Based on the results shown in Fig. 5, another important point should be noted, that is, pure ZrB2 powder can be synthesized by stoichiometric proportion of reaction (5) without excessive B4C in argon at 1500 °C for both the carbon black and graphite cases. The main reason for this phenomenon should be less or none B2O3 loss in Ar atmosphere. In vacuum atmosphere, intermediate product B2O3 in gaseous state was taken away easily with the pumping, which resulted in the lack of B2O3 in the powder mixtures. In argon atmosphere, on the other hand, although the vapor pressure of the intermediate product B2O3 was pretty high at 1500 °C, loss of the B2O3 must be much less than in vacuum atmosphere, because the vaporization of B2O3 was suppressed by Ar gas. The remained B2O3 may present in liquid trapped in pores or channels in the compacted raw powder disk. In addition, the escape of gaseous B2O3 in the block specimens would be more difficult.