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Transmission electron microscopy revealed that Al–Al metallic bonding without surface oxide layers was mainly achieved , although some oxide clusters were locally observed at the original interface .
 process characterization: Transmission electron microscopy; concept or principle: metallic bonding; concept or principle: surface; material: oxide; material: oxide; concept or principle: interface
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: manufacturing standard, machanical property, manufacturing process, material, engineering feature, process parameter, machine or equipment, application, process characterization, enabling technology, biomedical, concept or principle 
Text: {0} 
Answer:
 process characterization: Transmission electron microscopy; concept or principle: metallic bonding; concept or principle: surface; material: oxide; material: oxide; concept or principle: interface