以相干性X光顯微術即時觀察金屬電鍍層的成長

Abstract

電鍍是一種古老的鍍層製備工藝，要獲得品質佳的電鍍層必須透過其成長機構透析，觀察工具儼然成為瞭解電鍍層成長機構的關鍵。本論文使用新穎的同步輻射X光顯微術即時觀察電鍍的成長過程。即時X光顯微術可連續檢測成長中的金屬結構，由於它具有足夠的時間解析度來擷取快速的成長過程，及擁有高空間解析度以解析細微的結構。基於X光顯微術的獨特解析能力，本研究分為三個部份，以觀察不同金屬於電鍍時的成長情形，分別為電鍍鋅分歧結構的成長過程，電鍍銅分歧結構的成長過程，以及硼酸對電鍍鎳的影響。 首先，於定電壓與定電流下，探討氫氣泡發展對於鋅分歧結構的影響及其詳細的結構特性。結果顯示，氫氣泡發展程度隨著施以電壓和電流密度增加而增加。特別是分歧結構由緻密狀樹枝結構轉變成蕨類狀樹枝結構，而蕨類狀樹枝結構的形成原因，是因為此結構的成長完全受到氫氣泡的限制。電鍍鋅分歧結構的最後形貌會因定電壓或定電流的不同而改變。 第二部份以不同濃度的硫酸銅與硫酸溶液，及以定電壓和定電流模式探討分枝結構的成長情形。於未添加硫酸的電鍍銅溶液，施以的電壓在極限電流密度區域，銅鍍層不因電鍍時間而改變，自始至終都呈現緻密狀分枝結構。然而在遠超過極限電流密度的條件下，發現銅鍍層形貌隨著時間而轉變，由初始階段的針狀樹枝結構轉變成緻密狀分枝結構。當電鍍銅溶液加入硫酸，分歧結構的分枝變細，而構成高多孔性的鍍層，而氫的吸附扮演著重要的角色。 最後，針對硫酸浴和瓦玆浴探討硼酸對電鍍鎳的影響。未添加硼酸的硫酸浴於低電壓的條件下，電鍍鎳伴隨著強烈的氫氣泡發展，其形成附著性差的鍍鎳層，於電鍍的過程中造成裂紋的成長，而導致鍍層的破裂。在瓦玆浴電鍍鎳過程中，氫氣泡發展較為微弱，可於較高的電壓下製備鍍鎳層。實驗結果顯示，硼酸於電鍍鎳溶液中扮演著抑制氫氣泡發展的角色。

Metal electrodeposition is one of the oldest and most established technologies. However, there is a substantial margin for improving the electrodeposits by better understanding its basic mechanism. The observation tool is a quite important role for this objective. In this dissertation, the metal electrodeposition is studied by a novel real-time synchrotron X-ray microradiology. Real-time radiography permits continuous inspection of growth structure, which has sufficient time resolution to capture the fast evolution, adequate lateral resolution to relevant microstructure. There are three parts in this study, including the ramified formation in zinc electrodeposition, the ramified formation in copper electrodeposition, and the effect of boric acid in nickel electrodeposition. In the first part, it is showed that the effect of hydrogen bubble evolution on zinc ramified growth and detailed zinc ramified feature under potentiostatic and galvanostatic mode. The degree of hydrogen bubble evolution increases with applied potential and current density. In particular, the zinc ramified electrodeposits transit from dense-branching morphology to fern-like dendrite, it is because that the fern-like dendrite completely forms by constricting of hydrogen bubbles. The end of morphology is entirely different between potentiostatic and galvanostatic mode. In the second part, it is systematically investigated that the ramified structure of deposits plated from various concentrations of copper sulfate and sulfuric acid. In the sulfuric acid-free solution, the copper electrodeposits appear consistent in dense-branching morphology at the applied potential in the vicinity of limiting current density. In particular, the morphology transition from needle-like dendrite to dense-branching is observed at the applied potential far above limiting current density. The detail transition process is discussed in this study. When the sulfuric acid is added to the plating solution, the highly porous ramified structure constructed from thin-branching morphology is created in the deposition. Last part, the effect of boric acid on the growth of nickel deposits from sulfate bath and Watts bath is demonstrated. From boric acid-free of sulfate bath, the nickel deposition accompanied with very strong hydrogen bubble evolution, causing the interior of nickel deposits, crack formation and fracture during deposition. In the Watts bath, the hydrogen bubble evolution became very weak during nickel deposition. The deposition can perform at higher applied potential.