TiB₂ has been widely used as conductive ceramic materials,ceramic cutting tools and molds,composite ceramics,electrode materials and cathodes for electro-chemical processing of aluminum in Hall-Heroults process,heating ceramic materials and metal materials enhancer because of its unique and fascinating properties such as high melting temperature,hardness,elastic modulus,good electro-conductibility and thermal diffusivity,and excellent refractory properties and chemical inertness^{[1, 2, 3, 4, 5, 6, 7]}. At present,TiB₂ has been produced by the reduction of TiO₂ and B₂O₃ with carbon and active metal,solid state reduction of TiCl₄ and the self-propagating high-temperature synthesis method^{[8, 9, 10, 11, 12]}. From the viewpoint of reducing costs,combustion synthesis,also known as self-propagating high-temperature synthesis (SHS),widely used to prepare a great diversity of ceramic powders^[13],is a preferable technique owing to its lower energy consumption and simplified equipment.

However,due to the high temperature in the process of the self-propagating high-temperature synthesis which leads to the rapid particle growth rate,it is very difficult to obtain pure and ultrafine-particle TiB₂ powder. NaCl addition as diluent to prepare TiB₂ powder with high pure and ultrafine-particle size has been proved an efficient way^{[14, 15]}. The introduction of soluble salt into mixture precursor can effectively prevent particles aggregation and forming submicron TiB₂ and greatly improve the purity^[16]. However,the effect of NaCl addition on morphology,size and phase of submicron TiB₂ powders prepared by self-propagating high-temperature synthesis on a large scale has never been reported,to our knowledge.

Thus,we report the combustion synthesis of submicron TiB₂ powder on a large scale combining with the employment of soluble salt. Effect of NaCl addition on the microstructure,average particle size and phases of the final products are investigated. Moreover,according to the experimental results and characterizations,the reaction procedure of combustion synthesis and the formation mechanism of TiB₂ are discussed.

The raw materials used in this paper were commercial grade B₂O₃ (>99.0%,mass fraction,same as below,Liaoning Pengda Science and Technology Ltd.,Yingkou city),TiO₂ (>99.5%,Shanghai Jianghutaibai Chemical Products Co.,Shanghai city),Mg (99.5%,Kunshan Fuerbang New Material Technology Co.,Ltd.,Kunshan city) and NaCl powders (99.0%,Nanjing Dongde Chemical Technology Co.,Ltd.,Nanjing city). The reaction follows the formula (1). Considering the high combustion temperature and low-melting-point Mg volatile loss^[17],Mg excess 5% and k of NaCl increases by 0.5 increment until the system is incapable of self-propagating reaction. The total material quantity is 2000g. Reactant powders were mixed in planetary ball mill with the ball-to-powder mass ratio of 2 ∶1 for 8h. The obtained mixture was uniaxially pressed to form cylindrical pellets (25.4cm in diameter and about 2.5cm high) of preform at 15MPa. Original sample was then loaded into home-made reaction chamber of SHS reactor. The reaction chamber was evacuated and filled with argon gas at the pressure of 0.5MPa. The igniting tablet was placed in the copper crucible of the reaction chamber,and the sample was smoothly on the tablet. Autoclave was sealed and then heated,while argon was filled with 0.5MPa,released to eliminate the autoclave air after 10 min. Until the internal temperature was heated to about 180℃,argon was filled with 2 MPa. When the reaction chamber was about 260℃,the internal temperature and pressure increased sharply,and the self-propagating reaction started and completed within a few minutes. Reactants turned into block product containing TiB₂. After the block product was grinded into powders,NaCl was washed away with distilled water and byproduct MgO was leached out from the powder with dilute HCl ( 9.6mol·L^-1 ) excess of 50% following reaction (2). Leaching time lasted 72h,and the leaching liquid was stirred 1 time every 4h. TiB₂ was separated from leaching solution by filtration,washed 5 times with distilled water to remove the residue of HCl. Then the TiB₂ powders were put in a vacuum oven and dried for 8 hours under 100℃.

The phase analysis of the powder samples was investigated by X-ray diffraction (XRD,D/max-2400). The morphology and identification of TiB₂ powder was examined by scanning electron microscope (SEM,JSM-6700) equipped with an energy dispersive spectrometer (EDS). The content of Mg and O impurities in the leached TiB₂ powders was determined by atomic absorption spectrum(CAAM-2001,Haiguang GGX-6).

Samples were prepared with different ratio (k=0.5,1.0,1.5,2.0mol) of NaCl. Reactant mixture could be ignited under these k values. However,when k increased to 2.25,the combustion synthesis reaction system could not occur. Merzhanov,etc^[18]. proposed empirical criterion: only when the adiabatic temperature T_ad ≥1800K,can the SHS reaction be self-sustained,otherwise it needs supplementary energy. When k increases to 2.25,the combustion synthesis reaction system cannot occur,and thus it is estimated adiabatic temperature drops below 1800K.

The product morphology with different amount of NaCl is shown in Fig.1. As shown in Fig.1,weak agglomeration of spherical particles with a large size of about 200-500nm can be observed. Simultaneously,melting sintering phenomenon of particles can also be observed and small particles adhere to the surface of large particles. For the sample of k=2.0,small particles agglomerates into a flocculent. In the process of combustion synthesis,a certain amount of TiB₂ particles was generated and saturated in the NaCl melt. As the system temperature was reduced,it was difficult for the precipitated particles in a solid environment to grow up continuously,which resulted in a smaller size of TiB₂. At the same time,small particles agglomerated into a flocculent,which might contribute to the generation of MgO along with the formation of the products. When the temperature decreased,the resulting sample was coated with MgO and NaCl. Product and MgO were in the environment of NaCl melt. With the processing of the reaction,the system temperature decreased,and the final desired product particles were coated with MgO and NaCl,or MgO and NaCl existed between TiB₂ particles. This resulted in the occurrence of sintering phenomenon and the generation of large-size aggregates. For the sample of k = 1.5,a rod-like morphology is observed,which contribute to the presence of large quantity of NaCl melt in the combustion synthesis inducing the liquid phase sintering on the melt surface for large spherical particles precipitated by supersaturation. EDX analysis results show that TiB₂ purity after leaching is above 98%. It is consistent with the results of atomic absorption spectrum.

	Fig.1 SEM photos of combustion synthesis product with different amount of diluent (before leaching) (a)k=0.5；(b)k=1.0；(c)k=1.5；(d)k=2.0
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Fig.2 shows SEM micrographs of TiB₂ obtained from combustion synthesis with different amount of NaCl after leaching. Compared to large size aggregates composed of a large number of small particles before leaching,weak aggregates formed by submicron particles with the size of 0.2μm can be observed after leaching. Melting sintering phenomenon between the particles is also seen. The degree of sintering decreases with the increase of diluent. The particles show a state of loose agglomeration without significant sintering phenomenon and the size of aggregates drops dramatically. Submicron particles show a spherical or ellipsoidal shape with smooth surface and smaller gap between particles. The removal of surface layers of NaCl and MgO on the particles results in the decrease of grain size. With the increase of NaCl amount,which leads to the drop of combustion temperature in system,the lower particle formation temperature and weak melting sintering degree are obtained.

	Fig.2 SEM photos of combustion synthesis product with different amount of diluent (after leaching) (a)k=0.5；(b)k=1.0；(c)k=1.5；(d)k=2.0
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The statistics of submicron particle size distribution is shown in Fig.3. The particle size follows normal distribution,and the range of distribution becomes narrow gradually with the increase of the diluent. At k=0.5,the particle size is in the range of 350 to 750nm. When the value of k increases to 2.0,the particle size decreases to 100-450nm. The increase of diluent leads to the decrease of the average particle size,as shown in Fig.4. At k = 0.5,the average particle size is 496 nm and decreases to 268nm when the value of k increases to 2.0,therefore average particle size of the products decreases with the increase of diluent amount.

	Fig.3 Particle size of combustion synthesis product with different addition of diluent (after leaching) (a)k=0.5;(b)k=1.0;(c)k=1.5;(d)k=2.0
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	Fig.4 Dependence of mean particle size on NaCl addition (after leaching)
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Fig.5 shows the XRD patterns of the products with different addition of NaCl. As seen in Fig.5(a),all the samples before leaching consist of MgO,TiB₂,NaCl and Mg₃B₂O₆ phases. The diffraction peaks of MgO in the four samples are the strongest,and MgO is the main phase of the product. However,the relative intensity of diffraction peak of NaCl is different as the k values vary. The intensity of diffraction peak of NaCl becomes stronger gradually with the increase of k. In addition,all products also contain a small amount of Mg₃B₂O₆ phase with relatively weak diffraction peak. After leaching,the products are mainly composed of TiB₂ and Mg₃B₂O₆,as shown in Fig.5(b). The phases of MgO and residual diluent NaCl are not observed because of the removal of them in the process of leaching. Both the relative intensity of the diffraction peak shows that when the value of k is in the range of 0.5 to 1.5,diffraction peak of Mg₃B₂O₆ are all weak. However,when k increases to 2.0,diffraction peak of Mg₃B₂O₆ becomes stronger.

	Fig.5 XRD patterns of product with different addition of diluent (a)before leaching;(b)after leaching
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Green samples were preheated inside the reaction chamber of SHS reactor. When the temperature reached about 240℃,the ignition agents began to react and released a lot of heat. The adjacent reactant was heated to high temperature rapidly. When the temperature reached 723K,the reactant B₂O₃ began to melt. When the temperature continued to rise to 923K,the reducing agent Mg melted. The TiO₂ and NaCl in solid state were surrounded quickly by the liquid B₂O₃ and Mg. At the same time,B₂O₃ and Mg fully contacted,while redox reaction began to take place and generated intermediate product B. The reaction formula was shown in (4):

The reaction formula (4) was a strong exothermic reaction which released large amount of heat. The temperature of raw materials rose rapidly. When the temperature reached 1074K,NaCl began to melt,forming a liquid reaction medium. At the same time,due to the strengthening of the quality transmission,Mg liquid and TiO₂ were in full contact with each other,leading to the redox reaction of Mg liquid and TiO₂ to generate intermediate Ti,as indicated in reaction formula (5).

Reaction medium of NaCl liquid promoted the reaction (4) and (5),and the intermediate product B and Ti were rapidly combined to form the final product TiB₂. The reaction formula (6) was as follows.

Exothermic reaction (4),(5) and (6) rapidly released a lot of heat,which transferred through heat conduction to adjacent unreacted materials,and the green sample temperature suddenly rose. When the temperature reached the ignition temperature,the reaction (4),(5) and (6) also occurred. The combustion wave moved from the reaction zone to the unreacted zone by self-propagating. Simultaneously,due to the heat release,the temperature of the prior reaction zone decreased to room temperature.

Diluent significantly affected the average particle size of the product based on three points: reducing the combustion temperature of the system,restraining the particles from growing and sintering,and providing liquid medium for the formation of the particles. When reaching oversaturation,the resulting particles would precipitate from the melt and had difficulty in recrystallization,so that the growth would be inhibited. When the temperature was below the melting point of the diluent,the residual volatile diluent was coagulated,forming coating layer on the surface of the particles,which became an obstacle to the quality transmission. Thus,it was difficult for those particles in the melt to grow up.

Experimental results show that the introduction of soluble salt into mixture precursor can effectively prevent the aggregation of particles and forming submicron TiB₂. The average particle size of the powder product decreases with the increase of the k moles of the diluent. NaCl provides the liquid medium for the combustion synthesis reaction,promotes the mass transfer during the reaction,and improves the conversion rate of TiO₂ and product purity. Submicron powders TiB₂ can be prepared on a large scale via this self-propagating high-temperature combustion synthesis.