UC Irvine UC Irvine Previously Published Works Title A reaction study of sulfur vapor with silver and silver-indium solid solution as a tarnishing test method Permalink https://escholarship.org/uc/item/1gq3w8hj Journal JOURNAL OF MATERIALS SCIENCE-MATERIALS IN ELECTRONICS, 27(10) ISSN 0957-4522 Authors Huo, Yongjun Fu, Shao-Wei Chen, Yi-Ling et al. Publication Date 2016-10-01 DOI 10.1007/s10854-016-5124-y Peer reviewed eScholarship.org Powered by the California Digital Library University of California J Mater Sci: Mater Electron (2016) 27:10382–10392 DOI 10.1007/s10854-016-5124-y A reaction study of sulfur vapor with silver and silver–indium solid solution as a tarnishing test method 1 1 1 1 Yongjun Huo • Shao-Wei Fu • Yi-Ling Chen • Chin C. Lee Received: 17 February 2016 / Accepted: 3 June 2016 / Published online: 10 June 2016 Ó Springer Science+Business Media New York 2016 Abstract It is well known that pure silver (Ag) or sterling applications in electronics, jewelry, joining, optics, silver- silver can easily get tarnished under ordinary atmosphere by ware, and welding. sulfur-containing gases. Over past several decades, the industries have tried to formulate and produce silver-based materials that do not get tarnished for various applications 1 Introduction including electronics, optics, jewelry, and silverware. Recently, our group has grown silver and silver–indium Silver possesses the title of champion for its highest ther- solid solution ingots and studied their anti-tarnishing prop- mal conductivity, electrical conductivity, and reflectivity erty. Since there are no standard tarnishing test methods for among any known metal elements. Dry silver does not Ag-based alloys, we have designed and established an form a significant surface oxide at ambient temperature and accelerated tarnishing experiment using sulfur vapor as the pressure [1], so it is relatively chemical inert to oxidation reacting agent. Sulfur vapor, rather than other sulfur-con- under normal atmosphere. However, silver is very sensitive taining gases such as H2S, is chosen because sulfur vapor to the presence of sulfur-containing corrosive gases such as reacts with silver more easily to form silver sulfide (Ag2S). sulfur vapor, hydrogen sulfide, sulfur dioxide, and carbonyl After ingot growth, disk samples were cut from silver and sulfide, etc. Typically, the concentrations of those corrosive silver–indium solid solution ingots and examined with X-ray gases are very low, ranging from less than one parts per diffraction (XRD) and scanning electron microscope/energy billion (ppb) under normal atmosphere to hundreds of ppb dispersive X-ray spectroscopy (SEM/EDX). Commercially in heavy industrial environment [2]. However, the corro- available Argentium silver was also studied for comparison. sion phenomenon can be commonly observed from silver Disk samples were placed into the sulfur vapor reacting used in electronics. The corrosive reaction product is silver chamber at 120 °C for 15, 30, 45, and 60 min, respectively. sulfide which can be the killer of electronic products in The thickness of Ag2S film grown on the samples was several aspects. First of all, silver sulfide has poor measured. The results showed that Ag2Sgrowthrate mechanical strength, so it can be detached and crumbled depends on indium concentration. For example, with from its substrate easily. In addition, the resistivity of silver 19 at.% In, Ag2S growth rate on silver–indium solid solution sulfide is 7–10 order of magnitude higher than that of silver is only 4 % of that on pure silver and 7 % of that on [3], so the overall resistivity will continue to increase with Argentium silver. The excellent anti-tarnishing property of the formation of silver sulfide. Silver sulfide also poses a silver–indium solid solution should open up new threat to circuits because its dendritic growth nature might result in the formation of silver sulfide whiskers [4]. This could lead to the occurrence of short circuit since silver & Yongjun Huo sulfide is not a good insulating material either. Further- [email protected] more, the reflectance of silver mirror surface used in LEDs would drop significantly with corrosive degradation, 1 Electrical Engineering and Computer Science, Materials and Manufacturing Technology, University of California, Irvine, resulting in light output power reduction [5]. CA 92697-2660, USA 123 J Mater Sci: Mater Electron (2016) 27:10382–10392 10383 In order to understand the nature of silver sulfurization, structure is designated as a phase historically. In the lit- extensive research has been conducted over the decades. erature [25], the authors discovered a’ phase, which has Several experiments, using various corrosive gases, such as also cubic crystal structure, at the composition of 25 at.%, sulfur vapor [6], hydrogen sulfide [7], sulfur dioxide [8], and it is correspond to Ag3In which is transformed from f and organic sulfur [9], have been performed to see the phase at 187 °C. It is important to note that material influence on the rate of silver tarnishing. Other environ- composition may fluctuate from region to region, which mental parameters, such as temperature, relative humidity can be caused by various mechanism of segregation effects. [10], illumination [8], and silver surface state [11], have Therefore, it is necessary to keep enough margin to the also been studied to explore the silver tarnishing mecha- maximum solubility boundary in order to avoid entering nism, thereby searching possible scientific and engineering the mixture region of a and a’ phase in the phase diagram, solutions for anti-tarnishing purposes. In literatures, some thereby producing nearly homogeneous single phase sil- surface treatments for silver using different coating meth- ver–indium solid solution. As a consequence, (Ag)–xxIn ods or coating materials such as metallic coating by elec- with 19 at.% indium concentration has been chosen as the troplating [12], sputter-applied coatings [13], oxide coating primary research subject, which is designated as (Ag)–19In [14], organic coating [15], chromate conversion coating in this article. In addition, (Ag)–9.5In with 9.5 at.% indium [16], etc. However, such thin layer of coating material can has been also produced and investigated in order to find be worn off very easily after a short period of time. influence of the indium concentration on the properties of Another approach to improve the anti-tarnishing property (Ag)–xxIn phase. of silver is by alloying. Alloying with copper can The casting method was chosen for the production of the strengthen silver, but it will make silver less tarnish material of the interested phase, resulting in the form of resistant to corrosive gases [17]. It has been reported that ingots. The raw materials of silver and indium shots with adding other elements such as palladium [18], silicon [19], 99.99 % purity were weighed, uniformly mixed and loaded tin [20], germanium [21], and zinc [22], would have, more in quartz tube with one end closed. Since the native oxide or less, increased anti-tarnishing effects compared with forming on the surface of silver and indium metal is neg- sterling silver. However, to the knowledge of the authors, ligible thin, there is no treatment required to perform there is no standard test method existing to quantify the before loading the raw material. While the quartz tube is capability of anti-tarnishing for different silver alloy. being pumped by a vacuum pump, the other end of the tube Recently, the authors had presented some preliminary is sealed by hydrogen–oxygen torch operation to form a anti-tarnishing results of the silver solid solution phase capsule. The vacuum environment is necessary for the with indium [23]. In the present work, we have designed a casting production for reducing the number of voids or new quantitative test method and performed the reaction common defects created by trapped air bubbles. Next, the experiments with well-controlled and repeatable environ- capsule was placed into a furnace preheated at 1030 °C and ments. The authors hope to establish this method as a stayed there for 30 min. The temperature of the furnace standard anti-tarnishing test method for silver-based alloys. was then reduced gradually to room temperature in 7 days In the following sections, silver solid solution phase with in order to obtain the homogenized single phase solid indium, designated as (Ag)–xxIn in this paper, are pro- solution at different compositions. This is the preliminary duced in polycrystalline ingot form at two different com- process and optimization requires more time and many positions. After examining their chemical composition growth runs. using SEM/EDX and XRD, the prepared samples are tested After the successful production of (Ag)–xxIn solid in the designed sulfurization experiment. The details of the solution, the ingots were cut into disk samples, using slow design of the sulfurization test experiment have been speed diamond saw, for further chemical composition described. Experimental results are reported, analyzed, and examination. X-ray diffraction (XRD) and scanning elec- discussed. Finally, a summary is then given. tron microscope/energy dispersive X-ray spectroscopy (SEM/EDX) were used in a combination to identify (Ag)– xxIn phase, since their functionality are complementary to 2 Material preparation and characterization each other. While SEM/EDX can provide information about specific chemical composition at local areas within In order to understand Ag–In binary system and produce its electron-beam interaction volume, a few micron meters the single phase (Ag)–xxIn material, it is essential to or sub-micron range depending on the electron beam review the Ag–In binary phase diagram, as shown Fig. 1 energy and atomic number of the material under exami- [24]. The maximum solubility of indium element in silver nation. On the other hand, XRD is capable of interpreting crystal lattice is 20 at.% at room temperature, the silver– the crystallography nature of the material by generalizing indium solid phase with face-centered cubic (FCC) crystal data generated from macroscopic regions of the samples, 123 10384 J Mater Sci: Mater Electron (2016) 27:10382–10392 Fig.