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With increasing nitrogen content of the powder , the porosity within the L-PBF built specimens increased .
 nitrogen, powder, porosity, L-PBF
Please list all entity words in the text that fit the category. Output format is "word1, 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:
 nitrogen, powder, porosity, L-PBF