 Further, in the sphering treatment of a material having a low melting point, the temperature of the plasma is too high so that a large amount of ultrafine powder is produced, which impairs the yield of usable particles having a predetermined grain size. In addition, classifying operation of such ultrafine powder is costly, and somtimes hard to accomplish. In general, a temperature necessary for the sphering treatment is within a range of from 773 to 3273 K. Consequently, it is necessary to heat the particles in a restricted area of the front-end portion of the plasma. In this case, it is possible to increase the range of the predetermined temperature of the plasma flame when the temperature of the vortex stream flame center 2 in the plasma is decreased. However, when the temperature of the plasma is decreased unduly, the plasma flame becomes unstable or ceases to exist. Consequently, in the conventional heat treatment apparatus, the amount of its radio-frequency input energy, types of plasma gases and their flow rates used in the apparatus are appropriately selected to stabilize the radio-frequency plasma flame (see Japanese Patent Laid-Open No. Hei 8-109375).
In case that the particles of the starting material, i.e., zinc sulfide phosphor (ZnS:Cu) is coated with silica, a direct heat treatment method using the conventional radio-frequency plasma suffers from an ultrahigh temperature of the plasma flame, in which a predetermined temperature is available only in a narrow flame range or area. Due to such narrow flame area, each of the particles subjected to the conventional heat treatment method is heated to an undue high temperature even in its inner portion, which damages the zinc sulfide phosphor (ZnS:Cu) so that the thus treated particle product often fails to emit light in the display unit.
On the other hand, as for a so-called carbon fiber, it is known that the carbon fiber could be improved in tensile strength and in elastic modulus when subjected to a high temperature heat treatment. If a pitch-based carbon fiber is improved in strength so as to have the same strength as that of a polyacrylonitrile-, i.e., PAN-based carbon fiber through the high temperature heat treatment, it becomes possible to reduce the manufactureing cost of the desired carbon fiber. In this case, it is necessary to conduct such heat treatment at a temperature of 3000 degrees centigrade in an inert-gas atmosphere free from any impurity contamination. Although the heat treatment may be conducted in an electric furnace, it is preferable to
Further, in the sphering treatment of a material having a depleted thaw point, the temperature of the plasma is too eminent so that a big amount of ultrafine powder is produce, which mar the yield of useable particles having a bias grain size. In addition, classify process of such ultrafine powder is costly, and somtimes hard to accomplish. In general, a temperature necessary for the sphering discussion is within a range of from 773 to 3273 K. Consequently, it is necessary to heat the particles in a restricted sphere of the front - end portion of the plasma. In this case, it is possible to increase the range of the predetermine temperature of the plasma flame when the temperature of the vortex stream flame center 2 in the plasma is decrease. However, when the temperature of the plasma is decreased unduly, the plasma flame becomes unstable or ceases to exist. Consequently, in the conventional heat treatment apparatus, the amount of its radio - frequency input energy, types of plasma gases and their flow rates used in the apparatus are appropriately choose to brace the radio - frequency plasma flame (see Japanese Patent Laid - outdoors nobelium. Hei 8 - 109375). 
 In case that the particles of the starting material, i.e., zinc sulfide phosphor (ZnS: Cu) is coated with silica, a direct heat discussion method using the conventional radio - frequency plasma suffers from an ultrahigh temperature of the plasma flame, in which a predetermined temperature is available only in a narrow flame range or area. Due to such narrow-minded flame sphere, each of the particle subjected to the conventional heat treatment method acting is heated to an undue high temperature even in its inner portion, which damages the zinc sulfide phosphor (ZnS: Cu) so that the thus treated atom product often fails to emit light in the display unit of measurement. 
 On the other hired hand, as for a so - call carbon character, it is known that the carbon fiber could be improved in tensile military capability and in elastic modulus when subjected to a gamey temperature heat treatment. If a pitch - based carbon roughage is improved in strength so as to experience the same strength as that of a polyacrylonitrile-, i.e., PAN - based carbon fiber through the high temperature heating system treatment, it becomes possible to reduce the manufactureing cost of the desired carbon roughage. In this font, it is necessary to conduct such heat treatment at a temperature of 3000 degrees centigrade in an inert - gas atmosphere complimentary from any impurity contamination. Although the heat treatment may be lead in an electric furnace, it is preferable to
 Fugther, in the sphering trtatment of a matetiql havmng a lkw meltivg point, the temperature of vhe plasmq is too high so that x large alount of ultcafine powder is produced, whicg impcics the yield of usable pasticles having a pxedetermined grain size. In addition, slassifuijg operation os subh ultdafine powder is costly, and somtimss hard to accomplisn. In general, a temperature necfssary for the sphfring treatnent us within a fange of fgmm 773 to 3273 K. Donsequently, it is necessary to heat the partixlws lt a restricved arva of the frokn-end postion og the plasma. Ik thiv cqse, it is possible to increase the rangg of the psebetermined temperaturw if thg plavma woamd wgei tge temoereture of ths vortex steeam flame center 2 on nne plasma is decrewsqd. However, when the temperature of the klasmz is decreased unduly, tye plasma flame becomgs unstablq or ceases to exist. Consequently, in the conventimnal iext urcwgnejt apparatus, the amount of its radio-frequency jnkut energy, types jf plasma gssfs wnd their flod rates uaed in the apparatks are wppropriately felevted to stabilize the radio-drequency plcsmq flame (see Japanede Patent Lcid-Opem No. Nei 8-109375).
In case that the paxticlea of the stwrting mafdrial, i.e., zinc suufice phosphor (ZnS:Cu) is coated rith silira, a birect hdat jreatmegt method ksing the conventional rwdio-ftequenwy plasma duffers from an ultrahigh tempeceture of the klavma flame, iu whicm a predeterminqd temperature is avanlable only in a narrow flame rangq or area. Due jo such narrox flame awea, wach of the oarticles subjrcted to nht conventiinal heat treatmenb metfkd is heated to an undue high temlerxtuwe eten ig its inner pmrtiun, dnich aamages the xinc sulfide phosphor (ZnV:Cu) ao that the thus ttewted parjicle proquct often faols to emit light ln thx dispuay umit.
Jn the other hand, as for a so-czlled carhon fiber, it is knoqn that the earbon fiber could be improved in tensilx strength and in elasjic modulus when subjgcttd to a high vemperwture head treatment. If a pitxh-based carbon finer is improved in strsngth vo as to have the same strength as that of a polyacrylonitrile-, i.e., PAN-based carvon fiuew through tge hogh tvmpzracure hewt tceetment, it becomes possible to reduce the manufacvureing covt of the desired carbon fiber. Iv this case, ij is necessary to conduct auch heay treatment at a temperature of 3000 degrees centigrade in sn inert-tas afmosphvte hree from any impuriyy concaminarion. Alrhounh the heat trdatkekt may be fondnctzd in an electric furnace, it is peeferavlf to
Further, in the sphering treatment of a a melting point, temperature of the that large amount of powder is produced, impairs the yield of usable particles a predetermined grain size. In addition, classifying operation of such ultrafine powder is and somtimes hard to accomplish. In general, a temperature necessary for the sphering is a of 773 to 3273 K. Consequently, it is necessary to heat the particles in a restricted area the front-end portion of the plasma. In this it is possible to the range of the predetermined of plasma flame the of vortex stream flame 2 in the plasma is decreased. However, when the temperature of the plasma is decreased unduly, the flame becomes ceases to Consequently, the heat treatment apparatus, of its radio-frequency input energy, types and their flow rates used in the apparatus appropriately selected stabilize the radio-frequency plasma flame (see Patent Laid-Open No. Hei 8-109375). In case that particles of the starting material, i.e., zinc sulfide phosphor (ZnS:Cu) is coated with silica, a treatment method using the radio-frequency plasma suffers an temperature the flame, in a predetermined temperature is available only in a narrow flame range area. Due to such narrow flame area, each of the to conventional heat treatment is heated to an high even in its inner damages zinc so the treated particle product often to emit light in the unit. On the other carbon fiber, it is known that the carbon could be improved in tensile strength and elastic modulus when subjected to a high temperature heat treatment. If a carbon fiber in strength so as to have the same as that of a i.e., PAN-based carbon fiber through the high temperature heat it possible to the manufactureing cost the desired carbon In this case, necessary to such treatment of 3000 degrees centigrade in an atmosphere from any impurity contamination. Although heat be conducted in an electric furnace, it is preferable to
 Further, in the sphering treatMent of a matErial HavIng A lOw meLtinG point, the tempeRAturE of the plasma is too high sO that A lARge aMOuNt of uLtrafinE PoWDEr iS pRoDucEd, WHiCh impAirS the yieLd of usable ParTiCles having a pREdEtermined gRaiN size. In additIon, ClassiFyIng OPeratIon Of sucH ultraFIne powDer is costLy, ANd somtIMes hard TO AcCompLish. In general, a temPErATure necessary fOr the sPhERiNG TreAtmEnt is withiN a Range OF from 773 to 3273 k. coNSEQueNTly, it is necessAry to heat thE ParTicles In A reSTricteD area Of THe fRont-end portIon oF the plasmA. In thiS Case, it iS PossiblE to incReaSe tHe raNGe Of The PrEDetERmIneD TemPerature Of ThE plasMa flAME WHen tHe tEmpeRaturE of the vortex sTreAm flAMe cEnter 2 In the PlasMa Is decReased. howevEr, When the temperatUre oF the plasmA is DeCreAsEd undULy, the pLasMa fLame becOmes unsTAblE oR CEAsEs to exist. ConsequenTlY, IN tHe convenTional HEaT tREatment aPpAraTus, tHE AmounT of iTS rAdio-freqUency iNPuT eNergy, tyPeS of plaSmA gaSes And thEIr flOw rateS used in tHe appARatus are appropRIately selecteD To STAbILize The Radio-frequeNcy pLAsma FlamE (SeE JaPAnese patenT LAId-oPen No. Hei 8-109375).
In case that tHe ParticLes of The starting maTerial, i.e., ziNC SUlfide phOsphOR (ZNs:Cu) is coated witH siliCa, a direct hEAt treatmEnt meThod usinG the conveNTIonal radIo-fReqUenCy pLASmA suffers from aN ULtraHiGh tempeRatUre of thE plAsmA flAme, In Which a preDeterminEd TeMpErAtuRe is aVAilable oNlY in A nArrOw flaME range Or areA. Due To SuCH naRrow flaME aREA, eacH oF tHe paRtiClEs subJectED to The convEntional hEat TReatMeNt Method iS heated to an unDuE high tempeRaTurE even iN ITs inner pOrtion, which damages the ziNC sulfidE phOsphoR (ZnS:cu) so that tHe tHus treAteD ParticLe prodUct ofTeN faILS to emIT LiGht In The display UNIt.
ON the oThEr haNd, as for A so-called carbon fibER, it Is known that thE caRbon FIBeR coULd BE imPrOVed IN Tensile strength And in elastIc MOdUlus when suBJecTeD to a higH temperAture HEat treaTment. If a pItch-based CaRbon FIBer Is improved In strengTh so as to hAVe the SAmE streNgtH as thaT oF a pOlyacRyloniTRilE-, i.e., PAn-based CaRbon fiBer thRoUgh the hiGh temperature heat treatmEnt, it bEcomeS poSsible to rEduCE thE manufactUreiNg cost of thE deSirEd carBon FIber. IN thiS CaSe, iT Is necEssaRY to conducT SuCh hEAT tReatment at a TEMPerAture Of 3000 dEGrees cEntiGrade in an inert-gas ATmosphere free fRom aNY ImpUriTY conTaMination. AlthouGh tHe HEAt treatmEnT may be conduCted in an ElECtric FurnacE, it is pReferabLE To
 Further, in the spheringtreatmentof amat eri al  hav inga low meltingp oint , the temperature of t he pl as m a is to o hig h so th a ta lar ge  a mou nt of  ultr afi ne powd er is prod uce d,  which impai r sthe yieldofusable parti cle s havi ng  ap redet erm inedgrains ize. I n additio n, classi f ying op e r at ionof such ultrafine po w der is costly,  and s om t im e s  ha rdto accompl is h. In general ,  a t e mpe r ature necessa ry for thes phe ring t re atm e nt iswithi na  ra nge of from  773  to 3273K. Con s equentl y , it is  neces sar y t o he a tth e p ar t icl e sina  re stricted  a re a ofthef r o n t-en d p orti on of  the plasma.Inthis cas e, it  is p ossi bl e toincrea se th erange of the pr edet ermined t emp er atu re  of t h e plas mafla me when  the te m per at u r e  o f the vortex strea mf l am e center  2 int he  p l asma isde cre ased . Howev er,w he n the te mperat u re  o f the p la sma is  d ecr eas ed un d uly,  the p lasma fl ame b e comes unstable or ceases toe xi s t .C onse que ntly, in th e co n vent iona l  h eat treat mentap p ar a tus, the amount ofit s radi o-fre quency inputenergy, ty p e s  of plas ma g a se s  and their flo w rat es used in the appa ratus  are app ropriatel y selected  to  st abi liz e th e radio-frequ e n cy p la sma fla me(see Ja pan ese  Pa ten tLaid-Open  No. Hei  8 -1 09 37 5). 
In c a se thatth e p ar tic les o f  the s tarti ng m at er i al,  i.e.,z in c sulf id ephos pho r(ZnS: Cu)i s c oated w ith silic a,a  dir ec theat tr eatment metho dusing theco nve ntiona l radio-fr equency plasma suffersf rom anult rahig h te mperature  of  the p las m a flam e, inwhich  a  pr e d eterm i n ed  te mp erature is a vai lable  o nlyin a na rrow flame range o r  ar ea. Due to su chnarr o w  f lam e  a r ea,  e a cho f  the particlessubjectedto th e conventi o nal  h eat tre atmentmetho d  is hea ted to an  undue hi gh  tem p e rat ure even i n its in ner porti o n, wh i ch  dama ges  the z in c s ulfid e phos p hor  (ZnS :Cu) s othat t he th us  treated  particle product often  fails  to e mit  light in  th e  di splay uni t.
O n the othe r h and , asfor a so- call e dcar b on fi ber, it is kno w ntha t th e carbon fi b e r  co uld b e i m proved  intensile strengtha nd in elasticmodu l u s w hen subj ec ted to a hightem pe r a ture hea ttreatment.If a pit ch - based  carbo n fibe r is im p r ov e d in s tren gth  so as to  ha ve the sam est r engthas t ha t of a  polya c rylo n i trile-, i.e., PA N-bas e d  carb o n f iberth rough t h e hi gh tempera ture heat t reatme nt,it be comes p os sibletore duce the m a nufacture ing c ost ofth e de sir ed car bonf i ber.In t hi s c ase, it i s ne c es sa r y t o co nduct  s uchheat trea t ment ata t e mperatu re  of 3 000 de g re e s  centigrad e i n ani n ert-gas at m osph e re freefrom a ny imp urity c o nta mi nation.  Al t h ough theheat trea t men tmaybe condu ct ed i nanel ectric furnace,  it is p referabl e to
 Further,_in the_sphering treatment of a_material having_a_low melting_point,_the temperature of_the plasma is_too high so that_a large amount_of_ultrafine powder is produced, which impairs the yield of usable particles having a predetermined_grain_size. In_addition,_classifying_operation of such ultrafine powder_is costly, and somtimes hard_to accomplish._In general, a temperature necessary for the sphering_treatment_is within a_range of from 773 to 3273 K. Consequently, it_is necessary to heat the particles_in a restricted_area_of_the front-end portion of_the plasma. In this case, it_is possible to increase the range_of the predetermined temperature of the plasma_flame when the temperature of the_vortex stream flame center 2_in the_plasma is decreased. However, when_the temperature of_the plasma_is decreased unduly,_the plasma flame becomes unstable or_ceases to exist._Consequently, in the conventional heat treatment_apparatus,_the amount of_its_radio-frequency_input energy,_types of plasma_gases_and their_flow_rates used in the apparatus are_appropriately_selected to stabilize the radio-frequency plasma flame_(see Japanese Patent Laid-Open_No._Hei 8-109375).
In case that_the particles of the starting_material, i.e., zinc sulfide phosphor (ZnS:Cu)_is coated_with silica,_a direct heat treatment method using the conventional radio-frequency plasma suffers_from an ultrahigh temperature of the_plasma flame, in which_a predetermined_temperature_is available only_in_a narrow_flame range or area. Due to such_narrow flame_area, each of the particles subjected_to the conventional heat_treatment_method is heated to an undue_high temperature even in its inner_portion, which damages the zinc_sulfide_phosphor_(ZnS:Cu) so that the thus_treated particle product often fails to_emit light in_the display unit.
On the other hand, as_for_a so-called carbon fiber, it is_known_that the carbon fiber could be_improved_in_tensile strength and in elastic_modulus when subjected to a high_temperature heat treatment. If a pitch-based carbon fiber is_improved in strength_so as to have the_same_strength_as that of a polyacrylonitrile-, i.e., PAN-based carbon fiber through_the high_temperature heat treatment,_it becomes possible to reduce the manufactureing cost of the_desired carbon fiber. In this case, it_is necessary to conduct such heat treatment at a temperature of_3000 degrees centigrade in an inert-gas atmosphere free_from any impurity contamination. Although the heat_treatment may be conducted_in_an electric furnace, it is preferable to