T-SQL unicode concatenation issues

Hello,
While executing the below T-SQL code, it yields not results [the search word is a unicode text].
After exploring i realized that the Unicode concatenation is not working well. All the characters converting to question marks.
declare @search_word nvarchar(255)
declare @sql nvarchar(255)
set @search_word='اب';

set @sql=N'select * from users where [fi_name] like ''%'+@search_word+'%''';
print @sql
exec sp_executesql @sql

Any clues how to overcome this?

The code works perfectly well, don't worry.
The only problem is that the results are being shown in a font that does not support those characters. Just select a font that does.

Thanks for your reply.
I was missing the charactor N before the uniocde string assignment which yielded results in ???.
My assignement statement s changed as below to work this query in SQL Server Management Studio.
set @search_word=N'اب';