Lesson seven - Diễn đàn
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Diễn đàn » Hóa học đại học và sau đại học » Tiếng Anh chuyên ngành hóa học (English for Special Purposes) » Lesson seven (Electrochemical Art)

Lesson seven
ProfVietanhNgày: Thứ bảy, 2010-04-24, 10:59 PM | Tin nhắn # 1
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Electrochemical Art

If aluminum is a silvery metal, then why are some aluminum objects brightly colored? Aluminum bicycle parts, for instance, come in a spectrum of colors including red, blue, purple, yellow, and black.

Aluminum, chromium, titanium, and several other metals can be colored by an electrochemical process called anodizing. Unlike electroplating, in which a metal ion in the electrolyte is reduced and the metal is coated onto the surface of the cathode, anodizing oxidizes a metal anode to yield a metal oxide coat. In the oxidation of aluminum, for instance, the electrode reactions are:
Cathode (reduction): 6H+ (aq) + 6e -> 3H2 (g)
Anode (oxidation): 2 Al (s) + 3H2O (l) -> Al2O3 (s) + 6H+ (aq)+ 6e
Overall cell reaction: 2 Al (s) + 3H2O -> 3H2 (g) + Al2O3 (s)

The thickness of the aluminum oxide coating that forms on the anode can be controlled by varying the current flow during electrolysis. Typically, the coating is about 0.01 mm thick, which corresponds to about 4x104 atomic layers of Al2O3 on top of the underlying aluminum metal. The porosity of the Al2O3 coating can also be controlled by varying the electrolysis conditions, and it’s this porosity that makes coloring possible. When an organic dye is added to the electrolyte, dye molecules soak into the spongy surface coating as it forms and become trapped as the surface hardens.

Titanium anodizing proceeds much like that of aluminum, but the resultant coat of TiO2 is much thinner than (1x10-4 nm) the corresponding coat of Al2O3 (1x10-2 nm). Furthermore, the iridescent colors of anodized titanium result not from the absorption of organic dyes but from the interference of light as it is reflected by the anodized surface. When a beam of white light strikes the anodized surface, part of the light is reflected from the outer TiO2, while part penetrates through the semitransparent TiO2 and is reflected from the inner metal. If the two reflections of a particular wavelength are out of phase, they interfere destructively and that wavelength is canceled from the reflected light. As a result, the light that remains is colored. (Similar effects are responsible for the colors in oil slicks and peacock feathers.)

The exact color of anodized titanium depends on the thickness of the TiO2 layer, which in turn varies with the voltage used to produce the layer. A voltage of 5 V, for example, gives TiO2 a coating 3x10-5 nm thick with a yellow appearance, and a voltage of 30 V gives a coating 6x10-5 nm thick with a light blue appearance. Artists use these effects to produce striking metal sculptures whose vibrant colors appear to change depending on the viewing angle.

Vocabulary
for instance = for example: ví dụ
spectrum: quang phổ
electroplating: sự mạ điện
electrolyte: chất điện phân
electrode: điện cực
thickness: độ dày
typically: điển hình, đặc trưng
correspond: tương ứng, tương đương, ứng với
porosity: độ rỗng, độ xốp
dye: thuốc nhuộm
soak: thấm, ngấm
spongy: xốp, rỗng
trap: kẹt lại
furthermore: hơn nữa, vả lại
iridescent: (màu) óng ánh
interference: sự giao thoa
reflect: phản xạ, phản chiếu
beam: tia (ánh sáng)
penetrates through: xuyên qua
semitransparent: nửa trong suốt
inner: bên trong
oil slick: vết dầu loang trên mặt nước
peacock: con công
sculpture: tác phẩm điêu khắc
vibrant color = vibrant colour: màu sắc rực rỡ
viewing angle: góc nhìn


Phạm Bá Việt Anh

Department of Analytical Chemistry
Faculty of Chemistry
Hanoi National University of Education
Mobile - Tel: (84) 943 919 789
 
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