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Dear Liz,

    My stay in Thailand has certainly been the experience of my life.

    Life is busy and exciting.Bangkok is just like any other big city with a population of 10  million and heavy traffic.I'm very lucky because my host family is in a nice quiet area outside the city.There are Mr. and Mrs. Phairat, their son Sanan, who is 18, the daughter Chinda, who is 16, and Grandpa and Grandma.

    I go to an international school with Sanan and Chinda.The school teaches about 70 percent in English,and 30 percent in Thai. I've learned some spoken language,but Thai writing is very difficult.The cooking lesson is my favourite.I'm learning all about Thai food and culture People don't use chopsticks here,but spoons and forks.When I come back,I'm going to cook you a real Thai meal.

    Last weekend we visited some temples.We also drove to Pattaya beach near Bangkok.I thought it was great,but Sanan and Chinda say that next month they're taking me to Phuket  Island,where the beaches are even more beautiful.The month after next,we're going to travel to Mr.Phairat's hometown in the north of Thailand.The Phairats own land there,and they have two elephants.I'm going to ride those elephants — and even wash them.Sanan and  Chinda say it's really fun.I'm amazed by everything in this country,especially by the elephants.Elephants are an  important pan of Thai culture and way of life.They have been a traditional symbol(象征)of Thailand for many years in times of war and peace.In the seventh century,a Thai king trained 20,000 elephants for battle.

    I'll tell you all about my Thai boxing(拳击)lessons next time I write.

Love,

Mandy

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打不碎的韧性陶瓷

       ①陶瓷,是用物理化学方法制造出来的最早的人造材料。一万多年以前,它的诞生使人类由旧石器时代进入了新石器时代。然而,陶瓷的脆性一直是它的致命弱点,如果能够克服这个弱点,再加上它原有的耐磨、耐高温、耐腐蚀等非常有价值的性能,那么陶瓷材料的前途将远远超过现代的优质合金。

       ②可喜的是,经过科学家的不懈努力,终于找到了克服陶瓷脆性的“药方”。

       ③首先,从改善内部结构着手。研究表明,在陶瓷的原料中添加少量的氧化钇、氧化镁、氧化钙等粉末,经高温炼制成氧化锆陶瓷后,其中的氧化锆便生成两种晶体:立方晶体和四方晶体。当陶瓷受到外力作用时,四方晶体便变成一种单斜晶体,体积迅速“膨胀”。由于晶体的体积急速增大,进而可阻止陶瓷中原先存在的细微裂纹的扩展,陶瓷就不会破裂了。有人在氧化铝坯料中加入二氧化锆,当加入量为基体体积的15%时,陶瓷的硬度可以提高3倍,有人称之为“陶瓷钢”。

       ④其次,将纤维均匀地分布于陶瓷的原料中,以提高陶瓷的强度和韧性,其原理与我们在石灰中加入纸筋相类似。这是因为,将纤维加入陶瓷原料中,具有以下三大作用:纤维不易拉断,在工作时可承担大部分外加负荷,减轻陶瓷的负担,使裂纹不易产生;纤维与陶瓷体结合在一起后,具有很大的摩擦力,于是,陶瓷的韧性可以大大增加;即使陶瓷出现了细微裂纹,纤维也能将它们紧紧拉住,使裂纹不至于进一步扩展开来。

       ⑤第三,还在改善陶瓷表面状态方面下功夫。一般来说,陶瓷的断裂大都从表面开始,因此,改善陶瓷表面状态就犹如为防止陶瓷的破损设下了一道屏障。具体方法为:通过化学和机械抛光技术消除陶瓷的表面缺陷;对氮化硅、碳化硅等非氧化物,只要通过控制表面氧化技术,就可消除表面缺陷或使裂纹尖端变钝;通过热处理也可达到表面强化或增韧的目的。

       ⑥经过特殊加工的陶瓷,再也不是那种碰不得、摔不起的“瓷娃娃”了,即使把它丢在水泥地上也毫无损伤。这种打不碎的陶瓷,目前已经广泛应用于一些高科技领域,如用来制造汽车、飞机、快艇的发动机和其他重要零部件。这类陶瓷材料无疑将有广阔的发展前景。