was observed from 100 to 200°C in DTA curve and corresponded to the decomposition and volatile of gum Arabic. Weight loss of Ag90Pd10 and Ag40Pd60 powders in TG curve was 1.1 mass% and 1.0 mass%, respectively. Exothermic peak was observed from 300 to 400°C in DTA curve and corresponded to the oxidation of Pd. Weight gain of Ag90Pd10 and Ag40Pd60 powders in TG curve was 2 mass% and 5 mass%, respectively. It was found that as-prepared powder was easily oxidized so that the content of Pd increased.
DTA-TG curves of as-prepared Ag90Pd10 and Ag40Pd60 powder.
The influence of reaction temperature on D50 of as-prepared powder was investigated. The change of D50 and induction time as a function of reaction temperature is shown in Fig. 4(a) and (b), respectively. The time that the color of solution changed was defined as induction time. From Fig. 4(a), the D50 of Ag90Pd10 and Ag40Pd60 alloy powders decreased with increasing reaction temperature. It was considered that the solubility of starting solution was accelerated when the reaction temperature was elevated. Therefore, the higher reaction temperature lead to the promotion of much AgPd nuclei and smaller particles. The color of solution turns into dark gray from colorless when the AgPd particles were precipitated by chemical reduction reaction. From Fig. 4(b), the particles were rapidly precipitated so that the reaction temperature was high. It was found that the induction time was short so that the reaction temperature was high. Considering the result of Fig. 4(a), it was presumed that the D50 was reduced so that the induction time was short.
Change of D50 (a) and induction time (b) as a function of reaction temperature.
The influence of starting solution concentration on D50 of Ag90Pd10 powder was investigated. The change of D50, yield and particle number density at 30°C was listed in Table 3. The particle number density was determined according the eq. (2), where n was particle number density (n/m3), y was the yield (kg/m3), d was particle density (kg/dm3) and V was particle volume (m3). The density of silver (10.49 kg/m3) and palladium (12.023 kg/cm3) was used as used as the particle density. The particle volume was determined by the D50 since the particle was close to spherical morphology.
| \begin{equation} n = \frac{y}{d\times V} \end{equation} | (2) |
The D50 of Ag90Pd10 powder increased from 330 to 590 nm with increasing the starting solution concentration from 0.1 to 0.5 mol/dm3. The yield of Ag90Pd10 powder was more than 98% from 0.1 to 0.5 mol/dm3. It was found that the particle number density of Ag90Pd10 powder was order of 1018 and constant for the starting solution concentration. When AgPd particles grow up by the diffusion of Ag and Pd ion from the starting solution, the particle number density will be constant. On the other hand, when AgPd particles grow up by the aggregation of AgPd particles, the particle number density will be reduced. Hence, it was considered that the particle growth was supported by the diffusion of Ag and Pd ions.
3.4 Influence of gum Arabic content on the formation of AgPd particlesThe change of D50 and adsorption layer as a function of the gum Arabic content was shown in Fig. 5(a) and (b), respectively. When the D50 of Ag90Pd10 and Ag40Pd60 particles was 540 nm and 420 nm, respectively. The D50 of Ag90Pd10 and Ag40Pd60 particles decreased to 250 nm and 220 nm when gum Arabic content increased to 6 g/cm3, respectively. It was confirmed from DTA-TG analysis that the layer of gum Arabic molecule was absorbed on AgPd particles. Assuming a structure in which spherical AgPd particles are used as a core and are covered with gum Arabic layer as a shell, the thickness of gum Arabic layer was determined using the amount of weight loss determined from DTA-TG as the amount of gum Arabic coated. When gum Arabic content was 2 g/cm3, the thickness of gum Arabic layer absorbed on Ag90Pd10 and Ag40Pd60 alloy particles was 20 nm and 22 nm, respectively. The thickness of gum Arabic layer increased to 34 nm and 31 nm when gum Arabic content increased to 6 g/cm3, respectively.
Change of D50 (a) and adsorption layer (b) as a function of gum Arabic content.
The effect of dispersant on the particle size was reported.18–21) The dispersion of AgPd alloy particles in the solution is due to steric hindrance utilizing repulsion by the polymer molecule of the gum Arabic. When the gum Arabic content is low, a relatively thin adsorption layer of gum Arabic is formed. Therefore, it is considered that Ag and Pd ions can easily diffuse into the particles and AgPd particles can grow. The adsorption layer of gum Arabic increases with increasing the gum Arabic content. The thick adsorption layer is formed to suppress the diffusion of Ag and Pd ions at higher gum Arabic content. As a result, it was considered that the growth of AgPd alloy particles was restricted and then smaller particles are given.
3.5 Electrical properties of AgPd pasteThe electrical properties of AgPd paste sintered from 500 to 1400°C was investigated. The relation between sintering temperature and specific resistivity was shown in Fig. 6. The specific resistivity of AgPd paste decreased with increasing the sintering temperature. Also, the specific resistivity of AgPd paste increased with increasing the content of Pd. When Ag90Pd10 paste was sintered at 1100°C, it was melted on the LTCC substrate. The specific resistivity of Ag90Pd10 paste sintered at 1000°C was 6.2 × 10−8 Ω·m. The AgPd paste could be sintered at higher temperatures with increasing Pd content. It was possible for the Ag40Pd60 paste to sinter up to 1400°C. The specific resistivity of Ag50Pd50 paste and Ag40Pd60 paste at 900°C paste was 3.9 × 10−7 Ω·m and 6.3 × 10−7 Ω·m, respectively. The specific resistivity of Ag50Pd50 paste and Ag40Pd60 paste decreased to 2.6 × 10−7 Ω·m at 1400°C. However, the specific resistivity of Ag50Pd50 paste and Ag40Pd60 paste was still higher one order of magnitude than that of Ag90Pd10 paste. This results in that the volume resistivity of Pd (10 × 10−8 Ω·m at 20°C19)) is higher about 7 times than that of Ag (1.47 × 10−8 Ω·m at 20°C22)).
Relation between sintering temperature and specific resistivity.
Spherical AgPd alloy powder was prepared through wet chemical reduction method using an ascorbic acid as a reductant agent. As-prepared powder was crystallized and alloying regardless of molar ratio of Ag and Pd. The chemical composition of as-prepared powder was in good agreement with starting solution composition. The D50 was influenced due to the reaction temperature, starting solution concentration and gum Arabic content. The D50 decreased with increasing the reaction temperature. Also, the D50 decreased with increasing the content of gum Arabic. The layer of gum Arabic on the Ag90Pd10 and Ag40Pd60 alloy particles at 2 g/cm3 of gum Arabic content was 20 nm and 22 nm, respectively. The D50 increased with increasing the starting solution concentration. It was concluded from the change of the particle number density that the growth of AgPd alloy particles was presumed to be diffusion of Ag and Pd ions. The specific resistivity of Ag90Pd10 paste was 6.2 × 10−8 Ω·m at 1000°C. The specific resistivity of AgPd paste exhibited an order of magnitude higher, when Pd content was higher than 50%. The specific resistivity of Ag50Pd50 paste and Ag40Pd60 paste was 2.6 × 10−7 Ω·m at 1400°C.
