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Flaky copper powder, method for producing the same, and conductive pasteRelated Patent Categories: Specialized Metallurgical Processes, Compositions For Use Therein, Consolidated Metal Powder Compositions, And Loose Metal Particulate Mixtures, Compositions, Loose Particulate Mixture (i.e., Composition) Containing Metal ParticlesFlaky copper powder, method for producing the same, and conductive paste description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070209475, Flaky copper powder, method for producing the same, and conductive paste. Brief Patent Description - Full Patent Description - Patent Application Claims
TECHNICAL FIELD [0001] The present invention relates to a flaky copper powder, a method for producing the same and a conductive paste. More specifically a flaky copper powder useful as a raw material for a copper paste which is used for securing electrical continuity of electrical circuits on printed circuit substrate and external electrodes of ceramic condensers and the like, a method for producing the same and a conductive paste. BACKGROUND ART [0002] One of the convertible methods for forming electrodes or circuits in an electronic device or the like is, that printing a conductive paste in which a copper powder is dispersed as a conductive material on a substrate and then hardening by calcining or curing the paste to obtain electrodes or circuits. [0003] Recently, as the markets have required downsizing and higher density for electronic devices for multi functional electronic devices, so, a copper powder for a conductive paste is required to have distribution sharper in particle size with finer size so that it gives a well filling ability conductive paste. [0004] A copper powder for a conductive paste may be oxidized on contacting oxygen, e.g., in evaporation of volatile contents from the conductive paste at the calcining, in particular. Copper oxidation is an undesirable phenomenon, because it leads high resistance of a thick copper film when it is used as conductive paste. Therefore, the copper powder is preferred to have excellent resistance to oxidation. It is said that the resistance can be enhanced by increasing a diameter to decrease a grain boundaries of crystallites in the powder. Therefore it is preferable to increase the diameter of the crystallite in the copper powder as large as possible. [0005] One of a substrate on which a conductive paste is printed is a ceramic material, for use of IC packages, where large quantities of heat are generated. However, a ceramic substrate generally differs in thermal expansion rate coefficient from a thick copper film formed with a conductive paste, with the result that troubles, e.g., separation of the film from the substrate and deformation of the substrate itself, may occur when the paste is printed on the substrate at the calcinations process. Therefore, thermal expansion values are preferably as close to each other as possible. [0006] One of the causes for thermal expansion of the thick copper film by the calcinations may be minimized space of the copper particles with each other by sintering, which are left between the copper particles in evaporation of volatile contents from the conductive paste. It is therefore necessary to minimize the spaces remaining among the copper particles as small as possible in order to have a copper-containing conductive paste of low thermal expansion values. In other words, the copper particles preferably have a shape which makes it easy to fill the paste densely. It is also preferable that the copper particles in the conductive paste have a shape which increases contact area between them to improve conductivity of the thick copper film prepared by calcinating the conductive paste. Particle shape anisotropy decreases as it increases sphericity, or increases as it becomes flatter. Therefore, a flaky copper powder instead of spherical one has been studied to meet the copper particle requirements. [0007] Moreover, a flaky copper powder preferably has a sharper distribution particle size to be more dispersible in paste for a conductive paste. [0008] As discussed above, copper powder, in particular flaky copper powder, for conductive paste is preferably finer, sharper distribution in particle size and larger in crystallite diameter. [0009] Patent Document 1 (Japanese Patent Laid-Open No. 2003-119501) discloses flaky copper powder having the following characteristics; particle diameter: 10 .mu.m or less, SD/D.sub.50 ratio: 0.15 to 0.35 (where, SD is standard deviation of particle size distribution and D.sub.50 is 50% volumetric cumulative particle diameter), and aspect ratio: 0.3 to 0.7 (i.e., ratio of thickness of particles which constitute the powder to D.sub.50). The invention gives a flaky copper powder composed of fine, flat and flaky particles. [0010] [Patent Document 1]: Japanese Patent Laid-Open No. 2003-119501 (Page 2) DISCLOSURE OF THE INVENTION [0011] However, the flaky copper powder disclosed by Patent Document 1, although fine, is produced by breaking agglomerated copper particles and then deforming the broken particles by compression in a high-energy ball mill. As a result, it involves problems that it tends to be oxidized or strained during the compression deformation step and to have small crystallite diameter. [0012] It is therefore an object of the present invention to provide a flaky copper powder composed of fine particles, having a sharp distribution particle size, a large crystallite diameter and high oxidation resistance. It is another object to provide a conductive paste using the flaky copper powder. [0013] The inventors of the present invention have found, after having extensively studied to solve the above problems under these situations, that a P-containing flaky copper powder, in particular that composed of particles of specific shape, is suitable for a conductive paste. The inventors have also found that a flaky copper powder composed of small particle diameter, a large crystallite diameter, and high oxidation resistance, a sharp distribution particle size, can be produced without having to employ compression deformation treatment, like the method disclosed by Patent Document 1, by a wet process, adding a specific phosphoric acid and its salt in at least one of the steps, comprising a plurality of steps for reducing the copper ion (II) present in a copper salt in a raw material, achieving the present invention. [0014] The present invention provides a flaky copper powder which is characterized in that comprising P. [0015] The present invention also provides a flaky copper powder which is characterized in that comprising P at 10 to 200 ppm (D.sub.50 is a median diameter (.mu.m) calculated as 50% of volume cumulative distributions examined by a laser diffraction scattering particle size distribution measuring method). [0016] The flaky copper powder of the present invention preferably has an average particle diameter D.sub.50 of the powder is in the range from 0.3 to 7 .mu.m. [0017] In addition, the flaky copper powder of the present invention preferably has a crystallite diameter of the powder is 25 nm or more. [0018] The flaky particles which constitute the flaky copper powder of the present invention preferably have an aspect ratio (D.sub.1A/t), obtained by dividing an aspect ratio D.sub.1A/t of the powder, which is calculated by dividing an average particle diameter D.sub.1A (.mu.m) by a flaky copper particle thickness t (.mu.m), the average particle diameter D.sub.1A (.mu.m) and the thickness t (.mu.m) are measured from analyzing SEM images directly observed by a scanning electron microscope, is in the range from 2 to 50. [0019] Moreover, the flaky copper powder of the present invention preferably has an SD/D.sub.50 ratio of 0.45 or less, where D.sub.50 is a median diameter (.mu.m) calculated as 50% of volume cumulative distributions examined by a laser diffraction scattering particle size distribution measuring method. [0020] Still more, the flaky copper powder of the present invention preferably has a D.sub.90/D.sub.10 ratio of 3.0 or less, where D.sub.10 is diameter (.mu.m) at 10% of volume cumulative distributions and D.sub.90 is diameter (.mu.m) at 90% of volume cumulative distributions examined by a laser diffraction scattering particle size distribution measuring method, respectively, and SD is a standard deviation (.mu.m) of a particle size distribution examined by the same measuring method. [0021] As a method of the present invention for producing a flaky copper powder, a method comprising four steps: a first step of preparing an aqueous solution comprising a copper salt and a complexing agent; a second step of adding an alkali hydroxide to the aqueous solution to prepare a first slurry comprising cupric oxide; a third step of adding a first reducing agent which can reduce cupric oxide into cuprous oxide to the first slurry to prepare a second slurry comprising cuprous oxide; and a fourth step of adding a second reducing agent which can reduce the cuprous oxide into copper to the second slurry to provide a flaky copper powder, which is characterized in that phosphoric acid and its salt are added in at least one of the first to third steps and/or phosphoric acid and its salt are added in the second slurry in the fourth step is preferably adopted. Continue reading about Flaky copper powder, method for producing the same, and conductive paste... 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