I have this code for export to excel
    private static void FileDownloadPrompt(string FileName, SqlDataReader reader, string ContentType, string FileExtension)
    {
        //set the conttent type of the file to be downloaded
        System.Web.HttpContext.Current.Response.ContentType = ContentType;

        //add the response headers
        System.Web.HttpContext.Current.Response.AddHeader("content-disposition", "attachment; filename=\"" + FileName + FileExtension + "\"");

        //add the contents of the result set to the response stream
        System.Web.HttpContext.Current.Response.Write(ToHTMLTable(reader));

        //end the http response
        System.Web.HttpContext.Current.Response.End();
    }

    private static string ToHTMLTable(SqlDataReader iDr)
    {
        System.Text.StringBuilder sb = new System.Text.StringBuilder("");

        //check to see if there the reader has columns
        if (iDr.FieldCount != 0)
        {
            sb.Append("<table border=1><tr>");

            //loop thru each column
            for (int i = 0; i < iDr.FieldCount; i++)
            {
                sb.Append("<td>" + Convert.ToString(iDr.GetName(i)) + "</td>");
            }

            sb.Append("</tr><tr>");

            //loop until the record reaches eol(end of line)
            while (iDr.Read())
            {
                //loop thru each column, this is necessesary so that we could map
                //the columns to their proper place
                for (int i = 0; i < iDr.FieldCount; i++)
                {
                    sb.Append("<td>" + Convert.ToString(iDr.GetValue(i)) + "</td>");
                }

                sb.Append("</tr><tr>");
            }

            sb.Remove(sb.ToString().LastIndexOf("<tr>"), 4).Append("</table>");
        }
        //return our stringbuilder
        return sb.ToString();
    }

After export to excel is showing characters are exranos
1. NumeroDocumento column must be of type text
2. NumeroSentencia column is showing strange characters, the text to be displayed is "N° de la sentencia"
3. DecisionSuperior column is showing strange characters, the text to be displayed is "Decisión Superior"
I attached this file
errorExcel.JPG