KARAKTERISTIK BAHAN KERAMIK

KERAMIK

Keramik berasal dari bahasa yunani yaitu “ceramos”(pembuat barang tembikar tanah liat) dan bahasa inggris “ceramic” adalah material anorganik dan non-metal. Umumnya keramik adalah senyawa antara logam dan non logam. Keramik sudah digunakan sekitar sebelum  4000 SM yang sudah digunakan sebagai busi isolator listrik dan kekuatan terhadap suhu tinggi yang dapat menghasilkan jenis keramik yang sangat berkualitas.

Keramik sendiri dapat dibedakan menjadi 2 yaitu:

  1. keramik tradisional yang biasanya terbuat dari tanah liat. Contoh: porselen, batu ubin, gelas, whiteware, sewer piper,  dll.
  2. keramik modern yang biasanya mempunyai ruang lingkup yang lebih luas dan mempunyai efek dramaris yang lebih bermanfaat untuk kehidupan manusia atau biasanya disebut keramik industri, keramik teknologi tinggi dan keramik halus. Contoh: pemakaian pada benda-benda elektronik,komputer, aerospace, heat changers, turbin, dll.

JENIS IKATAN PADA KERAMIK

Dua jenis ikatan dapat terjadi dalam keramik, yakni ikatan ionik dan kovalen. Sifat keseluruhan material bergantung pada ikatan yang dominan. Bahan keramik dapat dibedakan menjadi dua kelas : kristalin dan amorf (non kristalin). Dalam material kristalin terdapat keteraturan jarak dekat maupun jarak jauh, sedang dalam material amorf mungkin keteraturan jarak pendeknya ada, namun pada jarak jauh keteraturannya tidak ada. Beberapa keramik dapat berada dalam kedua bentuk tersebut, misalnya SiO2. Jenis ikatan yang dominan (ionik atau kovalen) dan struktur internal (kristalin atau amorf) mempengaruhi sifat-sifat bahan keramik. Sifat termal penting bahan keramik adalah kapasitas panas, koefisien ekspansi termal, dan konduktivitas termal. Kapasitas panas bahan adalah kemampuan bahan untuk mengabsorbsi panas dari lingkungan. Panas yang diserap disimpan oleh padatan antara lain dalam bentuk vibrasi (getaran) atom/ion penyusun padatan. Keramik biasanya memiliki ikatan yang kuat dan atom-atom yang ringan. Jadi getaran-getaran atom-atomnya akan berfrekuensi tinggi dan karena ikatannya kuat maka getaran yang besar tidak akan menimbulkan gangguan yang terlalu banyak pada kisi kristalnya.

STRUKTUR KRISTAL KERAMIK

Sruktur kristal keramik bisa biasanya berupa kristal tunggal atau struktur polikristal yang mempunya banyak bijian. Struktur kristal terdiri dari ion-ion. Kation adalah ion negatif dan anion adalah ion positif.

SIFAT FISIS DARI KERAMIK

Kebanyakan dari jenis keramik mempunyai sifat rapuh atau britle, terutama dari jenis keramik tradisional. Meskipun ini tidak untuk semua jenis keramik, coba dibandingkan antara piring  yang terbuat dari keramik dengan membandingkan dengan piring yang terbuat dari logam. Tetapi berbeda dengan keramik yang terbuat dari sintering atau campuran dari sintering dan logam, seperti halnaya keramik tradisional hanya mampu berrtahan pada suhu 120 0C sedangkan yang dari enginering mapu sampai 20000C.

Setelah dilakukan beberapa penelitian keramik memeiliki sifat-sifat bahan sebagai berikut:

  1.   Mekanik

Keramik biasanya materias yang kuat dan tahan korosi, sifat-sifat ini bersamaan dengan kerapatan yang rendah dan tititk lelehnaya yang tinggi yang menjadikan keramik material yang menarik. Kerapuhan dalam keramik merupakan keterbatasan, karena keramik cenderung untuk patah secara tiba-tiba dengan deformasi plastik yang sedikit. Tetapi dalam logam memungkinkan atom tangganya dapat berubah-ubah tanpa memutus ikatan strukturnya, yang memungkinkan logam terdeformasi dibawah tekanan. Keramik akan bertahan karena iktan ion dan kovalennya yang tidak mudah geser. Dalam padatan kristalin, retakan tumbuh melalui butiran (trans granular) dan sepanjang bidang cleavage (keretakan) dalam kristalnya. Permukaan tempat putus yang dihasilkan mungkin memiliki tekstur yang penuh butiran atau kasar.

2.   Sifat Termal

Sifat termal penting bahan keramik adalah kapasitas panas, koefisien ekspansi termal, dan konduktivitas termal. Kapasitas panas bahan adalah kemampuan bahan untuk mengabsorbsi panas dari lingkungan. Panas yang diserap disimpan oleh padatan antara lain dalam bentuk vibrasi (getaran) atom/ion penyusun padatan tersebut. Keramik biasanya memiliki ikatan yang kuat dan atom-atom yang ringan. Jadi getaran-getaran atom-atomnya akan berfrekuensi tinggi dan karena ikatannya kuat maka getaran yang besar tidak akan menimbulkan gangguan yang terlalu banyak pada kisi kristalnya. Hantaran panas dalam padatan melibatkan transfer energi antar atom-atom yang bervibrasi. Vibrasi atom akan mempengaruhi gerakan atom-atom lain di tetangganya dan hasilnya adalah gelombang yang bergerak dengan kecepatan cahaya yakni fonon. Fonon bergerak dalam bahan sampai terhambur baik oleh interaksi fonon-fonon maupun cacat kristal. Keramik amorf yang mengandung banyak cacat kristal menyebabkan fonon selalu terhambur sehingga keramik merupakan konduktor panas yang buruk. Mekanisme hantaran panas oleh elektron, yang dominan pada logam, tidak dominan di keramik karena elektron di keramik sebagian besar terlokalisasi. Contoh paling baik penggunaan keramik untuk insulasi panas adalah pada pesawat ruang angkasa. Hampir semua permukaan pesawat tersebut dibungkus keramik yang terbuat dari serat silika amorf. Titik leleh aluminium adalah 660oC. Ubin menjaga suhu tabung pesawat yang terbuat dari Al pada atau dibawah 175oC, walaupun eksterior pesawat midcapi 140oC.

3.   Sifat Listrik

Sifat bahan listrik sangat bervariasi, karena keramik dikenal dengan isolator yang baik. Beberapa isolator keramik (seperti BaTiO3) dapat di polarisasi dan digunakan sebagai kapasitor. Keramik lain menghantarkan elektron bila energi ambangnya dicapai, dan oleh karena itu disebut semikonduktor, bahan ini memiliki hambatan=0. Akhirnya keramik yang disebut dengan piezoelektrik dapat menghasilkan respond listrik akibat tekanan mekanik atau sebaliknya. Bahan dielektrik adalah isolator yang dapat dipolarisasi pada tingkat molekuler. Material semacam ini di gunakan untuk menyimpan muatan listrik dan elektron pada tegangan tinggi. Bila kapasitor dalam keadaan bermuatan punuh, arus tidak akan mengalir. Namun dengan tegangan tinngi yang dapat mengeksitasielektron dari pita valensi ke pita konduksi. Ada beberapa keramik yang dapt menghasilkan kekuatan dielektrik yang sangt besar , seperti porselen sampai 160 kv/cm. Ada beberapa keramik yang memiliki sifat piezoelektrik, atau kelistriakan tekan, yang merupakan bahan canggih yang bisanya digunakan sebagai sensor, atau tranduser yang terdapat pada mikrofon, dan lain-lain. Dalam bahan keramik ada juga yang dapat juga di hantarkan oleh ion-ion. Kemampuan penghantar ion didasarkan pada anion oksigan yang bergerak, tetapi tetap berupa isolator. Contoh padatan ionik adalah Zirkonia ZrO2 yang distabilkan dengan kalsia (CaO).

4.   Sifat Optik

Keramik juga memiliki sifat optik, karena sifat bahanya yang transparann, translusen, atau opaque. Seperti gelas yang dapat mentransmisikan cahaya dengan difusi, seperti gelas terfrosted (bahan transfulen). Maka dari itu ketika cahaya mengenai keramik itu cahaya akan ditranmisiskan, diabsorpsi, atau dipantulkan. Dua mekanisme penting interaksi cahaya denegn partikel dalam adalah polarisasi elektronik dan transisi elektron antar tingkat energi. Polarisasi adalah distorsi awan elektron atom oleh medan listrik dari cahaya.

Dalam padatan elektron yang energinya tertinggi ada dalam orbital-orbital dalam pita valensi dan orbital-orbital yang tidak terisi biasanya dalam pita konduksi. Gap antara pita valensi dan pita konduksi disebut gap energi.

Demikianlah pembahasan tentang Karakteristik Bahan Keramik, semoga bermanfaat… 🙂

37 Responses to KARAKTERISTIK BAHAN KERAMIK

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