Why can not I open many websites of Sci-tech Journals?

It could be that they are pay sites where you have to be a paid member, with a unique username and password issued to you. Or they could be behind institute firewalls, and the same condition would apply as the username and password, plus you have to be logged onto the institute’s network at the time of access.

Geolocation. You are not in a country where the publisher has the right to distribute.

It is also possible your country/government is prohibiting access as well.

Here is a list of some of the 3000 sites mainland China is blocking since Sept 2015:
List

Here is a small segment of that list (in case the link above is blocked.)
1 Gmail google.com gmail.com Email English 2014, September (or earlier) to present[2] BLOCKED
1 Google google.com google.com Search English 2014, May (or earlier) to present[3] Partially Unblocked – Certain branches are blocked
1 Google Maps google.com maps.google.com Maps English 2014, May (or earlier) to present[4] BLOCKED
1 Google Docs google.com docs.google.com Sharing English 2011, May (or earlier) to present[5] BLOCKED
1 Google Encrypted google.com encrypted.google.com Search English 2011, March (or earlier) to present[6] BLOCKED
5 Google APIs google.com *.googleapis.com Search English 2014, September (or earlier) to present[7] BLOCKED
1 Google+ google.com plus.google.com Social English 2011, July to present[8] BLOCKED
1 Google Sites google.com sites.google.com Web Hosting English 2011, March (or earlier) to present[9] BLOCKED
1 Picasa google.com picasaweb.google.com Sharing English 2009, July to present[10] BLOCKED
2 Facebook facebook.com www.facebook.com Social English 2008, July to present BLOCKED
3 YouTube youtube.com www.youtube.com Sharing English 2009, March to present[11][12][13][14] BLOCKED
4 Yahoo! Taiwan yahoo.com tw.yahoo.com Portal Chinese 2011, May (or earlier) to 2014, Jan[15] UNBLOCKED
4 Yahoo! Hong Kong yahoo.com hk.yahoo.com Portal Chinese 2011, March (or earlier) to 2014, Jan[16] UNBLOCKED
6 Mobile Wikipedia wikipedia.org mobile.wikipedia.org Other English 2011, May (or earlier) to 2012[17] BLOCKED
8 Twitter twitter.com twitter.com Social English 2009, June to present[12][18][19] BLOCKED
11 Blogspot blogspot.com blogspot.com Blog English 2009, May to present[12][13][20] BLOCKED
16 Yahoo! Japan yahoo.co.jp yahoo.co.jp Portal Japanese 2012, 15–17 June UNBLOCKED
20 T.co t.co t.co Other English 2011, May (or earlier) to 2012[21] UNBLOCKED
21 Google Japan google.co.jp google.co.jp Search Japanese 2012, 15–17 June BLOCKED
32 Instagram instagram.com instagram.com Social English Sept 19, 2014 to present[22] BLOCKED
51 XVideos xvideos.com www.xvideos.com Porn English 2011, March (or earlier) to present[23] BLOCKED
52 FC2 fc2.com www.fc2.com Blog Japanese 2011, March (or earlier) to 2012[24] UNBLOCKED
54 xHamster xhamster.com www.xhamster.com Porn English 2011, March (or earlier) to 2012[25] BLOCKED
64 Pornhub pornhub.com www.pornhub.com Porn English 2012, May to 2016, April UNBLOCKED
71 The Pirate Bay thepiratebay.se thepiratebay.se Sharing English 2012, February (or earlier) to 2012[26] UNBLOCKED
75 Rakuten Market rakuten.co.jp rakuten.co.jp Shopping Japanese 2012, 15–17 June, September ?? to 19? UNBLOCKED
87 Dailymotion dailymotion.com dailymotion.com Sharing English Unknown to Present[2] BLOCKED
94 Amazon Japan amazon.co.jp amazon.co.jp Shopping Japanese 2012, 15–17 June UNBLOCKED
108 The New York Times nytimes.com nytimes.com Publication English 2012 to present BLOCKED
110 Dropbox dropbox.com www.dropbox.com Sharing English 2014, June (or earlier) to present[27] BLOCKED
112 Vimeo vimeo.com www.vimeo.com Sharing English 2009, October[28] to 2015, Sept[29][30] BLOCKED
188 SoundCloud soundcloud.com www.soundcloud.com Audio English September 2013 to present[31][32] BLOCKED
219 Nico Video nicovideo.jp nicovideo.jp Sharing Japanese Unknown to Unknown UNBLOCKED
223 Archive.org archive.org www.archive.org Other English 2011, April (or earlier) to 2012[33] BLOCKED
225 Scribd scribd.com scribd.com Sharing English 2011, February (or earlier) to unknown[34] UNBLOCKED
227 isoHunt isohunt.com isohunt.com Sharing English 2012, April to 2012; also previously blocked and unblocked[35] UNBLOCKED
257 XING xing.com www.xing.com Social German 2011, April (or earlier) to 2012[36] BLOCKED
277 hardsextube hardsextube.com www.hardsextube.com Porn English 2011, April to 2012[37] BLOCKED
282 drtuber drtuber.com www.drtuber.com Porn English 2011, April to 2012[38] BLOCKED
333 Bloomberg bloomberg.com bloomberg.com Publication English 2012 to present BLOCKED
429 Wretch wretch.cc www.wretch.cc Blog Chinese 2007, August to 2012[39] BLOCKED
541 The Independent independent.co.uk independent.co.uk Publication English 2012 to present UNBLOCKED
553 Google Appspot appspot.com appspot.com Web Hosting English Unknown to Unknown BLOCKED
564 Yomiuri News yomiuri.co.jp yomiuri.co.jp Publication Japanese 2012, 15–17 June UNBLOCKED
610 Ustream.tv ustream.tv www.ustream.tv Sharing English 2011, April (or earlier) to 2012[40] UNBLOCKED
623 excite (Japan) excite.co.jp excite.co.jp Portal Japanese 2012, 15 June to 17 June, September ?? – 18 UNBLOCKED
633 Sponichi News sponichi.co.jp sponichi.co.jp Publication Japanese 2012, 15–17 June UNBLOCKED
660 Nikkei BP Japan nikkeibp.co.jp nikkeibp.co.jp Publication Japanese 2012, 15–17 June UNBLOCKED
713 LazyGirls lazygirls.info lazygirls.info Publication and Pictures English 2011 or 2013 to present BLOCKED
1321 Technorati technorati.com www.technorati.com Search English 2008, July to 2012[41] UNBLOCKED
2056 Plurk plurk.com www.plurk.com Social English 2009, April to 2012[42] UNBLOCKED
2925 Norwegian Broadcasting Corporation nrk.no www.nrk.no News Norwegian 2011, May (or earlier) to 2012[43] UNBLOCKED
3876 PBworks pbworks.com pbworks.com Sharing English 2011, January to 2012[44] UNBLOCKED
4247 The Epoch Times theepochtimes.com www.theepochtimes.com News English 2003 to present BLOCKED
4480 Sony Japan Sony.co.jp Sony.co.jp Corporation Japanese 2012, 15–17 June UNBLOCKED

Feel free to try any of the links listed above and let us know what happens. You can tell us on Twitter (oops, it’s blocked) Instagram (oops, it’s blocked)