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Answer:
Dr. Takahito Terashima, who is studying membrane crystal, kindly answered this question:
The reason why the heating is necessary in the solid reaction method
The solid reaction method is normally used to synthesize inorganic compounds such as oxide. In this method, ingredients are weighed, mixed with mortar, and then heated by an electric furnace, causing chemical reactions. This synthetic method is based on the principle of thermodynamics; an energetically stable structure (crystal) is made, when some kinds of atoms are mixed in a certain ratio, headed at a certain temperature, and equilibrated. Therefore, a simple answer to this question is “heating is for producing the new crystal.”
Concerning the solid reaction
Inorganic compounds such as metal oxides are usually crystallized, and have individual crystal structure. When two kinds of compounds having different crystal structures are mixed well (ground powder is used if the ingredients are oxides) and heated at high temperature, a chemical reaction occurs and a completely new compound is produced.
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Let me give you an example. The powders of an oxide with NaCl
type structure and Al2O3 with corundum
type structure (corumdum : the name of the minerals. Sapphire
and ruby also has this structure. The slight difference of
impurities in these stones causes different colors. See photo at
left, from iStone, with permission) are mixed together and heated up, causing
diffusion of atoms at points of contact between the powders.
Then the ions of Mg, Al and O form MgAl2O4
with spinel type structure (spinel is also the name of a
mineral), which is energetically more stable than when it exists
as individual ions. This reaction is called the “solid
reaction”.
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Sapphire
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Ruby |
Substances usually become liquid when they are heated; however,
the solid reaction is not a reaction such that several substances in
liquid state are mixed to react with one another. The driving force of
the solid reaction is diffusion of atoms (ions) between solid
substances. Therefore, the reaction happens at a temperature less than
the melting points of each substance.
I heard that the solid reaction method was commonly used in the field of semiconductors. Could you explain why it is important in this field, and how is the method applied to other fields?
Semiconductors and thin membrane technology
Semiconductors, especially integrated circuits, have many parts. These include arithmetic, condenser, resistance, lead wire, and so on. These parts are piled up intricately inside the chip. In order to make this type of chip, layers, each having its own function, are grown up in order on a silicone board. The size of these has recently become smaller than 1
mm, and so it is made using various kinds of micro-fabrication technologies such as photolithography and ion beam etching. Therefore we can say that thin membrane technology is fundamental to semiconductor technology. There are other methods for making thin membranes also. I list some of them below:
To determine the details of these methods, check for yourself.
Application to other fields
Because the membrane has a flat and wide area, it is easily handled and modified. Therefore, the membrane is applied to many fields. I will give some examples of the application in the field of metal/metal oxide:
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Giant magnetoresistive head (It is applied to the high-density magnetic head)
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Infrared sensor (e.g., PbTiO3 membrane, a temperature sensor for air conditioners)
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SQUID (superconducting quantum interference device, e.g., Superconductive Nb is used)
Acknowledgement
Our thanks to
Dr. Takahito Terashima
His study is to make crystals of metal oxide having different structures/properties from ordinary substances, using methods for producing thin membranes. There are detailed explanations about his study on his website.
This article is translated by Chemistryquestion.com from the original article in Chemistryquestion.jp. Please let us know if you find any errors.
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