1998
This minor improvement may be related to the improved efficiency of the passivation layer that was attributed to the increased β-phase content and the reduction of α/β interfaces .
 material: be; concept or principle: passivation; process parameter: layer; concept or principle: reduction
Given options, please tell me the categories of all the listed entity words.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} 
 Entities: be, passivation, layer, reduction 
Answer:
 material: be; concept or principle: passivation; process parameter: layer; concept or principle: reduction