Electrically conductive particles, composition, article and method of manufacturing electrically conductive particles

Abstract

The invention is directed to electrically conductive particles comprising a metallic core, a dielectric layer encapsulating said metallic core, and a silver containing outer-layer, wherein said metallic core comprises or consists of elemental metal selected from the group consisting of aluminum, copper, iron, nickel, zinc, and alloys, and mixtures thereof, said dielectric layer comprises at least one metal oxide selected from the group consisting of the group consisting of silicon oxide, aluminum oxide, titanium oxide, zirconium oxide, tin oxide, organic polymer, and mixtures thereof, wherein said silver containing layer is a continuous and opaque layer, wherein a silver ion coordinating layer is arranged between said dielectric layer and said silver containing layer and, optionally a further outer surface modification layer in amount of 0 to 3 wt.-%, based on the total weight of the electrically conductive particles. The invention is also directed to a composition and an article comprising the electrically conductive particles as well as to a method for producing said electrically conductive particles.

Claims

1. Electrically conductive particles comprising metallic cores, a dielectric layer encapsulating the metallic cores, a silver containing outer-layer, and a silver ion coordinating layer arranged between the dielectric layer and the silver containing outer-layer, wherein the metallic cores comprise one or more of elemental aluminum, elemental copper, elemental iron, elemental nickel, elemental zinc, aluminum alloy, copper alloy, iron alloy, nickel alloy, zinc alloy, aluminum oxide, copper oxide, iron oxide, nickel oxide, and zinc oxide, the dielectric layer comprises one or more of an organic polymer and a metal oxide, the metal oxide including one or more of silicon oxide, aluminum oxide, titanium oxide, zirconium oxide, and tin oxide, and the silver containing outer-layer includes a continuous and opaque layer, the dielectric layer includes a silicon oxide layer, and the metallic cores have a median diameter d.sub.50 in the range of 2 μm to 100 μm.

2. The electrically conductive particles according to claim 1, wherein the silver ion coordinating layer includes one or more of an organofunctional silane comprising a silver ion coordinating moiety, an organofunctional titanate comprising a silver ion coordinating moiety, an organofunctional aluminate comprising a silver ion coordinating moiety, and an organofunctional zirconate comprising a silver ion coordinating moiety.

3. The electrically conductive particles according to claim 1, wherein the silver ion coordinating layer comprises at least one silane of formula (I):
(R.sup.1O).sub.(4-a-b-c)Si(R.sup.2X).sub.a(R.sup.3).sub.b(R.sup.4).sub.c  (I), wherein: a is an integer selected from 1, 2, or 3, b and c are independently selected integers from 0, 1, or 2, with the proviso that a+b+c is an integer from 1 to 3, R.sup.1 is alkyl with 1 to 4 carbon atoms, each R.sup.2 independently represents a branched-chain having 1 to 14 carbon atoms, a straight chain alkylene having 1 to 14 carbon atoms, or a cycloalkylene having 5 to 11 carbon atoms, and each R.sup.3 and R.sup.4 independently represents a branched-chain alkyl having 1 to 14 carbon atoms, a straight chain alkyl having 1 to 14 carbon atoms, a cycloalkyl having 5 to 11 carbon atoms, or H, and X is a silver ion coordinating moiety.

4. The electrically conductive particles according to claim 3, wherein the silver ion coordinating moiety X independently represents one or more of mercapto, thioalkylether having 1 to 6 carbon atoms, amino, thiosulfate, thiocyanate, cyanate, cyanide, ureido, carbamate, and bipyridyl.

5. The electrically conductive particles according to claim 1, wherein the silver ion coordinating layer comprises one or more of mercapto silane, amino silane, and thiocyanate silane.

6. The electrically conductive particles according to claim 1, wherein the silver ion coordinating layer comprises mercaptosilane.

7. The electrically conductive particles according to claim 1, wherein the dielectric layer is present in an amount in a range of 0.5 to 15 wt.-%, based on the total weight of the metallic cores.

8. The electrically conductive particles according to claim 1, wherein the dielectric layer comprises a hybrid mixture of silicon oxide and one or more of an organic oligomer and an organic polymer.

9. The electrically conductive particles according to claim 1, wherein the dielectric layer includes a first sublayer comprising the silicon oxide layer, the silicon oxide layer comprising silicon dioxide, and a second sublayer comprising a polymer, wherein the second sublayer is located either directly on the first sublayer or between the silver ion coordinating layer and the first sublayer.

10. The electrically conductive particles according to claim 1, wherein the silver containing outer-layer is present in an amount of 5 to 45 wt-%, based on the electrically conductive particles.

11. The electrically conductive particles according to claim 1, wherein the electrically conductive particles have an electrical powder resistivity in a range of 0.2 to below 100 mΩ cm.

12. The electrically conductive particles according to claim 1, wherein the metallic cores have one or more of a spherical form, plate-like form, and a dendritic form.

13. The electrically conductive particles of claim 1, wherein the particles further comprise an outer surface modification layer in amount of 0 to 3 wt.-%, based on the total weight of the electrically conductive particles.

14. The electrically conductive particles of claim 1, wherein the silver ion coordinating layer comprises one or more of 3-mercaptopropyl trimethoxysilane, mercaptopropyl methyl dimethoxysilane, 3-mercaptopropyl triethoxysilane, and 3-thiocyanatopropyl triethoxysilane.

15. The electrically conductive particles of claim 1, wherein the dielectric layer is present in an amount in a range of 0.5 to 15 wt. %, based on the total weight of the metallic core.

16. The electrically conductive particles of claim 1, wherein the d.sub.50 of the metallic core is in the range from 3 μm to 85 μm.

17. The electrically conductive particles of claim 1, wherein the d.sub.50 of the metallic core is in the range from 4 μm to 75 μm.

18. A method of manufacturing electrically conductive particles, the method comprising: providing metallic core particles, the metallic core particles having a median diameter d50 in the range of 2 μm to 100 μm, applying a dielectric layer on the metallic core particles, the dielectric layer including a silicon oxide layer, applying a coordinating layer for silver ions on the dielectric layer, contacting the coordinating layer of the coated particles with silver ions, the silver ions being provided by a silver compound dissolved in a solvent, to coordinate at least part of the silver ions on the coordinating layer, and reducing the silver ions coordinated on the surface of the coordinating layer and reducing additional silver ions to provide a continuous and opaque silver containing layer on the coordinating layer.

19. The method of claim 18, further comprising applying an outer surface modification layer to the particles.

Description

FIGURES

(1) FIG. 1 shows schematic drawings of electrically conductive particles. The drawings (a) to (c) show electrically conductive particles according to the prior art. The electrically conductive particle (d) illustrates the present invention.

(2) The electrically conductive particle (a) has a structure wherein the silver layer is directly applied on a spherical substrate (comparative example 1).

(3) The electrically conductive particle (b) has a structure wherein a silver ion coordinating layer is arranged between the spherical substrate and the silver layer (comparative example 2).

(4) The electrically conductive particle (c) has a structure wherein a dielectric layer is directly applied on the spherical substrate and a silver layer is directly applied on a dielectric layer (comparative example 3).

(5) The electrically conductive particle (d) of the invention has a structure, wherein a dielectric layer is applied on a spherical metallic core, a silver ion coordinating layer is applied on said dielectric layer, and finally a silver containing layer is applied on said silver ion coordinating layer (inventive example).

(6) FIG. 2 shows SEM images of the electrically conductive particles shown schematically in FIG. 1. The images A, B, C, D correspond to the embodiments illustrated by the schematic drawings (a), (b), (c), and (d), respectively. The scale bar in SEM images A, B, C reflects 2 μm; the scale bar in SEM images D reflects 1 μm.

(7) SEM A is a silver coated aluminum powder according to comparative example 1.

(8) SEM B is an aluminum powder coated with a silver ion coordinating layer and then coated with a silver layer, according to comparative example 2.

(9) SEM C is an aluminum powder coated with a dielectric layer and a silver layer, according to comparative example 3.

(10) SEM D is an aluminum powder coated with a dielectric layer, a silver ion coordinating layer and a silver layer, according to example 28.

(11) FIG. 3 is a plot showing the powder resistivity depending on the silver content of electrically conductive particles of the invention wherein the aluminum core particles have a d.sub.50=45 μm (squares) and d.sub.50=20 μm (circles).

(12) FIG. 4 is a plot showing the powder resistivity depending on the silver content of electrically conductive particles of the invention wherein the aluminum core particles have a d.sub.50=5 μm (triangles).

EXAMPLES

1. Preparations of Examples and Comparative Examples

1.1 Comparative Examples

1.1.1 Al—Ag (Comparative Example 1)

(13) 50 g of the aluminum powder (d.sub.50=20 μm) were dispersed in 183 g ethanol in a 1 L double-wall glass reactor equipped with a stirrer (250 rpm) and a condenser. The dispersion was heated below the boiling point of the solvent. Subsequently, 29.3 g glucose dissolved in 32.40 g water and 8.5 g diethanolamine were added (each solution as one batch). After dispersing for 5 min, 13.8 g silver nitrate (equals 15% Ag) dissolved in 6.4 g water were added over 15 min. The dispersion was stirred for 1 h. After cooling down to room temperature, the silver-coated aluminum was filtered and washed twice with water (250 ml each) and ethanol (250 ml each), respectively. The resulting material was dried under vacuum for 5 h.

1.1.2 Al-MTMO-Ag (Comparative Example 2; One-Pot Synthesis in Contrast to Method Described in Wang Y. et al., “Surface Thiolation of Al Microspheres to Deposit Thin and Compact Ag Shells for High Conductivity”, Langmuir 2015, 31, 13441-14451.)

(14) 50 g of the aluminum powder (d.sub.50=20 μm) were dispersed in 183 g ethanol in a 1 L double-wall glass reactor equipped with a stirrer (250 rpm) and a condenser. The dispersion was heated below the boiling point of the solvent. 0.25 g 3-mercapto propyltrimethoxysilane (MTMO) were added and the mixture was stirred for 2 h. 29.3 g glucose dissolved in 32.40 g water and 8.5 g diethanolamine were added (each solution as one batch). After dispersing for 5 min, 13.8 g silver nitrate (corresponding to 15 wt-% Ag) dissolved in 6.4 g water were added over 15 min. The dispersion was stirred for 1 h. After cooling down to room temperature, the silver-coated aluminum was filtered, washed with water (2 times, 250 ml each) and ethanol (2 times, 250 ml each). The resulting material was dried under vacuum for 5 h.

1.1.3 Al—SiO.SUB.2.—Ag (Comparative Example)

1.1.3.1 Al—SiO.SUB.2 .(Comparative Examples 12-14)

(15) 240 g of the aluminum powder (d.sub.50=20 μm) were dispersed in 293 g ethanol in a 1 L double-wall glass reactor equipped with a stirrer (250 rpm) and a condenser. After addition of 17.64 g tetraethoxysilane (corresponding to 2 wt-% SiO.sub.2) the mixture was heated below the boiling point of the solvent. Subsequently, 4.5 g NH.sub.3 (25% in water) and 20 g water were added in one batch. After 3 h of further stirring, the product was cooled to room temperature, filtered and washed with ethanol (3 times, 100 ml each). For further experiments the material was directly used in paste form.

(16) Starting materials with d.sub.50=5 μm (comparative example 14) and d.sub.50=45 μm (comparative example 13) were also coated with SiO.sub.2.

(17) TABLE-US-00001 TABLE 1 Coating of substrates with dielectric layer based on 240 g starting material. NH.sub.3 Comparative d.sub.50, Tetraethoxysilane/ (25%)/ Water/ examples μm SiO.sub.2 % g g g 12a 20 2 17.6 4.5 20 13a 45 2 17.6 4.5 20 14a  5 2 17.6 4.5 20

1.1.3.2 Al—SiO.SUB.2.—Ag (Comparative Examples 3-11)

(18) The material prepared in section 1.1.3.1 was coated with silver according to section 1.1.1.

(19) By adapting the amounts of silver nitrate, glucose and diethanolamine and the dosage rate, materials with other silver contents (10-40 wt.-%) were prepared (see table 2). Additionally, silver coatings on coarser (d.sub.50=45 μm) and finer (d.sub.50=5 μm) SiO.sub.2-coated aluminum particles were carried out.

(20) TABLE-US-00002 TABLE 2 Silver-coating of SiO.sub.2-coated substrates based on 50 g starting material. Compar- d.sub.50 SiO.sub.2 Ag ative (Al-core)/ [wt-% [wt-% Glucose/ DEA/ AgNO.sub.3, / Ex. μm ref. to Al] ref. to Al g g g 3 20 2 15 29.3 8.5 13.9 4 20 2 20 41.7 12.2 19.7 5 20 2 30 72.1 21 34.0 6 45 2 10 18.5 5.4 8.7 7 45 2 15 29.3 8.5 13.9 8 5 2 15 29.3 8.5 13.9 9 5 2 20 41.7 12.2 19.7 10 5 2 30 72.1 21 34.0 11 5 2 40 111.3 32.5 52.5

1.1.4 Electrically Conductive Particles According to Wang Y. et al., “Surface Thiolation of Al Microspheres to Deposite Thin and Compact Ag Shells for High Conductivity”, Langmuir 2015, 31, 13441-14451)

1.1.4.1 Al-MTMO (Comparative Examples 15a, 16a)

(21) 40 g of the starting material (d.sub.50=20 μm) were dispersed either in 200 ml ethanol and 600 ml water (comparative example 15) or in 400 ml ethanol and 400 ml water (comparative example 16) in 1 L double-wall glass reactors equipped with stirrers (250 rpm) and condensers. 20 g 3-mercapto propyltrimethoxysilane (equals 33% MTMO) were added to each experiment and the mixtures were heated under reflux. After further stirring for 6 h a the materials were cooled to room temperature, filtered and washed with ethanol (3 times, 100 ml each). The products were dried under vacuum for 4 h at 60° C.

1.1.4.2 Al-MTMO-Ag (Comparative Examples 17, 18)

(22) The materials prepared in section 1.1.4.1 were coated with silver. 30 g of the MTMO-modified material were dispersed in 110 g ethanol in a 1 L double-wall glass reactor equipped with a stirrer (250 rpm) and a condenser. The dispersion was heated below the boiling point of the solvent. Subsequently, 25 g glucose dissolved in 27.60 g water and 7.3 g diethanolamine were added separately. After dispersing for 5 min, 11.8 g silver nitrate (corresponding to 20 wt.-% Ag) dissolved in 5.5 g water were added over 60 min. The dispersion was stirred for 1 h. After cooling down to room temperature, the silver-coated aluminum was filtered, washed twice with water (250 ml each) and ethanol (250 ml each), respectively. The resulting material was dried under vacuum at room temperature for 5 h.

(23) Comparative example 17 was prepared by silver-coating comparative example 15 and likewise comparative example 18 corresponds to comparative example 16.

1.2 Examples According to the Invention

1.2.1 Al—Sio.SUB.2.-MTMO-Ag

1.2.1.1 Al—SiO.SUB.2.-MTMO (Examples 19a-23a)

(24) 240 g aluminum powder (d.sub.50=20 μm) were dispersed in 293 g ethanol in a 1 L double-wall glass reactor equipped with a stirrer (250 rpm) and a condenser. After addition of 17.64 g tetraethoxysilane (corresponding to 2 wt.-% SiO.sub.2) the mixture was heated below the boiling point of the solvent. Subsequently, 4.5 g NH.sub.3 (25% in water) and 20 g water were added in one batch. After 2 h of further stirring, 1.2 g 3-mercapto propyltrimethoxysilane (equals 0.5 wt.-% MTMO) were added and the mixture was stirred for 1 h. Finally, the product was cooled to room temperature, filtered and washed with ethanol (3 times, 100 ml each). For further experiments the material was directly used in paste form.

(25) The 3-mercapto propyltrimethoxysilane content in the final product was varied between 0.5 wt.-% and 1 wt.-% by adjusting the 3-mercapto propyltrimethoxysilane amount added to the reaction mixture. The above mentioned instructions were transferred to aluminum particles with other particle sizes (5 μm, 45 μm). The corresponding examples 19-23 are summarized in table 3.

(26) TABLE-US-00003 TABLE 3 Coating of aluminum powder with dielectric layer and interlayer based on 240 g starting material. SiO.sub.2 d.sub.50, wt.- MTMO MTMO/ Tetraethoxy- NH.sub.3 (25%)/ Water/ # μm % wt.-% g Silane/g g g 19 20 2 0.5 1.2 17.6 4.5 20 20 20 2 1.0 2.4 17.6 4.5 20 21 45 2 0.5 1.2 17.6 4.5 20 22 5 2 0.5 1.2 17.6 4.5 20 23 5 2 1.0 2.4 17.6 4.5 20

1.2.1.2 Al—SiO.SUB.2.-MTMO-Ag (Examples 24-34)

(27) The material prepared in section 1.2.1.1 was coated with silver according to section 1.1.1.

(28) By adapting the amounts of silver nitrate, glucose and diethanolamine and the dosage rate, materials with other silver contents (5-40 wt.-%) were prepared. Additionally, silver coatings on coarser (d.sub.50=45 μm) and finer (d.sub.50=5 μm) aluminum particles were carried out. In all examples the MTMO concentration used was 0.5 wt.-% referred to Al.

(29) TABLE-US-00004 TABLE 4 Silver coating of SiO.sub.2-interlayer-coated substrates based on 50 g starting material. d.sub.50/ SiO.sub.2 Ag Glucose/ DEA/ AgNO.sub.3/ Examples μm [wt.-%] [wt.-%] g g g 24 45 2 5 8.7 2.5 4.1 25 45 2 10 18.5 5.4 8.7 26 45 2 15 29.5 8.6 13.9 27 45 2 20 41.8 12.2 19.7 28 25 2 15 29.5 8.6 13.9 29 25 2 20 41.8 12.2 19.7 30 25 2 30 72.2 21 34.0 31 5 2 15 29.5 8.6 13.9 32 5 2 20 41.8 12.2 19.7 33 5 2 30 72.2 21 34.0 34 5 2 40 111.4 32.5 52.5

1.2.2 Al—SiO.SUB.2.-AMEO-Ag

1.2.2.1 Al—SiO.SUB.2.-AMEO (Examples 35a-38a)

(30) 240 g aluminum powder (d.sub.50=5 μm) were dispersed in 293 g ethanol in a 1 L double-wall glass reactor equipped with a stirrer (250 rpm) and a condenser. After addition of 17.64 g tetraethoxysilane (corresponding to 2 wt.-% SiO2) the mixture was heated below the boiling point of the solvent. Subsequently, 4.5 g NH3 (25% in water) and 20 g water were added in one batch. After 2 h of further stirring, 1.2 g 3-aminopropyltriethoxysilane (equals 0.5 wt.-% AMEO) were added and the mixture was stirred for 1 h. Finally, the product was cooled to room temperature, filtered and washed with ethanol (3 times, 100 ml each). For further experiments the material was directly used in paste form.

(31) The 3-aminopropyltriethoxysilane content in the final product was varied between 0.5 wt.-% and 4 wt.-% by adjusting the 3-aminopropyltriethoxysilane amount added to the reaction mixture. The corresponding examples 35a-38a are summarized in table 5.

(32) TABLE-US-00005 TABLE 5 Coating of aluminum powder with dielectric layer and interlayer based on 240 g starting material. NH.sub.3 d.sub.50, SiO.sub.2 AMEO AMEO/ Tetraethoxy- (25%)/ Water/ # μm wt.-% wt.-% g Silane/g g g 35a 5 2 0.5 1.2 17.6 4.5 20 36a 5 2 1.0 2.4 17.6 4.5 20 37a 5 2 2.0 4.8 17.6 4.5 20 38a 5 2 4.0 9.6 17.6 4.5 20

1.2.2.2 Al—SiO.SUB.2.-AMEO-Ag (Examples 39-42)

(33) 50 g of the Al—SiO2-AMEO aluminum powder (d.sub.50=5 μm) were dispersed in 183 g ethanol in a 1 L double-wall glass reactor equipped with a stirrer (250 rpm) and a condenser. The dispersion was heated below the boiling point of the solvent. Subsequently, 111.3 g glucose dissolved in 120 g water and 32.5 g diethanolamine were added (each solution as one batch). After dispersing for 5 min, 52.5 g silver nitrate (equals 40% Ag) dissolved in 25 g water were added over 15 min. The dispersion was stirred for 1 h. After cooling down to room temperature, the silver-coated aluminum was filtered and washed twice with water (250 ml each) and ethanol (250 ml each), respectively. The resulting material was dried under vacuum for 5 h.

(34) The corresponding examples 39-42 are summarized in table 6.

(35) TABLE-US-00006 TABLE 6 Silver-coating of SiO.sub.2-coated substrates based on 50 g starting material. d.sub.50/ SiO.sub.2 Ag Ex. μm [wt-%] [wt-%] Glucose/g DEA/g AgNO.sub.3, /g 39 5 2 40 111.3 32.5 52.5 40 5 2 40 111.3 32.5 52.5 41 5 2 40 111.3 32.5 52.5 42 5 2 40 111.3 32.5 52.5

1.2.3 Al—SiO.SUB.2.-VPSI363-Ag

1.2.3.1 Al—SiO.SUB.2.-VPSI363 (Examples 43a-44a)

(36) 240 g aluminum powder (d50=5 μm) were dispersed in 293 g ethanol in a 1 L double-wall glass reactor equipped with a stirrer (250 rpm) and a condenser. After addition of 17.64 g tetraethoxysilane (corresponding to 2 wt.-% SiO2) the mixture was heated below the boiling point of the solvent. Subsequently, 4.5 g NH3 (25% in water) and 20 g water were added in one batch. After 2 h of further stirring, 2.4 g 3-mercaptopropyl-di(tridecan-1-oxy-13-penta(ethyleneoxide)) ethoxysilane (equals 1.0 wt.-% VPSI363) were added and the mixture was stirred for 1 h. Finally, the product was cooled to room temperature, filtered and washed with ethanol (3 times, 100 ml each). For further experiments the material was directly used in paste form.

(37) The VPSI363 content in the final product was varied between 1.0 wt.-% and 4 wt.-% by adjusting the 3-aminopropyltriethoxysilane amount added to the reaction mixture. The corresponding examples 43a-44a are summarized in table 7.

(38) TABLE-US-00007 TABLE 7 Coating of aluminum powder with dielectric layer and interlayer based on 240 g starting material. NH.sub.3 d.sub.50, SiO.sub.2 VPSI363 VPSI363/ Tetraethoxy- (25%)/ Water/ # μm wt.-% wt.-% g Silane/g g g 43a 5 2 1.0 2.4 17.6 4.5 20 44a 5 2 4.0 9.6 17.6 4.5 20

1.2.3.2 Al—SiO.SUB.2.-VPSI363-Ag (Examples 45-46)

(39) 50 g of the Al—SiO2-VPSI363 aluminum powder (d50=5 μm) were dispersed in 183 g ethanol in a 1 L double-wall glass reactor equipped with a stirrer (250 rpm) and a condenser. The dispersion was heated below the boiling point of the solvent. Subsequently, 111.3 g glucose dissolved in 120 g water and 32.5 g diethanolamine were added (each solution as one batch). After dispersing for 5 min, 52.5 g silver nitrate (equals 40% Ag) dissolved in 25 g water were added over 15 min. The dispersion was stirred for 1 h. After cooling down to room temperature, the silver-coated aluminum was filtered and washed twice with water (250 ml each) and ethanol (250 ml each), respectively. The resulting material was dried under vacuum for 5 h.

(40) The corresponding examples 43-44 are summarized in table 8.

(41) TABLE-US-00008 TABLE 8 Silver-coating of SiO.sub.2-coated substrates based on 50 g starting material. d.sub.50/ SiO.sub.2 Ex. μm [wt-%] Ag[wt-%] Glucose/g DEA/g AgNO.sub.3, /g 45 5 2 40 111.3 32.5 52.5 46 5 2 40 111.3 32.5 52.5

2. Test Methods

2a. Silicon Dioxide Content of Pigments

(42) The silicon dioxide content of the samples was determined gravimetrically. 1 g of the sample was dissolved in 25 ml hydrochloric acid. The supernatant was evaporated in boiling heat. After the sample was filtrated and washed with water, the residue was treated at 800° C. for 1 h and weighted. The SiO.sub.2 content was determined with IPC as amount of Si and calculated as SiO.sub.2.

2b. Silver Content

(43) The silver content of the samples was determined gravimetrically. 1 g of the silver coated aluminum powder were mixed with 20 ml nitric acid and dissolved in boiling heat. After filtration, silver was precipitated as silver chloride from the filtrate with an aqueous sodium chloride solution (100 g NaCl in 1 L of water). After filtration, the precipitate was washed with water, dried and weighted. The Ag content was determined with ICP.

2c. Particle Size Distribution

(44) Particle size distributions an especially the d.sub.50 value of (coated) Aluminum powder were determined by laser diffraction (Sympatec—Helos/BF) as volume-averaged median of the particle size distribution curve. Dispersion of the dry particles took place in an airstream.

2d. Sysmex FPIA 3000S Measurement

(45) Flow particle image analysis was carried out with an FPIA 3000S device from Sysmex Corporation. For measurement of particle size distribution and circularity, a homogeneous sample was deposited in the sample chamber. In the sample chamber the material is mixed with isopropanol (5 ml) and dispersed by application of ultrasound. After 1 min the sample is flushed across the camera lens to generate the corresponding contrast images of individual particles.

2e. Powder Resistivity

(46) In order to determine the powder resistivity of electrically conductive granular materials, a defined amount of the sample was compacted in a cylindrical setup prior to determination of the resistivity between to contact points.

(47) The custom-made setup consisted of the following parts: lab shaker, custom-made adapter for brass base, PVC tube, brass plunger with scale, additional weight (2 kg), contact clips, Milliohmmeter Resistomat® 2316 (Burster Präzisionsmesstechnik GmbH & Co. KG, Germany).

(48) First, the PVC tube was attached to the brass base via a thread. Both parts were then attached to the lab shaker via the adapter. 30 g of each sample were loosened up until all visible agglomerates were broken up and filled into the PVC tube on the shaker. The brass plunger with scale was carefully lowered into the PVC tube and loaded with the additional weight. The material was compacted by shaking for 2 min at 1000 rpm. The scale on the brass plunger provided the filling height of the PVC tube before and after the compacting step. For the measurement of the electrical resistivity of the powder the contact clips were connected to the Resistomat and attached to brass base and plunger. The Resistomat® 2316 provided the corresponding resistivity values Rtotai:
R.sub.total=R.sub.System+R.sub.Sample

(49) R.sub.system corresponds to the resistivity of brass base and plunger and was determined to 0.18 mΩ.

(50) The specific powder resistivity (in Ωcm) was then defined as:

(51) $R_{\mathrm{spec}}=\frac{\left(R_{\mathrm{total}}-R_{\mathrm{System}}\right)*A}{d}$
with d as filling height of the PVC tube after compacting (.fwdarw.plunger scale) and A as area of brass base. The specific powder resistivity is inverse proportional to the specific conductivity.

3. Results

i. Conductivity: Comparative (Wang Y. et al. Vs. Inventive Example)

(52) TABLE-US-00009 TABLE 9 Conductivity of the electrically conductive particles of the comparative examples (Wang Y. et al.) in comparison to electrically conductive particles of the examples according to the invention. MTMO amount with regard to Aluminum. Ag amount with regard to MTMO- coated material. d.sub.50/ SiO.sub.2/ MTMO/ Ag/ R.sub.spec/ Examples μm wt.-% wt.-% wt.-% mΩcm comparative example 17 20 — 33 20 4.0 comparative example 18 20 — 33 20 1.2 comparative example 4 20 2 — 20 1.5 example 29 20 2 0.5 20 0.9

(53) Preparation of comparative examples 17 and 18 is described in section 1.1.4.2. Preparation of comparative example 4 is described in section 1.1.3.2. Preparation of example 29 was carried out according to section 1.2.1.2 with increased silver content of 20 wt.-%.

(54) The material according to the invention (example 4) provides improved conductivity compared to the comparative examples 17 and 18. Additionally, production costs for materials according to the invention are reduced due to the reduced amount of MTMO.

4. Influence of Particle Properties on Conductivity

(55) TABLE-US-00010 TABLE 10 Conductivity of silver-coated Aluminum particles with varying particle properties (silver content, particle size, dielectric layer, silver ion coordinating layer). Silver Prepa- Silver ion ration ion coordi- according coordi- nating to d.sub.50/ SiO.sub.2/ nating layer/ Ag/ R.sub.spec/ Ex. section: μm wt.-% layer wt.-% wt.-% mΩcm comparative 1.1.1 20 0 0 15 811 example 1 comparative 1.1.2 MTMO 0.5 15 5.0 example 2 comparative 1.1.3.2 45 2 0 10 1.7 example 6 comparative 15 1.0 example 7 example 24 1.2.1.2 MTMO 0.5 5 5.5 example 25 MTMO 10 1.6 example 26 MTMO 15 0.9 example 27 MTMO 20 0.9 comparative 1.1.3.2 20 2 0 15 4.0 example 3 comparative 20 1.5 example 4 comparative 30 1.0 example 5 example 28 1.2.1.2 MTMO 0.5 15 1.4 example 29 MTMO 20 0.9 example 30 MTMO 30 0.7 comparative 1.1.3.2 5 2 0 15 n.a. example 8 comparative 20 n.a. example 9 comparative 30 22.8 example 10 comparative 40 1.7 example 11 example 31 1.2.1.2 MTMO 0.5 15 n.a. example 32 MTMO 20 14.3 example 33 MTMO 30 1.7 example 34 MTMO 40 1.0 example 39 1.2.2.2 5 2 AMEO 0.5 40 3.6 example 40 1.2.2.2 AMEO 1.0 40 3.5 example 41 1.2.2.2 AMEO 2.0 40 3.9 example 42 1.2.2.2 AMEO 4.0 40 1.9 example 45 1.2.3.2 5 2 VPSI363 1.0 40 2.2 example 46 1.2.3.2 VPSI363 4.0 40 2.0 n.a. = powder resistivity too high to be measured with measurement device = material not conductive.
Discussion of Results Referring to Tables 9 and 10:

(56) The comparative examples 1 and 2 essentially reproduced the findings of Wang et al. in the sense that the conductivity of silver coated aluminum powder can be significantly increased by the use of an interlayer of mercapto silane (0.5% MTMO); see FIG. 1B. Comparing the SEM pictures of the particles of FIGS. 2A and B a smoother Ag-layer can be observed in case of FIG. 2B, i.e. the interlayer pigment.

(57) A comparison of the conductivities of comparative examples 1 and 3 show a significant increase of the conductivity by the silica interlayer (see table 10). Comparing the SEM pictures of FIG. 2A (comp. example 1) to FIG. 2C (comp. example 3) show a more homogeneous coating of the Al/SiO.sub.2/Ag system.

(58) If one compares the conductivities of example 28 with the conductivities of the comparative examples 1 to 3 (all systems are 20 μm spherical Al-shot coated with 2 wt.-% SiO.sub.2 and 15 wt.-% Ag) it can be seen that the example 28 has the highest conductivity. Thus the interlayer coating of first silica and then mercaptosilane according to the present invention exhibits a synergetic effect on the conductivity. Additionally much less mercaptosilane is needed as in the experiments of Wang et al.

(59) Similar results are obtained for 20 μm Al-shot coated according to the method of Wang et al (comparative examples 17 and 18, see table 9) with a solely SiO.sub.2 coated Al-shot (comparative example 4) and the inventive example 29, which again exhibits the highest conductivity (0.9 mΩ cm) of this series. In comparative examples 17 and 18 the statement of Wang et al. that the ratio of the ethanol/water solvent in the coating step of the Al-core with MPTMS could be verified, but a much better conductivity was achieved with the inventive pigments.

(60) Comparing further the conductivities of the particles according to this invention with Ag/SiO.sub.2/Al systems always a higher conductivity is obtained for the inventive particles. Directly comparable samples in table 10 are for instance example 33 with comparative example 10 (5 μm Al-shot; 30 wt.-% Ag) and example 34 with comparative example 11 (5 μm Al-shot; 40 wt.-% Ag) and example 26 (45 μm Al-shot; 15 wt.-% Ag) with comparative example 7 (45 μm Al-shot; 15 wt.-% Ag). Comparing the SEM picture of FIG. 2D (coating according to this invention) with all other pictures from FIG. 2A to C the pigments according to FIG. 2D exhibit the most uniform coating with lowest amount of secondary precipitations.

(61) Due to its higher efficiency the silver content of the pigments according to this invention can be reduced. For example, the pigment of example 28 (15 wt.-% Ag) has a similar conductivity compared to comparative example 4. Likewise the pigment of example 29 (20 wt.-% Ag) has a similar conductivity compared to comparative example 5. This reduction of the silver content means a significant reduction of costs for the inventive pigments. The dependence of the powder resistivity on the amount of silver is shown for inventive particles having core shots with a d.sub.50=45 μm and 20 μm in FIG. 3. For the 20 μm core a significant non-linear dependence is seen with a lower resistance with increasing amount of silver. For the 45 μm shot no significant dependence is seen. Here the specific surface of the core material is the lowest and thus the critical thickness of the Ag layer to impart high conductivity is reached at lower amounts than in case of finer shots.

(62) Regarding fine pigments (d.sub.50=5 μm) the inventive example 32 (20 wt.-% Ag) had a conductivity of 14.6 mΩcm whereas no conductivity could be measured for the corresponding Al/SiO.sub.2/Ag system of comparative example 9. The strong dependence of the powder resistivity on the amount of silver is shown for inventive particles with a core shot of d.sub.50=5 μm in FIG. 4.