Patent Application
20190071789
The present invention relates to an electrolyte containing suitable reducing agents for adjusting the composition of silver-palladium layers. Furthermore, these reducing agents contribute to improving the layer appearance and to increasing the luminance (L value, CIE Lab) of the deposited layers. The present invention also discloses a method for the electrolytic deposition of silver-rich silver-palladium alloys.
Electrical contacts are used today in virtually all electrical appliances. Their applications range from simple plug connectors to safety-relevant, sophisticated switching contacts in the communications sector, for the automotive industry or for aerospace technology. Here the contact surfaces are required to have good electrical conductivity, low contact resistance with long-term stability, as well as good corrosion and wear resistance with insertion forces that are as low as possible. In electrical engineering, plug contacts are often coated with a hard-gold alloy layer, consisting of gold-cobalt, gold-nickel or gold-iron. These layers have a good resistance to wear, a good solderability, a low contact resistance with long-term stability, and good corrosion resistance. Due to the rising price of gold, less expensive alternatives are being sought.
As a substitute for hard-gold plating, coating with silver-rich silver alloys (hard silver) has proven advantageous. Silver and silver alloys are amongst the most important contact materials in electrical engineering, not just on account of their high electrical conductivity and good oxidation resistance. These silver-alloy layers have, depending on the metal that is added to the alloy, layer properties similar to those of currently used hard-gold layers and layer combinations, such as palladium-nickel with gold flash. In addition, the price for silver is relatively low compared with other precious metals, in particular hard-gold alloys.
One constraint on the use of silver is, for example, the fact that in atmospheres containing sulfur or chlorine silver has lower corrosion resistance than hard gold. Apart from the visible surface change, tarnishing films of silver sulfide in most cases do not represent any great danger since silver sulfide is semi-conductive, soft, and is easily wiped away during the insertion process provided contact forces are strong enough. Tarnishing films of silver chloride, on the other hand, are non-conductive, hard and not easily displaced. A relatively high proportion of silver chloride in the tarnishing layer thus leads to problems with the contact properties (literature: Marjorie Myers: Overview of the use of silver in connector applications; Interconnect & Process Technology, Tyco Electronics, Harrisburg, February 2009).
U.S. Pat. No. 3,980,531 discloses a cyanide-free electrolyte for the galvanic deposition of alloys containing gold, silver and/or palladium. The baths contain a thiosulfate, a sulfite and a borate or phosphate. The alloys are deposited in the weakly acidic to highly alkaline pH range. The electrolyte can optionally include the salts of base metals, such as arsenic or cadmium. Deposition takes place at current densities of 0.1 to 5 A/dm2. In the baths in accordance with U.S. Pat. No. 3,980,531, the composition of the deposited alloy depends on the concentrations of the metal salts used and the current density used. The appearance of the alloys varies from matt to high gloss. Due to the use of arsenic and cadmium, this electrolyte is no longer acceptable today on account of existing regulations (REACH).
U.S. Pat. No. 6,251,249 B1 discloses electrolytes for the deposition of precious metals onto solid substrates. These electrolytes are iodide-free and contain the precious metal to be deposited in the form of alkane sulfonates, alkane sulfonamides and/or alkane sulfonimides. In addition, the electrolytes contain an organosulfur compound and/or a carboxylic acid. The precious metals are preferably deposited in a temperature range of 20° C. to 60° C. The pH value can be between 0 and 12. The electrolytes are suitable for electroless and electrolytic deposition of precious metal layers, as well as for immersion plating. The examples in U.S. Pat. No. 6,251,249 B1 relate exclusively to immersion plating, and either silver or palladium is deposited, but no silver-palladium alloy. No information is provided about the electrolytic deposition of silver-palladium alloys or about their composition.
In EP 0 065 100 A1 a galvanic palladium electrolyte is described which contains palladium sulfite and an acid. The electrolyte contains sulfuric acid and/or phosphoric acid and can be used at 20° C. to 40° C. 80 to 95% of the palladium content can be added as palladium sulfate, the rest as palladium sulfite. However, EP 0 065 100 A21 is silent about the deposition of palladium alloys.
DE 10 2013 215 476 B3 discloses a cyanide-free, acidic and aqueous electrolyte for the deposition of silver-palladium alloys. In addition to silver and palladium salts, the electrolyte contains a selenium or tellurium compound, urea and/or at least one amino acid and a sulfonic acid. With this electrolyte, silver-palladium alloys with a predominantly silver content can be deposited across a wide current density range. However, only semi-matt alloy coatings can be produced with this electrolyte. With increasing current density, the layers produced exhibit a distinct brownish tinge. At the same time the electrolyte shows a marked dependence of the alloy composition on the current density applied. The alloy can only be influenced by shifting the concentration of the alloying metals or by varying the temperature of the electrolyte during deposition.
The electrolytes known from prior art for the electrolytic deposition of silver-palladium alloys do not allow the deposition of silver-palladium alloys which, over a wide current density range, are not only highly glossy but also have a constant ratio of silver to palladium. Similarly, the alloy composition can only be adjusted to a very limited extent by shifting the bath parameters. In previously known baths the palladium content in the deposited layers decreases as current density rises. The appearance of the deposited layers changes at the same time: as the current density increases, the layers take on an increasingly marked brownish tinge. Inhomogeneities in the layer, such as haze and speckles, increase at the same time.
Despite the large number of electrolytes already known for the electrolytic deposition of silver-palladium alloys, there is, therefore, still a need for electrolytes which in practical use are superior to prior-art electrolytes. Such electrolytes should be stable enough for industrial use and permit the deposition of stable and bright alloy compositions over the widest possible current-density range. Simple adjustment of the alloy composition is equally important. The electrolytes should remain fully functional even after a high current density load, and the layers deposited with these electrolytes should be homogeneous and advantageous with regard to use in contact materials. The composition of the deposited alloy is especially advantageously 90±3 wt % silver, 10±3 wt % palladium and 0-3 wt % tellurium and/or selenium.
These and other problems obviously arising for the person skilled in the art from the closest pertinent prior art are solved by an electrolyte according to the present claim 1. Protection is sought for further preferred embodiments in the subordinate claims dependent on claim 1. Claim 9 relates to a preferred method for the deposition of silver-palladium alloys in which the electrolyte according to the invention is used. Claims 10 to 12 relate to preferred embodiments of the present process.
The problem of providing a cyanide-free, acidic and aqueous electrolyte for the electrolytic deposition of bright silver-palladium alloys with a predominantly silver content is solved according to the invention by an aqueous electrolyte which in its dissolved form contains the following components:
e) at least one sulfonic acid in a concentration of 0.25-4.75 mol/l, based on the total amount of sulfonic acids,
Surprisingly, it was found that with the electrolytes described here it is possible over a wide current density range to deposit on conductive substrates homogeneous and bright silver-palladium alloy layers which are eminently suitable for use in contact materials. As a result, the electrolytes according to the invention are suitable as a substitute for hard-gold alloys in contact materials. At the same time, the palladium content in the layer as a function of the amount of the reducing agent added can be simply adjusted by means of the reducing agents (brighteners) added. With increasing concentration of the reducing agents the palladium content of the deposited layer increases. The electrolyte according to the invention here shows comparatively high stability, which makes it appear especially advantageous in industrial application. With the present electrolytes, high-quality electrical contact materials can be advantageously produced even in rack and high-speed coating systems. The electrolyte preferably contains only the above constituents.
The electrolyte according to the invention can be used in a current density range of 0.1 to 100 A/dm2. A current density range of 0.5 to 20 A/dm2 is preferred.
In the present invention ‘homogeneous’ silver-palladium alloy coatings means such layers whose appearance is uniform as regards color and layer properties. Layer properties in this case are gloss, brightness, hardness, and corrosion resistance. The silver-palladium alloy layers are here homogeneous in two respects. Firstly, the silver-palladium alloy layer deposited on a particular electrically conductive substrate is homogeneous according to the above definition. Secondly, the appearance of the deposited silver-palladium alloys is homogeneous when layers are deposited on a plurality of identical electrically conductive substrates with different current densities from the same electrolyte, without change in the composition of the electrolyte, in temperature or in movement, said layers having an identical alloy composition and an identical visual appearance—in other words, the deposited layers are in this case homogeneous irrespective of the current density.
It is known to the person skilled in the art that the color and brightness of metallic coatings can be determined with the aid of the so-called L*a*b* measurement according to CIEL*a*b (www.cielab.de), wherein the L* value indicates brightness. The brightness values (L* values) of the silver-palladium alloy layers according to the invention lie between 80 and 90 L*a*b* (measuring instrument X-Rite SP62, illuminant D65/10).
Gloss can be assessed by measuring the reflectivity. In the silver-palladium alloy layers according to the invention the addition of the reducing agents causes a rise in reflectivity of 5-40% of the initial value depending on the current density applied and the concentration of the reducing agent. The reflectivity was measured with the BYK-Gardner micro-TRI-gloss meter. Measurement was carried out at a 20° angle of incidence and a 20° angle of reflection of the light beam according to EN ISO 7668. Measurement of the gloss of surfaces is known to the person skilled in the art and information in this regard may be found in, for example, ‘Schriftenreihe Galvanotechnik und Oberflächenbehandlung. Prüfung von funktionellen metallischen Schichten [Publication series: Electroplating and surface treatment: Inspecting functional metal coatings], Section 4.3: Glanz-und Reflexionsmessung an Oberflächen’ [Gloss and reflection measurement of surfaces], Eugen G Leuze-Verlag, Saulgau, 1st ed. 1997, pp. 117-125.
Galvanic baths are solutions containing metal salts from which electrochemically metallic precipitates (coatings) can be deposited on substrates (objects). Galvanic baths of this kind are often also termed ‘electrolytes’. Accordingly, the cyanide-free and aqueous galvanic baths according to the invention are hereinafter referred to as ‘electrolytes’.
The electrolyte according to the invention for the electrolytic deposition of bright homogeneous silver-palladium alloys with a predominant content of silver, and also the method for depositing such silver-palladium alloys are explained below, wherein the invention comprises all embodiments listed below, either individually or in combination with each other.
The person skilled in the art will generally be familiar with the metal compounds which can be added to the electrolyte.
The silver compound contained in the electrolyte according to the invention is preferably a silver salt which is soluble in this electrolyte. The silver salts are here preferably selected from the group consisting of silver methanesulfonate, silver carbonate, silver sulfate, silver phosphate, silver pyrophosphate, silver nitrate, silver oxide, silver lactate, silver fluoride, silver bromide, silver chloride, silver iodide, silver azide, silver sulfide and silver sulfate. Silver nitrate, silver carbonate, silver methanesulfonate, silver chloride and silver oxide are particularly preferably used in the electrolyte according to the invention. Here the person skilled in the art should be guided by the principle that as few additional substances as possible should be added to the electrolyte. For this reason, the person skilled in the art will give the utmost preference to selecting silver methanesulfonate, silver carbonate or silver oxide as the silver salt to be added. As regards the concentration of the silver compound employed, the person skilled in the art should be guided by the limit values given above. The silver compound in a concentration of 1-300 g/l of silver is preferable, more preferable is 2-100 g/l of silver and most preferable is between 4-15 g/l of silver in the electrolyte.
The palladium compound to be employed is preferably also a salt soluble in the electrolyte or a soluble complex. The palladium compound used here is preferably selected from the group consisting of palladium hydroxide, palladium chloride, palladium sulfate, palladium pyrophosphate, palladium nitrate, palladium phosphate, palladium bromide, palladium P salt (diammine dinitrito palladium (II); ammoniacal solution), palladium glycinates, palladium acetates, tetramminepalladium (II) chloride, tetramminepalladium (II) bromide, palladium methanesulfonate, diamminedinitropalladium (II) chloride, diamminedinitropalladium (II) bromide, diamminedinitropalladium (II) sulfate, potassium di-oxalatopalladate, palladium iodide, tetramminepalladium (II) sulfate, bis (ethylenediamino) palladium (II) bromide, bis (acetylacetonato) palladium (II), diammine dichloropalladium (II), palladium oxide hydrate, tetramminepalladium (II) hydrogen carbonate, bis(ethylenediamine) palladium (II) chloride, bis(ethylenediamine) palladium (II) sulfate and bis(ethylenediamine) palladium (II) carbonate. The palladium compound is advantageously selected from palladium hydroxide, palladium chloride, palladium glycinate, palladium methanesulfonate and palladium sulfate.
The palladium compound is in this case added to the electrolyte in a concentration as indicated above. The palladium compound is preferably used in a concentration of 0.1 to 100 g/l palladium, most preferably in a concentration of 2-20 g/l palladium in the electrolyte.
The electrolyte according to the invention is aqueous. The silver and palladium compounds to be employed are preferably salts soluble in the electrolyte or soluble complexes. The terms ‘soluble salt’ and ‘soluble complex’ therefore refer to such salts and complexes as dissolve in the electrolyte at the working temperature. Here the working temperature is that temperature at which the silver-palladium alloy is deposited. In the context of the present invention, a substance is deemed soluble when at least 0.002 g/l of this substance dissolves in the electrolyte at the working temperature.
The deposited alloys, which contain silver, palladium and also selenium and/or tellurium, here have a composition comprising 70-99 wt % silver, 1-30 wt % palladium and 0.1-5 wt % selenium and/or tellurium. The proportions of silver, palladium and selenium and/or tellurium here add up to 100 wt %. According to the invention the concentrations in the electrolyte of the metals to be deposited are set within the framework given above in such a way that the result is a silver-rich alloy. It should be noted that not only does the concentration of the metals to be deposited have an influence on the silver concentration and brightness of the deposited alloy but so also do the current density set, the quantity of tellurium compound and/or selenium compound used, and the addition of the reducing agents. The person skilled in the art will know how the corresponding parameters must be set in order to obtain the target alloy desired, or will be able to determine this by routine experimentation. Efforts are preferably made to obtain an alloy in which silver has a concentration of 70-99 wt %, more preferably 80-95 wt % and most preferably 87-94 wt %. The palladium content of the alloys according to the invention is 1-30 wt %, preferably 5-20 wt % and particularly preferably 6-13 wt %. The selenium or tellurium content of the alloy according to the invention is 0.1-5 wt %, preferably 0.5-4 wt % and particularly preferably 1-3 wt %.
The alloys according to the invention which contain silver, palladium and also selenium and/or tellurium are hereinafter referred to as ‘silver-palladium alloys’.
The selenium or tellurium compound which is used in the electrolyte can be appropriately selected by the person skilled in the art within the framework of the concentrations indicated above. A concentration of 0.002-10 g/l tellurium and/or selenium can be selected as the preferred concentration range and a concentration of 0.1-5 g/l tellurium and/or selenium as the most preferred range. The concentration data here relate to the total amount of tellurium and selenium in the electrolyte. Suitable selenium and tellurium compounds are those in which selenium or tellurium is present in oxidation states +4 or +6. Selenium and tellurium compounds are advantageously used in the electrolyte in which selenium or tellurium in oxidation state +4 is present. The selenium and tellurium compounds are particularly preferably selected from tellurites, selenites, tellurous acid, selenious acid, telluric acid, selenium acid, selenocyanates, tellurocyanates and selenate and tellurate. The use of tellurium compounds rather than selenium compounds is generally preferred here. More particularly preferable is the addition of tellurium to the electrolyte in the form of a salt of the tellurous acid in, for example, the form of potassium tellurite.
The electrolyte according to the invention contains a compound selected from the group consisting of urea, urea derivatives, thiourea, thiourea derivatives and mixtures thereof and/or one or more α-amino acids which serve as complexing agents for the palladium and contribute to increasing the stability of the present electrolyte.
Urea derivatives are selected from dimethyl urea, ethylene urea, N, N′-dimethyl propylene urea, and N-(2-hydroxyethyl) ethylene urea. The thiourea derivatives are, for example, 3-S-isothiuronium propane sulfonate and n-ethyl thiourea.
In one advantageous embodiment, component (d) of the electrolyte according to the invention, that is, the complexing agent for the palladium, is urea.
The one or more α-amino acids are here selected from the group consisting of alanine, aspartic acid, cysteine, glutamine, glutamic acid, glycine, lysine, leucine, methionine, phenylalanine, phenylglycine, proline, serine, tyrosine and valine. Preferably the amino acids used here are those which have only alkyl groups in the variable residue. In one advantageous embodiment, the α-amino acid is selected from alanine, glycine and valine. The use of glycine and/or alanine is most preferable.
Urea, urea derivatives, thiourea, thiourea derivatives and mixtures thereof are used in a concentration of 0.05 to 2 mol/l, preferably 0.2 to 1.5 mol/l, based on the total amount of urea and urea derivatives in the electrolyte. The concentration of the one or more α-amino acids in the electrolyte according to the invention is here 0.005 to 0.5 mol/l, preferably 0.01-0.2 mol/l. In the case of α-amino acids, these concentration data refer to the total amount of α-amino acid or α-amino acids, regardless of whether the electrolyte contains one or more α-amino acids.
Within the concentration framework given above the person skilled in the art can freely select the optimum concentration for the amino acid used. He will be guided by the fact that if the quantity of amino acid is too small it will not produce the stabilizing effect desired, while too high a concentration can inhibit the deposition of palladium.
The electrolyte according to the invention is used within an acidic pH range. Optimal results can be obtained with pH values of <2 in the electrolyte. The person skilled in the art will know how he can set the pH value of the electrolyte. He will be guided by the idea of introducing as few additional substances into the electrolyte as possible which could adversely affect the deposition of the alloy in question. In one most preferable embodiment the pH value is determined solely by the addition of sulfonic acid. This then preferably yields strongly acidic deposition conditions where the pH value is less than 1 and possibly may even reach 0.1 or even in marginal cases 0.01. In the optimum case the pH value will be 0.3-0.6.
In the electrolyte according to the invention, at least one sulfonic acid is used in addition in a concentration of 0.25-4.75 mol/l, wherein the concentration is based on the total amount of sulfonic acids used. The concentration is preferably 0.5-3 mol/1 and most preferably 0.8-2.0 mol/l. The at least one sulfonic acid serves firstly to establish an appropriate pH value in the electrolyte. Secondly, its use leads to a further stabilization of the electrolyte according to the invention. The upper limit of the sulfonic acid concentration is due to fact that at too high a concentration only silver is deposited. In principle the sulfonic acids known to the person skilled in the art for use in electroplating technology can be used. Sulfonic acids are preferably selected from the group consisting of ethanesulfonic acid, propanesulfonic acid, benzenesulfonic acid, and methanesulfonic acid. Here they can be used individually or as mixtures. Propanesulfonic acid and methanesulfonic acid are more particularly preferred in this context. Most particularly preferred is methanesulfonic acid.
The at least one reducing agent is selected from formic acid, oxalic acid, ascorbic acid, hydrazine, hexamethylenetetramine, salts and/or esters of sulfurous acid, gaseous sulfites, sulfinic acids and their salts and/or esters, formaldehyde, sodium formaldehyde sulfoxylate, benzaldehyde, benzaldehyde derivatives, hydroxybenzenes and their esters, polyphenols and their esters, phenolsulfonic acids and their salts and/or esters, and glutathione and also its salts and/or esters.
In one advantageous embodiment, the reducing agent is selected from hydroxybenzolenes, sodium formaldehyde sulfoxylate and ascorbic acid.
In another advantageous embodiment, the reducing agent is selected from salts and/or esters of sulfurous acid.
The salts of sulfurous acid can be sulfites or hydrogen sulfites. The sulfites and hydrogen sulfites are advantageously lithium, sodium, potassium, or ammonium salts.
The esters of sulfurous acid are compounds with the general formula R1-O—S(═O)—O—R2, where R1 and R2 are independently selected from linear or branched acyclic alkyl groups with 1 to 10 carbon atoms, cyclic alkyl groups with 3 to 10 carbon atoms, aryl groups and benzyl groups. Within the context of the present invention the linear or branched acyclic alkyl groups with 1 to 10 carbon atoms are selected from methyl, ethyl, n-propyl, isopropyl, 1-butyl, 2-butyl, tert-butyl, 1-pentyl, 2-pentyl, 3-pentyl, 3-methylbutyl, 2,2-dimethylpropyl and all isomers of hexyl, heptyl, octyl, nonyl, and decyl. It is known to the person skilled in the art that cyclic alkyl groups must contain at least three carbon atoms. In the context of the present invention, cyclic alkyl groups will advantageously include propyl, butyl, pentyl, hexyl, heptyl and octyl rings. A cyclic alkyl group for the purpose of the present invention is selected from the aforenamed cyclic alkyl groups which carry no other substituents, and from the aforenamed cyclic alkyl groups which for their part are bound to one or more acyclic alkyl groups. In the latter case, the cyclic alkyl group can according to the above formula be bound to the oxygen atom via a cyclic or an acyclic carbon atom of the cyclic alkyl group. According to the definition given above of the term ‘alkyl group’, cyclic alkyl groups also contain a maximum of 10 carbon atoms. If, in the case of groups R1 and R2, an aryl group is concerned, this will be selected from phenyl, naphthyl and anthracenyl.
In the case of gaseous sulfites, the gas which is introduced into the electrolyte is SO2.
The sulfinic acids are compounds with the general formula R3-S(═O)—OH, where R3 is a linear or branched acyclic alkyl group with 1 to 10 carbon atoms, a cyclic alkyl group with 3 to 10 carbon atoms, an aryl or benzyl group and where these groups are defined as described above for R1 and R2.
Benzaldehyde derivatives are selected from benzaldehyde sulfonic acid, its salts and esters, for example, benzaldehyde-2-sulfonic acid sodium salt, dimethylaminobenzaldehyde, 3-chlorobenzaldehyde, 4-chlorobenzaldehyde, 2-methoxybenzaldehyde, 2-methyl benzaldehyde, 2-nitrobenzaldehyde, 3,5-dibromobenzaldehyde, 3-nitrobenzaldehyde and 3.5-dimethoxybenzaldehyde.
Hydroxybenzenes are selected from phenol, catechol, resorcinol, hydroquinone, pyrogallol, hydroxyquinone and phloroglucinol.
If the at least one reducing agent is a salt of an organic compound, a sodium, potassium, lithium or ammonium salt will be advantageously selected. In the case of organic acids with multiple protons, a single, several or all acidic hydrogen atoms can be replaced by sodium, potassium, lithium, or ammonium ions. If more than one acidic hydrogen atom is replaced by sodium, potassium, lithium, or ammonium ions, these cations can be identical or different.
In the case of the at least one reducing agent, this may also be an ester of an organic compound. It is known to the person skilled in the art that esters are the condensation products of an alcohol and a carboxylic acid. Esters of the alcohols in the list given above of suitable reducing agents are therefore a condensation product of one of the aforementioned alcohols and a carboxylic acid R4-COOH, and esters of carboxylic acids in the list given above are condensation products of one of the aforementioned carboxylic acids with an alcohol R5-OH.
Here R4 and R5 are selected from linear or branched acyclic alkyl groups with 1 to 10 carbon atoms, cyclic alkyl groups with 3 to 10 carbon atoms, and aryl or benzyl groups, where these groups are defined as described above for R1 and R2.
Particularly advantageously the at least one reducing agent is selected from salts or esters of sulfurous acid and gaseous sulfites.
The at least one reducing agent is contained in the electrolyte in a concentration of 1-100 mmol/l, advantageously in a concentration of 5-30 mmol/l, wherein the concentration is based on the total amount of the aforementioned reducing agents in the electrolyte.
The electrolyte according to the invention furthermore contains at least one sulfonic acid in a concentration of 0.25-4.75 mol/l. The concentration is preferably 0.5-3 mol/1 and most preferably 0.8-2.0 mol/l. The at least one sulfonic acid serves firstly to establish an appropriate pH value in the electrolyte. Secondly, its use leads to a further stabilization of the electrolyte according to the invention. The upper limit of the sulfonic acid concentration is due to the fact that at too high a concentration only silver is deposited. The sulfonic acids have the general molecular formula R6-S(═O)2-OH, wherein R6 represents a linear or branched acyclic alkyl group with 1 to 10 carbon atoms, a cyclic alkyl group with 3 to 10 carbon atoms, or an aryl or benzyl group, wherein these groups are defined as described above for R1 and R2. Sulfonic acids are preferably selected from the group consisting of methanesulfonic acid, ethanesulfonic acid, propanesulfonic acid and benzenesulfonic acid. Methanesulfonic acid and propanesulfonic acid are more particularly preferred in this context. Most particularly preferred is methanesulfonic acid.
Optionally, the electrolyte according to the invention can additionally contain a surfactant. This surfactant is selected from anionic and non-ionic surfactants. Examples include polyethylene glycol adducts, fatty alcohol sulfates, alkyl sulfates, alkyl sulfonates, aryl sulfonates, alkylaryl sulfonates and heteroarylsulfonates, betaines, fluorosurfactants and their salts and derivatives. Suitable surfactants are known to the person skilled in the art, for example, as in N. Kanani: Galvanotechik [Electroplating], Hanser-Verlag, Munich and Vienna, 2000, pp. 84 ff. Before the addition of a surfactant the electrolyte according to the invention has a surface tension greater than or equal to 70 mN/m. If a surfactant is added, its concentration will be advantageously chosen such that the surface tension of the electrolyte decreases to a value less than or equal to 50 mN/m. The surface tension can be measured with a bubble pressure tensiometer.
In a further embodiment, the present invention relates to a method for the electrolytic deposition of silver-palladium alloys with a predominant content of silver from an electrolyte according to the invention, wherein an electrically conductive substrate is immersed in the electrolyte and a flow of current established between an anode in contact with the electrolyte and the substrate as cathode. It should be noted that the embodiments mentioned as preferable for the electrolyte apply mutatis mutandis to the method addressed here.
The temperature prevailing during the deposition of the silver-palladium alloy can be selected as desired by the person skilled in the art. He will be guided on the one hand by an adequate deposition rate and an applicable current density range and on the other hand by cost aspects or the stability of the electrolyte. A temperature of 25° C. to 75° C. is set advantageously in the electrolyte, preferably between 30° C. and 65° C. The use of the electrolyte at a temperature between 45° C. and 55° C. appears more particularly preferable.
The current density which is established in the electrolyte between the cathode and the anode during the deposition process can be selected by the person skilled in the art according to the efficiency and quality of deposition. Depending on the application and coating plant type, the current density in the electrolyte is advantageously set to 0.1 to 100 A/dm2. If necessary, current densities can be increased or reduced by adjusting the system parameters, such as the design of the coating cell, flow rates, the anode or cathode set-ups, and so on. A current density of 0.5-20 A/dm2 is advantageous, 1-20 A/dm2 is preferable, and 1.5-15 A/dm2 most preferable.
As has already been indicated, the electrolyte according to the invention is an acidic type. The pH value should preferably be <2, and particularly preferably <1. It may be the case that fluctuations occur in the pH value of the electrolyte during electrolysis. In one preferred embodiment of the present method the person skilled in the art will therefore take steps to monitor the pH value during electrolysis and, if necessary, adjust it to the setpoint value.
Various anodes can be employed when using the electrolyte. Soluble or insoluble anodes are just as suitable as the combination of soluble and insoluble anodes. If a soluble anode is used, a silver anode is particularly preferred.
As insoluble anodes preference is given to those made of a material selected from the group consisting of platinized titanium, graphite, iridium-transition metal mixed oxide and special carbon material (DLC or diamond-like carbon) or combinations of these anodes. Particularly preferred for implementation of the invention are mixed-oxide anodes composed of iridium-ruthenium mixed oxide, iridium-ruthenium-titanium mixed oxide or iridium-tantalum mixed oxide. More particularly preferred are platinum-titanium anodes. More information may be found in Cobley, A. J et al. (The use of insoluble anodes in acid sulphate copper electrodeposition solutions, Trans IMF, 2001, 79(3), pp. 113 and 114).
The present invention presents a silver-palladium alloy electrolyte with an added reducing agent for alloy adjustment and as a brightener and also for the electrolytic deposition of silver-palladium layers, and a corresponding method. The electrolyte contains at least one reducing agent for alloy adjustment and brightening: by adding the at least one reducing agent the palladium content of the deposited silver-palladium alloy can be adjusted. As already indicated above in the text, the alloys deposited according to the invention have a composition comprising 70-99 wt % silver, 1-30 wt % palladium and 0.1-5 wt % selenium and/or tellurium, wherein the proportions of silver, palladium and selenium and/or tellurium add up to 100 wt %. In addition, the electrolyte according to the invention leads to a more homogeneous deposition in comparison with conventional silver-palladium alloy electrolytes.
Layers deposited from conventional silver-palladium electrolytes have, depending on the current density applied, L* values of 67-78. With the new electrolyte system according to the invention, markedly higher L* values are achieved for the deposited layers which are also uniform over the current density range applied. These values lie between 80 and 90, depending on the reducing agent used.
Against the background of the known prior art, this was not to be expected.
Various basic electrolytes were prepared and in each case a reducing agent in two different concentrations added. From these electrolytes, both with and without a reducing agent, silver-palladium layers were then deposited, characterized and compared to each other.
Basic Electrolyte:
100 ml/l methanesulfonic acid 70%
3 g/l glycine
10 g/l palladium (as palladium hydroxide)
5 g/l silver (as silver nitrate)
0.5 g/l tellurium (as tellurous acid)
Reducing Agents:
Temperature: 30° C.
Anodes: PtTi
The palladium content of the deposited layers was measured using an X-ray fluorescence analysis method (XRF) (Fischerscope XDV-SDD, software WIN-FTM Version 6.28-S-PDM).
Measurement Results for Palladium Content:
The brightness of the deposited layers was measured in the form of the L* value according to CIEL*a*b.
Measurement Results:
Basic Electrolyte:
80 ml/l methanesulfonic acid 70%
5 g/l urea
10 g/l palladium (as palladium chloride)
6 g/l silver (as silver methanesulfonate)
1.0 g/l tellurium (as potassium tellurite)
Reducing Agents:
Temperature: 60° C.
Anodes: PtTi
The palladium content of the deposited layers was measured using an X-ray fluorescence analysis method (XRF).
Measurement Results for Palladium Content:
The brightness of the deposited layers was measured in the form of the L* value according to CIEL*a*b.
Measurement Results for Brightness:
Basic Electrolyte:
100 ml/l methanesulfonic acid 70%
5 g/l valine
12 g/l palladium (as palladium hydroxide)
25 g/l silver (as silver nitrate)
1.5 g/l tellurium (as tellurous acid)
Reducing Agents:
Temperature: 60° C.
Anodes: Graphite
The palladium content of the deposited layers was measured using an X-ray fluorescence analysis method (XRF).
Measurement Results for Palladium Content:
The brightness of the deposited layers was measured in the form of the L* value according to CIEL*a*b.
Measurement Results for Brightness:
Basic Electrolyte:
200 ml/l methanesulfonic acid 70%
2 g/l glycine
15 g/l palladium (as palladium sulfate)
8 g/l silver (as silver carbonate)
0.5 g/l tellurium (as tellurous acid)
Reducing Agents:
Temperature: 40° C.
Anodes: PtTi
The palladium content of the deposited layers was measured using an X-ray fluorescence analysis method (XRF).
Measurement Results for Palladium Content:
The brightness of the deposited layers was measured in the form of the L* value according to CIEL*a*b.
Measurement Results for Brightness:
| Number | Date | Country | Kind |
|---|---|---|---|
| 15190885.2 | Oct 2015 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2016/075096 | 10/19/2016 | WO | 00 |
