Electrical trees are widely grown in laboratories from metallic needle electrodes to enable the degradation process to proceed rapidly at voltages of ∼10 kV AC. Here it is shown that cone-shaped air gaps may form at a needle tip, even when they are not visible optically. Using X-ray imaging techniques, samples with and without air gaps up to 55 μm long have been characterized. The presence of air gaps led to partial discharge (PD) patterns resembling void-discharges. Prior to tree initiation, discharge characteristics evolved with increased voltage increments and increased phase lag behind the applied voltage. These changes to PD characteristics reflect a change in the physical state of the air gap. Although samples with air gaps generally initiated trees quicker than those with no gap, PD activities did not appear to play a decisive role in tree initiation times. However, initial voids were found to have major impact on the subsequent tree growth. In samples with persistent pre-tree air-gap PDs, trees grew larger branchy structures with higher PD magnitudes. In addition, the PDs became more asymmetric between positive and negative half-cycles than occurred in samples with no air gaps. It is shown that understanding the interface between the needle and polymer at a micron scale is critical to interpretation of laboratory tree growth experiments.

中文翻译：

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针和聚合物之间的气隙对针平面样品中电树的影响

电树在实验室中由金属针状电极广泛种植，以使降解过程在约 10 kV AC 的电压下快速进行。此处表明，即使在光学上不可见时，也可能在针尖处形成锥形气隙。使用 X 射线成像技术，已经表征了长达 55 μm 的具有和不具有气隙的样品。气隙的存在导致类似于空放电的局部放电 (PD) 模式。在树启动之前，放电特性随着电压增量的增加和施加电压后相位滞后的增加而演变。PD 特性的这些变化反映了气隙物理状态的变化。虽然有空气间隙的样本通常比没有间隙的样本更快地启动树木，PD 活动似乎对树木萌生时间没有起到决定性作用。然而，发现初始空隙对随后的树木生长有重大影响。在具有持久性树前气隙 PD 的样本中，树木长出更大的枝状结构，具有更高的 PD 幅度。此外，与没有气隙的样品相比，PD 在正负半周期之间变得更加不对称。结果表明，在微米尺度上了解针和聚合物之间的界面对于解释实验室树木生长实验至关重要。与没有气隙的样品相比，PD 在正半周和负半周之间变得更加不对称。结果表明，在微米尺度上了解针和聚合物之间的界面对于解释实验室树木生长实验至关重要。与没有气隙的样品相比，PD 在正半周和负半周之间变得更加不对称。结果表明，在微米尺度上了解针和聚合物之间的界面对于解释实验室树木生长实验至关重要。