Google Scholar is a search engine dedicated to academic work. Not everything listed on Google Scholar will be freely available in full. But it is a good place to start if you’re looking for a specific paper, and many papers can be downloaded for free.
PubMed is a search engine maintained by the NCBI, part of the United States National Library of Medicine. It provides access to more than 29 million citations of biomedical research from MEDLINE, life science journals, and online books. The NCBI runs a similar search engine for research in the chemical sciences called PubChem, too, which is also free to use.
Bielefeld Academic Search Engine :
The (BASE) is operated by the Bielefeld University Library in Germany, and it offers more than 100 million documents from more than 4,000 sources. Sixty percent of its content is open access, and you can filter your search accordingly.
WorldWideScience is a global academic search engine, providing access to national and international scientific databases from across the globe. One interesting feature is that it offers automatic translation, so users can have search results translated into their preferred language.
Semantic Scholar is an “intelligent” academic search engine. It uses machine learning to prioritize the most important research, which can make it easier to find relevant literature. Or, in Semantic Scholar’s own words, it uses influential citations, images, and key phrases to “cut through the clutter.
A multidisciplinary search engine, JURN provides you with links to various scholarly websites, articles, and journals that are all free access or open access. Specifically covering the fields of the arts, humanities, business, law, nature, science, and medicine, JURN has indexed almost 5,000 repositories to help you find exactly what you're looking for.