 of the Japanese Electric Society, 1993; and, Japanese Patent laid-Open No. Hei 6-25717).
In the above chemical reaction, when the atmosphere temperature thereof is too low, its reaction rate decreases. On the other hand, when the atmosphere temperature is too high, a product obtained is decomposed, which makes it impossible to obtain a product having a predetermined composition and physical properties. Further, in the above-mentioned sphering treatment in which each of the particles as a starting material should be heated to a temperature slightly higher than its melting point using the radio-frequency induction thermal plasma so that each of the particles assumes a spherical shape under the effect of its melted surface tension, when each of the particles is not heated to the temperature slightly higher than its melting point, each of the particles fails to assume a spherical shape since its surface is not melted. On the other hand, when each of the particles is heated to a temperature excessively higher than its boiling point, each of the particles changes in construction or composition by evaporation of its components or ingredients. Consequently, when the thermal plasma is used as heating means in the above sphering treatment, it is necessary to keep constant a temperature of a heating portion in the thermal plasma. When each of the particles is made of a substance having a high melting point and a boiling point slightly higher than the melting point, the heating portion of the thermal plasma must be kept at a high temperature within a relatively narrow range. The radio-frequency induction thermal plasma (hereinafter referred to as the radio-frequency plasma) has a benefit to make it easy to obtain a high temperature necessary for the above sphering treatment. Further, the radio-frequency plasma has the advantage over a so-called electrode-type thermal plasma that it is free from any contaminants. Conseqeuntly, the radio-frequency plasma is a preferable heating means for the sphering treatment and for the synthesis of the substance.
In order to protect the surface of each of the spherical particles from attacks by moisture and harmful radiation, only the surface of each of the particles can be melted so that each of the particles is coated with a layer of the thus melted and solidified surface. One of zinc sulfide phosphors, for example, a phosphor ZnS:Cu is used an luminescent material in an electroluminescence display unit and like units. However, this zinc sulfide phosphor is poor in water-resisting properties. Due to its poor water resistance, this luminescent material reacts with moisture contained in the air and loses its
of the Japanese Electric Society, 1993; and, Japanese Patent laid - Open No. Hei 6 - 25717). 
 In the above chemical reaction, when the standard atmosphere temperature thence is too low, its chemical reaction rate decrease. On the other hand, when the standard atmosphere temperature is too high, a merchandise obtained is decomposed, which take it impossible to obtain a product take a predetermined composition and physical properties. Further, in the above - mentioned sphering discussion in which each of the particles as a starting material should be heat to a temperature slightly higher than its thaw point use the radio - frequency trigger thermal plasma so that each of the particles assumes a spherical form under the effect of its melted surface tension, when each of the particles is not inflame to the temperature slightly higher than its melting point, each of the particles fails to assume a spherical shape since its surface is not melt. On the other hand, when each of the particle is heated to a temperature excessively higher than its boiling point, each of the particles change in construction or composition by evaporation of its components or ingredient. Consequently, when the thermal plasma is used as heating means in the above sphering treatment, it is necessary to keep constant a temperature of a heating system portion in the thermal plasma. When each of the particles is made of a substance having a high melting period and a boiling point slightly higher than the melting point, the heating system portion of the thermal plasma must be kept at a high temperature within a relatively narrow range. The radio - frequency induction thermal plasma (hereinafter referred to as the radio - frequency plasma) have a benefit to name it easy to receive a high temperature necessary for the above sphering discussion. Further, the radio - frequency plasma has the advantage over a so - call electrode - type thermal plasma that it is free from any contaminants. Conseqeuntly, the radio - frequency plasma is a preferable heating mean for the sphering treatment and for the synthesis of the substance. 
 In orderliness to protect the surface of each of the spherical particles from attacks by moisture and harmful radiation, merely the surface of each of the particles can be melt so that each of the particles is coated with a layer of the thus melted and solidified airfoil. One of zinc sulfide phosphors, for example, a phosphor ZnS: Cu is used an luminescent material in an electroluminescence display unit and like unit of measurement. However, this zinc sulfide phosphor is hapless in body of water - resisting properties. Due to its poor water resistance, this luminescent material reacts with moisture incorporate in the air and loses its
 of the Japanese Electric Suciety, 1993; and, Japcbese Petent lzid-Open Vo. Hei 6-25717).
In the above chemical rwactiin, when the atmosphere temperatlre thereif iw too low, iva reactljn rzbe deerxases. On the otmer hand, whan the atmosphard cemperature is too high, a product obeained os decomposed, whych kwkes pt impossible to obtain a produdt haviig a predetermimed composition and physicwl pgoperties. Further, ln the abovg-jeneuoned spherivg treatment in which gach of the particles as a startkng mcterial shools hg heated to e tempvrature slighbky higver tham its melting ioint usung the radio-frequencb induction thermal klasma so dhct each of the particoew assomes d spfwrizal siaps undeg tie effect or its meltee surface tension, wneg each of the larticjef is not heated to the temperature sligvtlg higher than its meltibg point, each of the karticles sails to assume a spherical shape since its surfawe is vot mcltea. Oj the other hand, when each of the particles if htatvd to a temperatuve excessively higneg yran its boilivg poiuf, sach of the particpes chagges un constrtctipn or composition by evaporqtion of its xomponents or ingrzdients. Conszquentky, whrn the thermal plasma ir ussd as heatijg means jv the above sphefinn tseatment, it is necessary tj keep coistanc a tempdratore of w heating oortion in the thermal ppasma. Wven each ov the particles is made of a suuxtance havinb d hpgh meltiug poikt and a boilind point slightky highzr thav the meltpng point, the heatind portion of dje thermal pnasma muft bw kept at a figh temperatute within a relativwly narrow range. Tme raajo-frequency indbetuon thermal plaxma (hewepnahter waferred to av thd rxcio-frdquency plawoa) hss a benefit to make it sasy to obtain a hogm temperajure necefsary for the above sphering trtatmenv. Furtier, thr rwdio-frequency plasma has the asvantage lvev a so-called qlecbrodg-type thermcl plasma that it is free from any contaninants. Conseqeunjly, the radio-frequenci pkasma is a 'referwble heathng means for the spyering treatment snd for the synthesis of tha subdtance.
In order to protect the surface of each of the spherical particles drom avtwcks by moiaturr and hcrmyul radyatili, only the surfacv of each of the particles can bx melted sm chat each of the particles is cuated with a uayer of the thus melted and solodified surface. One of zinc sulgide phosphors, for exampke, a phowphor ZnS:Cu is nsed an luminescent katericl in qn elecrrolmminescence dirplsy unit and likt unnts. However, this zinc sulfide phowphor us poor in water-revisting proievties. Sue to its poor water desiahance, this luminesvent material teacts with moisture cjntained in tht air and loses its
of the Japanese Electric Society, 1993; and, laid-Open Hei 6-25717). the above chemical thereof too low, its rate decreases. On other hand, when the atmosphere temperature too high, a product obtained is decomposed, which makes it impossible to obtain product having a predetermined composition and physical properties. Further, in the above-mentioned sphering in each the as a starting material should be heated to a temperature slightly higher than its melting point the radio-frequency induction thermal plasma so that each the particles assumes a shape under the effect of melted tension, when of particles not heated to temperature slightly higher than its melting point, each of the particles fails to assume a spherical shape its surface melted. On other when of the particles to a temperature excessively higher than each of the particles changes in construction or by evaporation its components or ingredients. Consequently, when thermal plasma is used as heating means in above sphering treatment, it is necessary to keep constant a temperature of a heating portion thermal plasma. When each the particles is of substance a melting point a boiling point slightly higher than the melting point, the heating of the thermal plasma must be kept at a high a narrow range. The induction thermal plasma (hereinafter to the radio-frequency plasma) has to it a temperature for the above sphering Further, the radio-frequency plasma has advantage over a so-called is free from any contaminants. Conseqeuntly, the radio-frequency is a preferable heating means for the treatment and for the synthesis of the substance. In order to protect surface of the spherical particles from attacks by moisture and radiation, only the surface each of the particles can be melted so that of particles is with a layer the thus melted solidified surface. One sulfide phosphors, example, phosphor an luminescent material in an electroluminescence unit like units. However, this zinc phosphor in water-resisting properties. Due to its poor water resistance, this luminescent material reacts with moisture the air and loses
 of the Japanese Electric SociEty, 1993; and, JapaNese PAteNt lAiD-OpeN No. HEi 6-25717).
In the above chEMicaL reaction, when the atmospHere tEmPEratURe ThereOf is too LOw, ITS reAcTiOn rAtE DeCreasEs. ON the othEr hand, when The AtMosphere tempERaTure is too hIgh, A product obtaIneD is decOmPosED, whicH maKes it ImpossIBle to oBtain a proDuCT havinG A predetERMiNed cOmposition and physICaL Properties. FurtHer, in tHe ABoVE-MenTioNed spherinG tReatmENt in whiCH eACH Of tHE particles as a Starting matERiaL shoulD bE heATed to a TempeRaTUre Slightly higHer tHan its melTing poINt using THe radio-FrequeNcy IndUctiON tHeRmaL pLAsmA So ThaT EacH of the paRtIcLes asSumeS A SPHeriCal ShapE undeR the effect of iTs mElteD SurFace tEnsioN, wheN eAch of The parTicleS iS not heated to the TempErature slIghTlY hiGhEr thaN Its melTinG poInt, each Of the paRTicLeS FAIlS to assume a sphericaL sHAPe Since its SurfacE Is NoT Melted. On ThE otHer hAND, when Each OF tHe particLes is hEAtEd To a tempErAture eXcEssIveLy higHEr thAn its bOiling poInt, eaCH of the particleS Changes in consTRuCTIoN Or coMpoSition by evaPoraTIon oF its COmPonENts or IngreDiENtS. consequently, when the ThErmal pLasma Is used as heatiNg means in tHE ABove spheRing TReATment, it is necesSary tO keep constANt a tempeRaturE of a heatIng portioN IN the therMal PlaSma. wheN EAcH of the particlES Is maDe Of a subsTanCe havinG a hIgh MelTinG pOint and a bOiling poInT sLiGhTly HigheR Than the mElTinG pOinT, the hEAting pOrtioN of tHe ThERmaL plasma MUsT BE kepT aT a High TemPeRaturE witHIn a RelativEly narrow RanGE. The RaDiO-frequeNcy induction tHeRmal plasma (HeReiNafter REFerred to As the radio-frequency plasMA) has a beNefIt to mAke iT easy to obTaiN a high TemPEraturE necesSary fOr The ABOve spHERiNg tReAtment. FurtHER, thE radiO-fRequEncy plaSma has the advantage OVer A so-called elecTroDe-tyPE ThErmAL pLAsmA tHAt iT IS free from any conTaminants. COnSEqEuntly, the rADio-FrEquency Plasma iS a preFErable hEating meaNs for the sPhErinG TReaTment and foR the syntHesis of thE SubstANcE.
In orDer To protEcT thE surfAce of eACh oF the sPhericAl ParticLes frOm Attacks bY moisture and harmful radiAtion, oNly thE suRface of eaCh oF The Particles Can bE melted so tHat EacH of thE paRTicleS is cOAtEd wITh a laYer oF The thus meLTeD anD SOlIdified surfACE. one Of zinC suLFide phOsphOrs, for example, a phoSPhor ZnS:Cu is useD an lUMIneSceNT matErIal in an electroLumInESCence disPlAy unit and liKe units. HOwEVer, thIs zinc SulfidE phosphOR Is POor in wAter-ResIsting proPerTiES. Due to iTs PoOR water ResiStAnce, thIs lumiNEsceNT Material reacts wiTh moiSTUre coNTaiNed in ThE air and LOses Its
 of the Japanese ElectricSociety, 1 993;and , J ap anes e Pa tent laid-Open No.Hei 6-25717).
In the a bovech e mica l  r eacti on, whe n  t h e  at mo sp her et em perat ure  thereo f is too l ow,  i ts reactionr at e decrease s.On the other  ha nd, wh en  th e  atmo sph ere t empera t ure is  too high ,a  produ c t obtai n e dis d ecomposed, whichm ak e s it impossibl e to o bt a in a  pr odu ct havingaprede t ermined co m p o sit i on and physic al properti e s.Furthe r,  in the ab ove-m en t ion ed sphering  tre atment in  which each of the par ticles  as  astar t in gmat er i als ho uld beheated t oatempe ratu r e s ligh tly  hig her t han its melti ngpoin t  us ing t he ra dio- fr equen cy ind uctio nthermal plasmaso t hat eachofth e p ar ticle s  assum esa s pherica l shape und er t h eeffect of its melt ed s ur face ten sion,w he ne ach of t he  pa rtic l e s isnoth ea ted to t he tem p er at ure sli gh tly hi gh ertha n its melt ing po int, eac h oft he particles f a ils to assume as p he r ical  sh ape since i ts s u rfac e is no t m e lted.  On t he ot h er hand, when eachof  the p artic les is heated  to a temp e r a ture exc essi v el y  higher than i ts bo iling poin t , each o f the  particl es change s in const ruc tio n o r c o m po sition by eva p o rati on  of its  co mponent s o r i ngr edi en ts. Conse quently,  w he nth e t herma l  plasmais  us ed  as  heat i ng mea ns in  the  a bo v e s phering tr e a tmen t,  i t is  ne ce ssary  tok eep  consta nt a temp era t ureof  a  heatin g portion inth e thermalpl asm a. Whe n each ofthe particles is made o f  a subs tan ce ha ving  a high m elt ing po int and aboilin g poi nt  sl i g htlyh i gh erth an the mel t i ngpoint ,theheating  portion of the th e rma l plasma must  be  kep t at  ah ig h  te mp e rat u r e within a rela tively nar ro w  r ange. Ther adi o- frequen cy indu ction thermal  plasma ( hereinaft er  ref e r red  to as the  radio-f requencyp lasma )  h as aben efit t omak e iteasy t o  ob taina high  t empera turene cessaryfor the above spheringtreatm ent.Fur ther, the  ra d io- frequency  pla sma has th e a dva ntage  ov e r a s o-ca l le d e l ectro de-t y pe therma l  p las m a  t hat it is f r e e  fr om an y c o ntamin ants . Conseqeuntly, t h e radio-freque ncyp l asm a i s  a p re ferable heatin g m ea n s  for the  s phering tre atment a nd for t he syn thesis  of the s ub s tance. 
Inord er to pro tec tt he surf ac eo f each  ofth e sphe ricalp arti c l es from attacksby mo i s turea ndharmf ul  radiat i on,only the s urface of e ach of  the  part icles c an  be me lte dso that ea c h of theparti cles is  c oate d w ith alaye r of th e th us  me lted ands o li d if ie d  su rfac e. On eof z inc sulfi d e phosph ors ,  for ex am ple , a phos p ho r ZnS:Cu isuse d anl u minescentm ater i al in an  elect rolumi nescenc e  di sp lay uni t a n d  like uni ts. Howev e r,th is z inc sulf id e ph os pho ris poor in wate r-resist ing prop e r ties.Due to i t s poor  w ater resista nce,th i s lumin e s cen t mater i alrea cts  wit h m oist u re contained in the airandloses its 
 of_the Japanese_Electric Society, 1993; and,_Japanese Patent_laid-Open_No. Hei_6-25717).
In_the above chemical_reaction, when the_atmosphere temperature thereof is_too low, its_reaction_rate decreases. On the other hand, when the atmosphere temperature is too high, a_product_obtained is_decomposed,_which_makes it impossible to obtain_a product having a predetermined_composition and_physical properties. Further, in the above-mentioned sphering treatment_in_which each of_the particles as a starting material should be heated_to a temperature slightly higher than_its melting point_using_the_radio-frequency induction thermal plasma_so that each of the particles_assumes a spherical shape under the_effect of its melted surface tension, when_each of the particles is not_heated to the temperature slightly_higher than_its melting point, each of_the particles fails_to assume_a spherical shape_since its surface is not melted._On the other_hand, when each of the particles_is_heated to a_temperature_excessively_higher than_its boiling point,_each_of the_particles_changes in construction or composition by_evaporation_of its components or ingredients. Consequently, when_the thermal plasma is_used_as heating means in_the above sphering treatment, it_is necessary to keep constant a_temperature of_a heating_portion in the thermal plasma. When each of the particles is_made of a substance having a_high melting point and_a boiling_point_slightly higher than_the_melting point,_the heating portion of the thermal plasma_must be_kept at a high temperature within_a relatively narrow range._The_radio-frequency induction thermal plasma (hereinafter referred_to as the radio-frequency plasma) has_a benefit to make it_easy_to_obtain a high temperature necessary_for the above sphering treatment. Further,_the radio-frequency plasma_has the advantage over a so-called electrode-type_thermal_plasma that it is free from_any_contaminants. Conseqeuntly, the radio-frequency plasma is_a_preferable_heating means for the sphering_treatment and for the synthesis of_the substance.
In order to protect the surface of each_of the spherical_particles from attacks by moisture_and_harmful_radiation, only the surface of each of the particles can_be melted_so that each_of the particles is coated with a layer of the_thus melted and solidified surface. One of_zinc sulfide phosphors, for example, a phosphor ZnS:Cu is used an_luminescent material in an electroluminescence display unit and_like units. However, this zinc sulfide phosphor_is poor in water-resisting_properties._Due to its poor water resistance, this_luminescent_material_reacts_with moisture contained_in the air_and loses its