EP3159435B1 - Additive for silver palladium alloy electrolytes - Google Patents

Description

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 the improvement of the layered appearance and the increase in the brightness (L value, CIE-Lab) of the deposited layers. The present invention also discloses a process for the electrodeposition of silver-rich silver-palladium alloys.

Electrical contacts are installed today in virtually all electrical devices. Their application ranges from simple plug-in connectors to safety-relevant, sophisticated switch contacts in the communications sector, for the automotive industry or the aerospace industry. Here are required by the contact surfaces good electrical conductivities, low and long-term stable contact resistance, and good corrosion and wear resistance with the lowest possible insertion forces. 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 good wear resistance, good solderability, low and long-term stable contact resistance and good corrosion resistance. Due to the rising gold price is looking for cheaper alternatives.

As a replacement for the hard gold coating, the coating with silver-rich silver alloys (hard silver) has proven to be advantageous. Also due to their high electrical conductivity and good oxidation resistance, silver and silver alloys are among the most important contact materials in electrical engineering. These silver alloy layers have, depending on the metal to be alloyed, similar layer properties as the previously used hard gold layers or layer combinations such. Palladium-nickel with gold flash. In addition, the price of silver is relatively low compared to other precious metals, especially hard gold alloys.

A limitation for the use of silver is, for example, the lower corrosion resistance of silver in sulfur and chlorine atmospheres compared to hard gold. tarnish made of silver sulfide are in addition to the visible surface change usually no great danger, since silver sulfide is semiconducting, soft and easily displaced by the wiping mating process with sufficient contact forces. In contrast, start-up layers of silver chloride are non-conductive, hard and not easily displaceable. Thus, a higher proportion of silver chloride in the tarnish layer 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, Feb. 2009 ).

The US 3,980,531 discloses a cyanide-free electrolyte for the electrodeposition of alloys containing gold, silver and / or palladium. The baths contain a thiosulphate, a sulphite and a borate or phosphate. The deposition of the alloys takes place in the slightly acidic to strongly alkaline pH range. Optionally, the electrolyte may contain salts of base metals such as arsenic or cadmium. The deposition takes place at current densities of 0.1 to 5 A / dm ² . According to the baths US 3,980,531 h 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 or cadmium, this electrolyte is no longer up-to-date due to applicable regulations (REACH).

The US 6,251,249 B1 discloses electrolytes for the deposition of precious metals on solids. These electrolytes are free of iodides and contain the noble metal to be deposited in the form of alkanesulfonates, alkanesulfonamides and / or alkanesulfonimides. Furthermore, the electrolytes contain an organosulfur compound and / or a carboxylic acid. The deposition of the noble metals is preferably carried out in a temperature range of 20 ° C to 60 ° C. The pH can be between 0 and 12. The electrolytes are suitable for electroless and electrolytic depositions of noble metal layers as well as for dip plating. The examples in the US 6,251,249 B1 refer exclusively to dip plating and deposit either silver or palladium but no silver-palladium alloy. There is no information on the electrolytic deposition of silver-palladium alloys and their nature.

In the EP 0 065 100 A1 is a galvanic, palladium sulfite and an acid-containing palladium electrolyte described. The electrolyte contains sulfuric and / or phosphoric acid and can be used at 20 ° C to 40 ° C. 80 to 95% of the palladium content is added as palladium sulfate, the remainder as palladium sulfite. However, that does EP 0 065 100 A21 no statements on the deposition of palladium alloys.

The DE 10 2013 215 476 B3 discloses a cyanide-free, acidic and aqueous electrolyte for 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. This electrolyte allows the deposition of predominantly silver-containing silver-palladium alloys over a large current density range. However, only semi-matt alloy coatings can be made with this electrolyte. With increasing current density, the generated layers show a distinct brownish color. At the same time, the electrolyte shows a significant dependence of the alloy composition on the applied current density. The alloy can only be influenced by concentration shift of the alloy metals or by variation of the electrolyte temperature during the deposition.

Known in the art electrolytes for the electrodeposition of silver-palladium alloys do not allow to deposit silver-palladium alloys, which are both high gloss over a large current density range and also have a constant ratio of silver to palladium. Likewise, the alloy composition can be adjusted only very limited by shifting the bath parameters. In the previously known baths, the proportion of palladium in the deposited layers decreases with increasing current density. At the same time, the appearance of the deposited layers changes: As the current density increases, the layers show an increasingly strong browning. At the same time, inhomogeneities, such as veils and stains, increase in the layer.

Thus, despite the numerous electrolytes already known for the electrolytic deposition of silver-palladium alloys, there continues to be a need to offer electrolytes which are superior in practical use to prior art electrolytes. For industrial use, such electrolytes should have sufficiently high stability and allow stable and light alloy compositions to be deposited over as wide a current density range as possible can. It is also important to be able to easily adjust the alloy composition. The electrolytes should remain fully functional even after high current density loading, and the deposits made with these electrolytes should be homogeneous and advantageous for use in contact materials. Particularly advantageously, the composition of the deposited alloy 90 ± 3 wt .-% silver, 10 ± 3 wt .-% palladium and 0 - 3 wt .-% tellurium and / or selenium.

These and other objects to be deduced from the closest prior art by those skilled in the art are solved by an electrolyte according to the present invention. Further preferred embodiments are sought in the dependent of claim 1 dependent claims. 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 subject method.

1. a) eine Silberverbindung in einer Konzentration von 1 - 300 g/l Silber;
2. b) eine Palladiumverbindung in einer Konzentration von 0,1 - 100 g/l Palladium;
3. c) eine Tellur- und/oder Selenverbindung in einer Konzentration von 0,002 - 10 g/l Tellur und/oder Selen, bezogen auf die Gesamtmenge von Tellur und Selen im Elektrolyten;
4. d) eine Verbindung ausgewählt aus der Gruppe Harnstoff, Harnstoffderivate, Thioharnstoff und Thioharnstoffderivate und Gemischen davon in einer Konzentration von 0,05 - 2 mol/l, bezogen auf die Gesamtmenge von Harnstoff, Harnstoffderivaten, Thioharnstoff und Thioharnstoffderivaten im Elektrolyten und/oder eine oder mehrere Aminosäuren, ausgewählt aus der Gruppe bestehend aus Alanin, Asparaginsäure, Cystein, Glutamin, Glutaminsäure, Glycin, Lysin, Leucin, Methionin, Phenylalanin, Phenylglycin, Prolin, Serin, Tyrosin und Valin in einer Konzentration von 0,005 - 0,5 mmol/l, bezogen auf die Gesamtmenge von Aminosäuren im Elektrolyten;
5. e) mindestens eine Sulfonsäure in einer Konzentration von 0,25 - 4,75 mol/l, bezogen auf die Gesamtmenge der Sulfonsäuren,
6. f) mindestens ein Reduktionsmittel, ausgewählt aus der Gruppe Ameisensäure, Oxalsäure, Ascorbinsäure, Hydrazin, Urotropin, Salzen und/oder Estern der schwefligen Säure, gasförmigen Sulfiten, Sulfinsäuren und deren Salzen und/oder Estern, Formaldehyd, Natriumformaldehydsulfoxylat, Benzaldehyd, Benzaldehydderivaten, Hydroxybenzolen und deren Estern, Polyphenolen und deren Estern, Phenolsulfonsäuren und deren Salzen und/oder Estern und Glutathion sowie dessen Salzen und/oder Estern

in einer Konzentration von 0,1 mmol/l - 1 mol/l, bezogen auf die Gesamtmenge dieser Reduktionsmittel.The object of providing a cyanide-free, acidic and aqueous electrolyte for the electrolytic deposition of bright, predominantly silver-containing silver-palladium alloys is achieved according to the invention by an aqueous electrolyte which has the following constituents in dissolved form:

1. a) a silver compound in a concentration of 1 - 300 g / l silver;
2. b) a palladium compound in a concentration of 0.1-100 g / l palladium;
3. c) a tellurium and / or selenium compound in a concentration of 0.002 to 10 g / l tellurium and / or selenium, based on the total amount of tellurium and selenium in the electrolyte;
4. d) a compound selected from the group of urea, urea derivatives, thiourea and thiourea derivatives and mixtures thereof in a concentration of 0.05 to 2 mol / l, based on the total amount of urea, urea derivatives, thiourea and thiourea derivatives in the electrolyte and / or one or more amino acids selected from the group consisting of alanine, aspartic acid, cysteine, glutamine, glutamic acid, glycine, lysine, leucine, methionine, phenylalanine, phenylglycine, proline, serine, tyrosine and valine in a concentration of 0.005-0 , 5 mmol / l, based on the total amount of amino acids in the electrolyte;
5. e) at least one sulfonic acid in a concentration of 0.25-4.75 mol / l, based on the total amount of sulfonic acids,
6. f) at least one reducing agent selected from the group consisting of formic acid, oxalic acid, ascorbic acid, hydrazine, urotropin, 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 its salts and / or esters

in a concentration of 0.1 mmol / l - 1 mol / l, based on the total amount of these reducing agents.

Surprisingly, it has been found that homogeneous and bright silver-palladium alloy layers can be deposited on electrically conductive substrates with the electrolyte described here over a wide current density range, which are excellent for use in contact materials. As a result, the electrolytes according to the invention are suitable as replacement material for hard gold alloys in contact materials. At the same time, the palladium content in the layer can be easily adjusted by the added reducing agents (brightener additives), depending on the amount of reducing agent added. As the concentration of the reducing agents increases, the palladium content in the deposited layer increases. In this case, the electrolyte of the invention exhibits a comparatively high stability, which makes it appear particularly advantageous in industrial applications. With the present electrolyte, high-quality electrical contact materials can advantageously also be produced in frame and high-speed coating systems. The electrolyte preferably contains only the constituents specified above.

The electrolyte according to the invention can be used in a current density range of 0.1 to 100 A / dm ² . A current density range of 0.5 to 20 A / dm ² is preferred.

By "homogeneous" silver-palladium alloy layers in the present invention are meant those layers whose appearance is uniform in color and layer properties. Layer properties are gloss, brightness, hardness and corrosion resistance. The silver-palladium alloy layers are homogeneous in two respects. On the one hand, the silver-palladium alloy layer deposited on a specific electrically conductive substrate is homogeneous according to the above definition. On the other hand, the appearance of the deposited silver-palladium alloys is homogeneous when depositing layers of the same electrolyte, without change of electrolyte composition, temperature and motion, on a plurality of the same electrically conductive substrates at different current densities, which have the same alloy composition and optical appearance have, regardless of the current density, the deposited layers are homogeneous in this case.

It is known to the person skilled in the art that 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), where the L * value is the brightness indicates. The brightness values (L * values) of the silver-palladium alloy layers according to the invention are between 80 and 90 L * a * b * (measuring device X-Rite SP62, type of light D65 / 10).

To assess the gloss, a measurement of the reflectivity can be used. In the case of the silver-palladium alloy layers according to the invention, the reflectivity is increased by 5 to 40% from the starting value as a function of the applied current density and the concentration of the reducing agent due to the addition of the reducing agents. The reflectivity was measured with the BYK Gardner - mirror TRI-gloss meter. The measurement was carried out at 20 ° incidence and 20 ° angle of incidence of the light beam) in accordance with EN ISO 7668. The gloss measurement of surfaces is known to the person skilled in the art and can be described, for example, in US Pat. Series Electroplating and Surface Treatment. Testing of Functional Metallic Layers, Chap. 4.3: Gloss and Reflection Measurement on Surfaces ", Eugen G. Leuze-Verlag, Saulgau, 1, 1997 Edition, pp. 117-125 "be looked up.

Galvanic baths are solutions containing metal salts from which electrochemical metallic precipitates (coatings) can be deposited on substrates (objects). Frequently, such galvanic baths are also referred to as "electrolytes". Accordingly, the cyanide-free and aqueous electroplating baths of the present invention will hereinafter be referred to as "electrolytes".

The electrolyte according to the invention for the electrolytic deposition of light, homogeneous and predominantly silver-containing silver-palladium alloys and the process for depositing such silver-palladium alloys are explained below, the invention encompassing all embodiments listed below individually and in combination with one another.

The metal compounds that can be added to the electrolyte are generally known to those skilled in the art.

The silver compound contained in the electrolyte according to the invention is advantageously a silver salt which is soluble in this electrolyte. The silver salts are 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 expert should be guided by the sentence that as few additional substances as possible should be added to the electrolyte. Therefore, the skilled person will most preferably choose as the silver salt to be added the silver methanesulfonate, the silver carbonate or the silver oxide. In the concentration of the silver compound used, the skilled person will have to orientate to the limits given above. The silver compound is preferably present in the electrolyte in a concentration of 1 to 300 g / l of silver, more preferably 2 to 100 g / l of silver, and most preferably between 4 to 15 g / l of silver.

Also, the palladium compound to be used is preferably used as the electrolyte-soluble salt or soluble complex. Preferably, the palladium compound used here is selected from the group consisting of palladium hydroxide, Palladium chloride, palladium sulfate, palladium pyrophosphate, palladium nitrate, palladium phosphate, palladium bromide, palladium P salt (diamminedinitritopalladium (II), 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 ), Diamminedichloro-palladium (II), palladium oxide hydrate, tetramminepalladium (II) bicarbonate, bis (ethylenediamine) palladium (II) chloride, bis (ethylenediamine) palladium (II) sulfate and bis (ethylenediamine) palladium (II ) carbonate. Advantageously, the palladium compound is selected from palladium hydroxide, palladium chloride, palladium glycinate, palladium methanesulfonate and palladium sulfate.

The palladium compound is added to the electrolyte in a concentration as indicated above. The palladium compound is preferably used in a concentration of 0.1-100 g / l of palladium, and the concentration is very preferably 2-20 g / l of palladium in the electrolyte.

The electrolyte according to the invention is aqueous. The silver and palladium compounds to be used are preferably salts soluble in the electrolyte or soluble complexes. The terms "soluble salt" and "soluble complex" therefore refer to those salts and complexes which dissolve in the electrolyte at the working temperature. The working temperature is the temperature at which deposition of the silver-palladium alloy takes place. In the context of the present invention, a substance is considered to be soluble if 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 selenium and / or tellurium, have a composition which comprises 70-99% by weight of silver, 1-30% by weight of palladium and 0.1-5% by weight of selenium and / or tellurium. The sum of the proportions of silver, palladium and selenium and / or tellurium is 100 wt .-%. According to the invention, the concentrations of the metals to be deposited in the electrolyte in the above-mentioned frame so that a silver-rich alloy results. It should be noted that in addition to the concentration of the metals to be deposited, the current density used, the amount of sulfonic acid used, the amount of added tellurium compound and / or selenium compound and the addition of the reducing agents have an influence on the silver concentration and the brightness of the deposited alloy. The person skilled in the art knows how to set the corresponding parameters in order to obtain the desired target alloy or can determine this by routine experiments. Preferably, an alloy is desired in which the silver has a concentration of 70-99% by weight, more preferably 80-95% by weight and most preferably 87-94% by weight. The palladium content of the alloys according to the invention is 1 to 30% by weight, preferably 5 to 20% by weight and more preferably 6 to 13% by weight. The selenium or tellurium content of the alloy according to the invention is 0.1-5 wt .-%, preferably 0.5 to 4 wt .-% and particularly advantageously 1-3 wt .-%.

Hereinafter, the alloys according to the invention which contain silver, palladium and selenium and / or tellurium are referred to as "silver-palladium alloys".

The selenium or tellurium compound which is used in the electrolyte can be selected appropriately by the person skilled in the art within the scope of the above-indicated concentration. As a preferred concentration range, a concentration of between 0.002 and 10 g / l of tellurium and / or selenium and most preferably between 0.1 and 5 g / l of tellurium and / or selenium can be selected. The concentration data relate to the total amount of tellurium and selenium in the electrolyte. Suitable selenium and tellurium compounds are those in which selenium or tellurium are present in the oxidation states +4 or +6. Selenium and tellurium compounds in the electrolyte are advantageously used in which selenium or tellurium in the oxidation state +4 are present. The selenium and tellurium compounds are particularly preferably selected from tellurites, selenites, telluric acid, selenious acid, telluric acid, selenic acid, selenocyanates, tellurocyanates and selenate, and also tellurate. The use of tellurium compound over selenium compounds is generally preferred. Very particular preference is given to adding the tellurium to the electrolyte in the form of a salt of the telluric acid, for example in the form of potassium tellurite.

The electrolyte according to the invention contains a compound selected from the group of urea, urea derivatives, thiourea, thiourea derivatives and mixtures thereof and / or one or more α-amino acids which serve as a complexing agent for the palladium and contribute to increasing the stability of the present electrolyte.

Urea derivatives are selected from dimethylurea, ethyleneurea, N, N'-dimethylpropyleneurea and N- (2-hydroxyethyl) ethyleneurea. Thiourea derivatives are, for example, 3-S-isothioroniumpropanesulfonate and N-ethylthiourea.

In an advantageous embodiment, component d) of the electrolyte according to the invention, i. the complexing agent for the palladium to urea.

The one or more α-amino acids are 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, in the present case, those amino acids are used which have only alkyl groups in the variable radical. In an advantageous embodiment, the α-amino acid is selected from alanine, glycine and valine. Very particularly preferred is the use of glycine and / or alanine.

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 0.005 to 0.5 mol / l, preferably 0.01 to 0.2 mol / l. In the case of α-amino acids, these concentrations 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.

In the concentration range given above, the skilled person can freely choose the optimum concentration for the amino acid used. He will be guided by the fact that a too small amount of amino acid does not lead to the desired stabilizing effect, while their use in too high a concentration can inhibit the deposition of palladium.

The electrolyte according to the invention is used in an acidic pH range. Optimal results can be achieved at pH values in the electrolyte of <2. The person skilled in the art knows how to adjust the pH of the electrolyte. He will be guided by the idea of introducing as few additional substances into the electrolyte as possible, which may have a negative effect on the deposition of the corresponding alloy. In a very particularly preferred embodiment, the pH is determined solely by the addition of the sulfonic acid. Strongly acidic deposition conditions then preferably result in which the pH is less than 1 and, where appropriate, even up to 0.1 in borderline cases can also reach up to 0.01. In the optimal case, the pH is around 0.3-0.6.

In addition, at least one sulfonic acid in a concentration of 0.25-4.75 mol / l is used in the electrolyte according to the invention, the concentration being based on the total amount of the sulfonic acids used. The concentration is preferably 0.5-3 mol / l and most preferably 0.8-2.0 mol / l. The at least one sulfonic acid serves on the one hand to establish a corresponding pH in the electrolyte. On the other hand, their 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. As sulfonic acid, sulfonic acids known in principle to those skilled in the art for use in electroplating can be used. Preferably, sulfonic acids are selected from the group consisting of ethanesulfonic acid, propanesulfonic acid, benzenesulfonic acid, methanesulfonic acid used. They can be used individually or as mixtures. Very particular preference is given to mentioning propanesulfonic acid and methanesulfonic acid in this context. Most preferably, methanesulfonic acid is used.

The at least one reducing agent is selected from formic acid, oxalic acid, ascorbic acid, hydrazine, urotropin, 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 its salts and / or esters.

In an advantageous embodiment, the reducing agent is selected from hydroxybenzenes, Na-formaldehydsulfoxylat and ascorbic acid.

In a further advantageous embodiment, the reducing agent is selected from salts and / or the esters of sulfurous acid.

The sulphurous acid salts can be sulphites or hydrogen sulphites. Advantageously, the sulfites and hydrogen sulfites are lithium, sodium, potassium or ammonium salts.

The esters of sulfurous acid are compounds of the general formula R1-OS (= O) -O-R2, wherein R1 and R2 are independently selected from linear or branched acyclic alkyl groups having 1 to 10 carbon atoms, cyclic alkyl groups having 3 to 10 carbon atoms, aryl groups and benzyl groups.

In the context of the present invention, the linear or branched acyclic alkyl groups having 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. Cyclic alkyl groups in the context of the present invention advantageously include propyl, butyl, pentyl, hexyl, heptyl, and octyl rings. A cyclic alkyl group in the context of the present invention is selected from the said ring-shaped alkyl groups which carry no further substituents, and from the said ring-shaped alkyl groups, which in turn are bonded to one or more acyclic alkyl groups. In the latter case, the bonding of the cyclic alkyl group to the oxygen atom according to the above formula may be via a cyclic or an acyclic carbon atom of the cyclic alkyl group. Cyclic alkyl groups according to the above definition of the term "alkyl group" also contain a maximum of 10 carbon atoms. If one of the radicals R 1 and R 2 is an aryl group, this is selected from among phenyl, naphthyl and anthracenyl.

Gaseous sulfites are SO ₂ gas which is introduced into the electrolyte.

The sulfinic acids are compounds of the general formula R3-S (= O) -OH, where R3 is a linear or branched acyclic alkyl group having 1 to 10 Carbon atoms, a cyclic alkyl group of 3 to 10 carbon atoms, an aryl or benzyl group, and wherein these groups are defined as described above for R1 and R2.

Benzaldehyde derivatives are selected from benzaldehydesulfonic acid, their salts and esters, e.g. Benzaldehyde-2-sulfonic acid sodium salt, dimethylaminobenzaldehyde, 3-chlorobenzaldehyde, 4-chlorobenzaldehyde, 2-methoxybenzaldehyde, 2-methylbenzaldehyde, 2-nitrobenzaldehyde, 3,5-dibromobenzaldehyde, 3-nitrobenzaldehyde and 3,5-dimethoxybenzaldehyde.

Hydroxybenzenes are selected from phenol, catechol, resorcinol, hydroquinone, pyrogallol, hydroxyquinone and phloroglucin.

If the at least one reducing agent is a salt of an organic compound, it is advantageous to choose a sodium, potassium, lithium or ammonium salt. In the case of polybasic organic acids, a single acidic hydrogen atom or several or all may 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 may be identical or different.

The at least one reducing agent may further be an ester of an organic compound. It is known to those skilled in the art that esters are the condensation products of an alcohol and a carboxylic acid. Accordingly, esters of alcohols according to the above list of suitable reducing agents are condensation product of one of the abovementioned alcohols and a carboxylic acid R4-COOH, and esters of carboxylic acids according to the above list are condensation products of one of the abovementioned carboxylic acids with an alcohol R5-OH.

R 4 and R 5 are here selected from linear or branched acyclic alkyl groups having 1 to 10 carbon atoms, cyclic alkyl groups having 3 to 10 carbon atoms, aryl or benzyl groups, these groups being defined as described above for R 1 and R 2.

Particularly advantageous is the at least one reducing agent selected from salts and / or esters of sulfurous acid and gaseous sulfites.

The at least one reducing agent is contained in the electrolyte in a concentration of 1 to 100 mmol / l, advantageously in a concentration of 5-30 mmol / l, this concentration refers to the total amount of the above-mentioned reducing agent in the electrolyte.

The electrolyte of the invention further contains at least one sulfonic acid in a concentration of 0.25 to 4.75 mol / l. The concentration is preferably 0.5 to 3 mol / l and most preferably 0.8 to 2.0 mol / l. The at least one sulfonic acid serves on the one hand to establish a corresponding pH in the electrolyte. On the other hand, their 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 empirical formula R6-S (= O) 2-OH, wherein R6 represents a linear or branched acyclic alkyl group of 1 to 10 carbon atoms, a cyclic alkyl group of 3 to 10 carbon atoms, or an aryl or benzyl group these groups are defined as described above for R1 and R2. Preferably, sulfonic acids are selected from the group consisting of methanesulfonic acid, ethanesulfonic acid, propanesulfonic acid and benzenesulfonic acid. Very particular preference is given to mentioning methanesulfonic acid and propanesulfonic acid in this context. Most preferably, methanesulfonic acid is used.

Optionally, the electrolyte according to the invention further contains a surfactant. This surfactant is selected from anionic and nonionic surfactants. Examples thereof are polyethylene glycol adducts, fatty alcohol sulfates, alkyl sulfates, alkyl sulfonates, aryl sulfonates, alkylaryl sulfonates and heteroaryl sulfonates, betaines, fluorosurfactants and their salts and derivatives. Suitable surfactants are known to the person skilled in the art, for example from N. Kanani: Galvanotechik, Hanser-Verlag, Munich-Vienna, 2000, p. 84 ff , Before addition of a surfactant, the electrolyte according to the invention has a surface tension of greater than or equal to 70 mN / m. If a surfactant is added, its concentration is advantageously chosen so that the surface tension of the electrolyte drops 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 process for the electrolytic deposition of predominantly silver-containing silver-palladium layers of an electrolyte according to the invention, wherein an electrically conductive substrate is immersed in the electrolyte and between an anode in contact with the electrolyte and the substrate as a cathode established a current flow. It should be mentioned that the embodiments mentioned as preferred for the electrolyte also apply mutatis mutandis to the method mentioned here.

The temperature which prevails during the deposition of the silver-palladium alloy can be chosen at will by the person skilled in the art. It will orientate itself on a sufficient deposition rate and applicable current density range on the one hand and on the other hand on economic aspects or the stability of the electrolyte. It is advantageous to set a temperature of 25 ° C to 75 ° C in the electrolyte, preferably between 30 ° C and 65 ° C. Most preferably, the use of the electrolyte at temperatures of 45 ° C to 55 ° C.

The current density established during the deposition process in the electrolyte between the cathode and the anode can be selected by one skilled in the art in accordance with the efficiency and the quality of the deposition. Advantageously, the current density in the electrolyte is set to 0.1 to 100 A / dm ² , depending on the application and coating system type. Optionally, the current densities can be increased or decreased by adjusting the system parameters such as the structure of the coating cell, flow rates, anode, cathode ratios, etc. Is advantageously a current density 0.5 to 20 A / dm ^2, preferably 1 - 20 A / dm ² and very particularly preferably from 1.5 to 15 A / dm ^2.

As already indicated above, the electrolyte according to the invention is an acidic type. The pH should preferably be <2, more preferably <1. It may be that with respect to the pH of the electrolyte during the electrolysis fluctuations occur. In a preferred embodiment of the subject method, the person skilled in the art therefore proceeds in such a way that he controls the pH during the electrolysis and, if necessary, sets it to the desired value.

When using the electrolyte, various anodes can be used. Soluble or insoluble anodes are also suitable, as is the combination of soluble and insoluble anodes. If a soluble anode is used, it is particularly preferred if a silver anode is used.

Preferred insoluble anodes are those made of a material selected from the group consisting of platinized titanium, graphite, iridium-transition metal mixed oxide and special carbon material ("Diamond Like Carbon" DLC) or combinations of these anodes. Mixed oxide anodes of iridium-ruthenium mixed oxide, iridium-ruthenium-titanium mixed oxide or iridium-tantalum mixed oxide are particularly preferably used for carrying out the invention. Very particular preference is given to using platinum-titanium anodes. Others can be added Cobley, AJ et al. (The use of insoluble anodes in Acid Sulphate Copper Electrodeposition Solutions, Trans. IMF, 2001, 79 (3), pp. 113 and 114 ) being found.

The present invention provides a silver-palladium alloy electrolyte with an added reducing agent for alloying and brightening, and for the electrodeposition of silver-palladium layers, and a corresponding method. The electrolyte contains at least one reducing agent for alloy adjustment and lightening: By adding the at least one reducing agent, the palladium content of the deposited silver-palladium alloy can be adjusted. As already stated above, the deposited alloys according to the invention have a composition which contains 70-99% by weight of silver, 1-30% by weight of palladium and 0.1-5% by weight of selenium and / or tellurium. wherein the sum of the proportions of silver, palladium and selenium and / or tellurium is 100 wt .-%. In addition, the electrolyte according to the invention leads to a more homogeneous deposition compared to conventional silver-palladium alloy electrolytes.

Layers deposited from conventional silver palladium electrolytes have L * values of 67-78, depending on the applied current density. With the new electrolyte system according to the invention, significantly higher L * values of the deposited layers, uniform over the applied current density range, are achieved. These are between 80 and 90, depending on the reducing agent used.

This was not to be expected against the background of the known state of the art.

embodiments

Various basic electrolytes were prepared and one reducing agent each was added in two different concentrations. From these electrolytes, with and without reducing agent, silver-palladium layers were then deposited, characterized and compared with each other.

Embodiment 1

• Supporting 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 telluric acid)
• Reducing agent:
  • 0 g / l Na formaldehyde sulfoxylate
  • 0.95 g / l Na formaldehyde sulfoxylate (8 mmol)
  • 7.1 g / l Na formaldehyde sulfoxylate (40 mmol)
• Temperature: 30 ° C
• Anodes: PtTi

The palladium content of the deposited layers was determined by means of an X-ray fluorescence analysis method (RFA, XRF) (Fischerscope XDV-SDD, software WIN-FTM version 6.28-S-PDM). Measurement results of palladium content: Content of Na formaldehyde sulfoxylate [g / l] Current density [A / dm2] Pd content [% by weight] 0 1 4.2 0 2 3.2 0 3 3.0 0.95 1 5,7 0.95 2 3.5 0.95 3 3.4 4.7 1 9.1 4.7 2 6.8 4.7 3 5.4

The results of the determination of the palladium content are in Fig. 1 displayed.

The brightness of the deposited layers was measured in terms of the L * value according to CIEL * a * b. Measurement results: Content of Na formaldehyde sulfoxylate [g / l] Current density [A / dm2] Brightness [L *] 0 1 78.3 0 2 73.4 0 3 73.0 0.95 1 73.6 0.95 2 83.0 0.95 3 80.5 4.7 1 75.6 4.7 2 77.2 4.7 3 78.8

Embodiment 2

• Supporting 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 agent:
  • 0 g / l ascorbic acid
  • 0.14 g / l ascorbic acid
  • 0.42 g / l ascorbic acid
• Temperature: 60 ° C
• Anodes: PtTi

The palladium content of the deposited layers was determined by an X-ray fluorescence analysis method (RFA). Measurement results of palladium content: Ascorbic acid content [g / l] Current density [A / dm2] Pd content [% by weight] 0 1 3.8 0 2 2.9 0 3 2.7 0.14 1 4.2 0.14 2 3.1 0.14 3 2.7 0.42 1 5.3 0.42 2 3.6 0.42 3 3.3

The results of the determination of the palladium content are in Fig. 2 displayed.

The brightness of the deposited layers was measured in terms of the L * value according to CIEL * a * b. Measurement results Brightness: Ascorbic acid content [g / l] Current density [A / dm2] Brightness [L *] 0 1 81.8 0 2 67.9 0 3 64.5 0.14 1 83.6 0.14 2 76.6 0.14 3 71.0 0.42 1 83.0 0.42 2 79.0 0.42 3 73.6

Embodiment 3

• Supporting 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 telluric acid)
• Reducing agent:
  • 0 g / l hydroquinone
  • 0.5 g / l hydroquinone
  • 1 g / l hydroquinone
• Temperature: 60 ° C
• Anodes: graphite

The palladium content of the deposited layers was determined by an X-ray fluorescence analysis method (RFA). Measurement results of palladium content: Hydroquinone content [g / l] Current density [A / dm2] Pd content [% by weight] 0 1 1.4 0 2 2.9 0 3 2.8 0.5 1 6.8 0.5 2 5.5 0.5 3 6.0 1.0 1 16.8 1.0 2 15.0 1.0 3 14.4

The results of the determination of the palladium content are in Fig. 3 displayed.

The brightness of the deposited layers was measured in terms of the L * value according to CIEL * a * b. Measurement results Brightness: Hydroquinone content [g / l] Current density [A / dm2] Brightness [L *] 0 1 81.7 0 2 77.8 0 3 72.5 0.5 1 83.1 0.5 2 81.6 0.5 3 77.1 1.0 1 76.5 1.0 2 77.7 1.0 3 73.8

Embodiment 4

• Supporting 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 telluric acid)
• Reducing agent:
  • 0 g / l sodium sulfite
  • 1 g / l sodium sulfite
  • 2 g / l sodium sulfite
• Temperature: 40 ° C
• Anodes: PtTi

The palladium content of the deposited layers was determined by an X-ray fluorescence analysis method (RFA). Measurement results of palladium content: Sodium sulphite content [g / l] Current density [A / dm2] Pd content [% by weight] 0 1 6.2 0 2 4.9 0 3 3.5 1.0 1 10.0 1.0 2 8.1 1.0 3 8.1 2.0 1 15.6 2.0 2 12.3 2.0 3 11.7

The results of the determination of the palladium content are in Fig. 3 displayed.

The brightness of the deposited layers was measured in terms of the L * value according to CIEL * a * b. Measurement results Brightness: Sodium sulphite content [g / l] Current density [A / dm2] Brightness [L *] 0 1 80.0 0 2 76.3 0 3 71.1 1.0 1 81.8 1.0 2 82.2 1.0 3 81.2 2.0 1 78.4 2.0 2 77.8 2.0 3 78.3

Claims (12)

1. Cyanide-free, acidic, and aqueous electrolyte for the electrolytic deposition of bright, predominantly silver-containing, silver-palladium alloys which, in their dissolved form, contains the following components:

   a) a silver compound in a concentration of 1 - 300 g/L silver;

   b) a palladium compound in a concentration of 0.1 - 100 g/L palladium;

   c) a tellurium and/or selenium compound in a concentration of 0.002 - 10 g/L tellurium and/or selenium, based on the total amount of tellurium and selenium in the electrolyte;

   d) urea and/or urea derivatives in a concentration of 0.05 - 1.5 mol/L, based upon the total amount of urea and urea derivatives in the electrolyte and/or one or more α-amino acids, selected from the group consisting of alanine, aspartic acid, cysteine, glutamine, glutamic acid, glycine, lysine, leucine, methionine, phenylalanine, phenylglycine, proline, serine, tyrosine, and valine in a concentration of 0.005 - 0.5 mol/L, based on the total amount of amino acids in the electrolyte;

   e) at least one sulfonic acid in a concentration of 0.25 - 4.75 mol/L, based upon the total amount of sulfonic acids,

   characterized in that the electrolyte additionally has

   f) at least one reducing agent selected from the group of 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, as well as its salts and/or esters

   in a concentration of 1 - 100 mmol/L, based upon the total amount of these reducing agents.
2. Electrolyte according to claim 1, characterized in that the silver compound is selected from silver nitrate, silver carbonate, silver methane sulfonate, silver chloride, and silver oxide.
3. Electrolyte according to any one of claims 1 and 2, characterized in that the palladium compound is selected from palladium hydroxide, palladium chloride, palladium glycinate, palladium methane sulfonate, and palladium sulfate.
4. Electrolyte according to any one of claims 1 to 3, characterized in that the selenium and/or tellurium compounds are selected from tellurites, selenites, tellurous acid, selenious acid, telluric acid, and selenate as well as tellurate.
5. Electrolyte according to any one of claims 1 to 4, characterized in that the α-amino acid is selected from alanine, glycine, and valine.
6. Electrolyte according to any one of claims 1 to 4, characterized in that component (d) is urea.
7. Electrolyte according to any one of claims 1 to 6, characterized in that the at least one sulfonic acid is selected from ethane sulfonic acid, propane sulfonic acid, benzene sulfonic acid, and methane sulfonic acid.
8. Electrolyte according to any one of claims 1 to 7, characterized in that the at least one reducing agent is selected from hydroxyphenols, ascorbic acid, and salts and/or esters of sulfurous acid.
9. Method for the electrolytic deposition of predominantly silver-containing silver-palladium layers from an electrolyte according to any one of claims 1 to 8, characterized in that 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.
10. Method according to claim 9, characterized in that the electrolyte temperature is 25 to 70 °C.
11. Method according to any one of claims 9 and 10, characterized in that the current strength is between 0.5 and 20 A/dm² during the electrolysis.
12. Method according to any one of claims 9 to 11, characterized in that the pH value is continuously set to a value of <2 during the electrolysis.

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