here. Scroll down to "Audio samples".
/j/ is a semi-vowel. It's produced from the tip of the tongue, but it has the characteristic of a consonant.
/i/ is exactly a full vowel--giving an extra syllable.
When you pronounce /ju:nɪ'vɜ:sətɪ/, the tip of the tongue slides from /i/ position to /u/ position.
Some old dictionary used to give the pronunciation with /'ʌnjən/ because of the blend of /i/ and /ə/. However, if you give the pronunciation of /'ʌnɪən/, the word is divided into 3 syllables, rather than 2.
Hope that helps.
The IPA uses /i/ in diphthongs, such as /iә/ or /aiә/. I can't recall most of the excerpt from those books, but you might want to search publications by Oxford or Cambridge, esp. those older than 1990s.
Nowadays many RP speakers only pronounce /i:/ and /I/.
Oh, and I forget. Words in plural, such as cases or passes, are used to be represented as /'keisIz/ and /'pɑ:sIz/ for the RP, but for Americans, they use the /'keisәz/ and /'pæ:sәz/
It seems to be hard to distinguish the vowels. I will try to find out more about it. Anyway, thank to all of you! :)